v3.20.4

Retirement Plans - Additional Information (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Defined Contribution Plan Disclosure [Line Items] |  |  |
Employer contribution amount to plan | $ 700,000 | $ 500,000 | $ 400,000
Savings Plan |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Employer contribution amount to plan | $ 0 | $ 0 | $ 0

X
- 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_DefinedContributionPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_RetirementPlanNameAxis=roku_TaxDeferredSavingsPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |