Business Combinations - Additional Information (Details) - USD ($) $ in Millions	Apr. 15, 2021	Mar. 19, 2021
Other Non-current Assets
Business Acquisition [Line Items]
Content assets acquired		$ 22.5
Nielsen's Advanced Video Advertising Business
Business Acquisition [Line Items]
Preliminary purchase consideration	$ 53.4
Business combination, aggregate cash consideration	38.5
Business combination, noncash consideration	21.4
Business combination, service agreements	$ 6.5
Business combination, obligation to deliver services period	6 years
Business combination, contract terms for goods and services to be received period	6 years
Nielsen's Advanced Video Advertising Business \| General and Administrative Expenses
Business Acquisition [Line Items]
Acquisition related cost	$ 3.9
TOH Intermediate Holdings, LLC
Business Acquisition [Line Items]
Preliminary purchase consideration		97.8
TOH Intermediate Holdings, LLC \| General and Administrative Expenses
Business Acquisition [Line Items]
Acquisition related cost		$ 2.4

Business combination contract terms for goods and services to be received period.

No definition available.

Name:	roku_BusinessCombinationContractTermsForGoodsAndServicesToBeReceivedPeriod
Namespace Prefix:	roku_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Business combination obligation to deliver services period.

+ References

No definition available.

+ Details

Name:	roku_BusinessCombinationObligationToDeliverServicesPeriod
Namespace Prefix:	roku_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Business combination service agreements.

+ References

No definition available.

+ Details

Name:	roku_BusinessCombinationServiceAgreements
Namespace Prefix:	roku_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Content assets acquired during period.

+ References

No definition available.

+ Details

Name:	roku_ContentAssetsAcquiredDuringPeriod
Namespace Prefix:	roku_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_BusinessAcquisitionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 23
-URI http://asc.fasb.org/extlink&oid=123586518&loc=d3e1043-128460

+ Details

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

- Definition

Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 30
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=116859721&loc=d3e6613-128477

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e6927-128479

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 30
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=116859721&loc=d3e6578-128477

+ Details

Name:	us-gaap_BusinessCombinationConsiderationTransferred1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of tangible or intangible assets, including a business or subsidiary of the acquirer transferred by the entity to the former owners of the acquiree. Excludes cash.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e6927-128479

+ Details

Name:	us-gaap_BusinessCombinationConsiderationTransferredOther1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e6927-128479

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3213-108585

+ Details

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

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherNoncurrentAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=roku_NielsenSAdvancedVideoAdvertisingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_BusinessAcquisitionAxis=roku_ThisOldHouseHoldingsLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: