Retirement Plans (Details 8) (Pension [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Level 3 Rollforward [Line Items]
Fair value of plan assets at beginning of year			$ 2,875
Fair value of plan assets at end of year	3,654	3,429	2,875
Significant Unobservable Inputs (Level 3) [Member]
Level 3 Rollforward [Line Items]
Fair value of plan assets at beginning of year	352	322
Realized gain/(loss)	25	15
Unrealized gain	11	27
Purchases	73	66
Sales	(88)	(78)
Fair value of plan assets at end of year	373	352
Venture Capital And Buyout Partnerships [Member]
Level 3 Rollforward [Line Items]
Fair value of plan assets at end of year	234	213
Venture Capital And Buyout Partnerships [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Level 3 Rollforward [Line Items]
Fair value of plan assets at beginning of year	213	179
Realized gain/(loss)	17	7
Unrealized gain	5	24
Purchases	54	43
Sales	(55)	(40)
Fair value of plan assets at end of year	234	213
Real Estate Partnerships And Funds [Member]
Level 3 Rollforward [Line Items]
Fair value of plan assets at end of year	139	139
Real Estate Partnerships And Funds [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Level 3 Rollforward [Line Items]
Fair value of plan assets at beginning of year	139	143
Realized gain/(loss)	8	8
Unrealized gain	6	3
Purchases	19	23
Sales	(33)	(38)
Fair value of plan assets at end of year	$ 139	$ 139

Name:	unp_Level3RollforwardLineItems
Namespace Prefix:	unp_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Actual return on plan assets, separately identifying the amount related to assets sold during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (d)(iv)(02)(A)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 5

-Subparagraph (c)(iv)(02)(A)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920

+ Details

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

- Definition

Actual return on plan assets, separately identifying the amount related to assets still held at the reporting date.

+ References

+ Details

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

- Definition

Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 30

-Section 35

-Paragraph 50

-URI http://asc.fasb.org/extlink&oid=29635418&loc=d3e12355-114930

+ Details

Name:	us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

+ Details

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

- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration