Leases - Additional Information (Details) $ in Millions	6 Months Ended
Jun. 30, 2021 USD ($)
Lessee Lease Description [Line Items]
Lessee, operating lease, existence of option to extend	true
Lessee, operating lease, existence of option to terminate	true
Commitment relating to operating lease, that have not yet commenced	$ 4.4
Minimum
Lessee Lease Description [Line Items]
Remaining lease term, operating lease	2 years
Operating leases that have not yet commenced, lease terms	4 years
Maximum
Lessee Lease Description [Line Items]
Remaining lease term, operating lease	11 years

Commitment relating to operating lease, that have not yet commenced.

No definition available.

Name:	roku_CommitmentRelatingToOperatingLeaseThatHaveNotYetCommenced
Namespace Prefix:	roku_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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

- Definition

Indicates (true false) whether lessee has option to extend operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=121609121&loc=SL77918673-209980

+ Details

Name:	us-gaap_LesseeOperatingLeaseExistenceOfOptionToExtend
Namespace Prefix:	us-gaap_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

- Definition

Indicates (true false) whether lessee has option to terminate operating lease.

+ References

+ Details

Name:	us-gaap_LesseeOperatingLeaseExistenceOfOptionToTerminate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

- Definition

Term of lessee's operating lease not yet commenced, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121609121&loc=SL77918673-209980

+ Details

Name:	us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

Remaining lease term of operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=121609121&loc=SL77918673-209980

+ Details

Name:	us-gaap_LesseeOperatingLeaseRemainingLeaseTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- 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: