Represents the number of grants that will be made on achieving specified percentage of performance target.

No definition available.

Name:	amch_AwardsToBeGrantedOnAchievingSpecifiedPercentageOfPerformanceTarget
Namespace Prefix:	amch_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of grants that will be made if target goal are not achieved.

+ References

No definition available.

+ Details

Name:	amch_AwardsToBeGrantedSpecifiedGoalsNotAchieved
Namespace Prefix:	amch_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of days from termination of service under certain termination scenarios as defined in the award agreement for settlement of fully vested restricted stock unit.

+ References

No definition available.

+ Details

Name:	amch_NumberOfDaysFromTerminationOfServiceForSettlementOfFullyVestedEquityInstrumentsOtherThanOptions
Namespace Prefix:	amch_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of Tranches.

+ References

No definition available.

+ Details

Name:	amch_NumberOfTranches
Namespace Prefix:	amch_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the percentage of PSUs vesting expressed as a percentage of performance target.

+ References

No definition available.

+ Details

Name:	amch_PercentageOfPerformanceStockUnitAwardVestingAsPercentageOfPerformanceTarget
Namespace Prefix:	amch_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the percentage of performance target for grant of performance stock unit award.

+ References

No definition available.

+ Details

Name:	amch_PercentageOfPerformanceTargetForGrantOfPerformanceStockUnitAward
Namespace Prefix:	amch_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the period of time of cumulative adjusted EBITDA, diluted earnings per share, and net profit performance target conditions for vesting of the PSU awards to occur.

+ References

No definition available.

+ Details

Name:	amch_PeriodOfCumulativeAdjustedEbitdaDilutedEarningsPerShareNetProfitPerformanceToMeetTargetCondition
Namespace Prefix:	amch_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Value of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan.

+ References

No definition available.

+ Details

Name:	amch_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodValue
Namespace Prefix:	amch_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the fair value of restricted stock units granted during the period.

+ References

No definition available.

+ Details

Name:	amch_ShareBasedCompensationArrangementByShareBasedPaymentAwardOtherThanOptionsAggregateFairValue
Namespace Prefix:	amch_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Aggregate grant date fair value of equity instruments other than options.

+ References

No definition available.

+ Details

Name:	amch_ShareBasedCompensationArrangementByShareBasedPaymentAwardsEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	amch_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Volume weighted average stock price.

+ References

No definition available.

+ Details

Name:	amch_ShareBasedCompensationVolumeVolumeWeightedAverageExercisePrice
Namespace Prefix:	amch_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Volume weighted average stock price threshold trailing days.

+ References

No definition available.

+ Details

Name:	amch_VolumeWeightedAverageStockPriceThresholdTrailingDays
Namespace Prefix:	amch_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5047-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_AllocatedShareBasedCompensationExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Number of securities into which each warrant or right may be converted. For example, but not limited to, each warrant may be converted into two shares.

+ References

No definition available.

+ Details

Name:	us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByEachWarrantOrRight
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Price of a single share of a number of saleable stocks of a company.

+ References

No definition available.

+ Details

Name:	us-gaap_SharePrice
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Number, after forfeiture, of shares or units issued under share-based payment arrangement. Excludes shares or units issued under employee stock ownership plan (ESOP).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_GeneralAndAdministrativeExpenseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=amch_TwoThousandTwentyOnePerformanceShareAwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=amch_TwoThousandTwentyPerformanceShareAwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=amch_TwoThousandNineteenPerformanceShareAwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=amch_SpecialPerformanceStockUnitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=amch_ShareBasedPaymentArrangementTrancheFiveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=amch_ShareBasedPaymentArrangementTrancheSixMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=amch_RestrictedSharesAndPerformanceSharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_PerformanceSharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GranteeStatusAxis=amch_ManagementAndExecutiveOfficersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=amch_StockOptionPlan2013Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GranteeStatusAxis=srt_ManagementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockUnitsRSUMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_EmployeeStockOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GranteeStatusAxis=srt_DirectorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

STOCKHOLDERS' EQUITY - Awards granted in 2021 (Details) $ / shares in Units, $ in Millions		1 Months Ended	3 Months Ended
	Feb. 23, 2021 USD ($) shares	Jan. 31, 2021 $ / shares	Mar. 31, 2021 USD ($) tranche $ / shares shares	Mar. 31, 2020 USD ($)
STOCKHOLDERS' EQUITY
Stock-based compensation expense \| $			$ 5.4	$ 2.7
General and administrative: other
STOCKHOLDERS' EQUITY
Stock-based compensation expense \| $			$ 5.4	$ 2.7
2021 Performance share award
STOCKHOLDERS' EQUITY
Granted (in shares)	895,951
Stock value \| $	$ 6.9
2020 Performance share award
STOCKHOLDERS' EQUITY
Granted (in shares)	438,244
Stock value \| $	$ 3.4
2019 Performance share award
STOCKHOLDERS' EQUITY
Granted (in shares)	181,916
Stock value \| $	$ 1.4
Special Performance Stock Unit
STOCKHOLDERS' EQUITY
Volume weighted average stock price threshold trailing days		20 days
Special Performance Stock Unit \| Tranche 5
STOCKHOLDERS' EQUITY
Weighted Average Stock Price (in dollars per share) \| $ / shares		$ 4
Special Performance Stock Unit \| Tranche 6
STOCKHOLDERS' EQUITY
Weighted Average Stock Price (in dollars per share) \| $ / shares		$ 8
RSU and PSU Units
STOCKHOLDERS' EQUITY
Price per share (in dollars per share) \| $ / shares			$ 7.70
Performance Vesting \| Members of management and executive officers
STOCKHOLDERS' EQUITY
Granted (in shares)			2,687,813
Number of tranches \| tranche			3
Awards to be granted if target not achieved (in shares)			0
Number of days from the termination of service for settlement of fully vested units			30 days
Period of cumulative adjusted EBITDA, diluted earnings per share, and net profit results to meet the performance target condition			3 years
Performance Vesting \| Members of management and executive officers \| Minimum
STOCKHOLDERS' EQUITY
PSUs vesting as a percentage of performance target			80.00%
Percentage of performance target			50.00%
Performance Vesting \| Members of management and executive officers \| Maximum
STOCKHOLDERS' EQUITY
PSUs vesting as a percentage of performance target			120.00%
Percentage of performance target			200.00%
2013 Equity Incentive Plan \| Members of management
STOCKHOLDERS' EQUITY
Granted (in shares)			2,687,813
Grant date fair value (in dollars) \| $			$ 20.7
Vesting period (in years)			3 years
2013 Equity Incentive Plan \| Restricted stock unit \| Members of management and executive officers
STOCKHOLDERS' EQUITY
Number of shares to be received for each unit			1
2013 Equity Incentive Plan \| Restricted stock unit \| Members of management
STOCKHOLDERS' EQUITY
Number of days from the termination of service for settlement of fully vested units			30 days
2013 Equity Incentive Plan \| Performance Vesting \| Members of management and executive officers
STOCKHOLDERS' EQUITY
Awards to be granted upon achieving 100% of performance target (in shares)			2,687,813
Class A common stock \| 2013 Equity Incentive Plan \| Stock options \| Board of Director
STOCKHOLDERS' EQUITY
Grant date fair value \| $			$ 0.9
Shares granted			124,054