Number of warrants or rights cancelled.

No definition available.

Name:	roku_ClassOfWarrantOrRightCancelled
Namespace Prefix:	roku_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Class of warrant or right, exercisable period.

+ References

No definition available.

+ Details

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

- Definition

Number of warrants or rights exercised.

+ References

No definition available.

+ Details

Name:	roku_ClassOfWarrantOrRightExercised
Namespace Prefix:	roku_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt instrument, covenant required minimum current ratio.

+ References

No definition available.

+ Details

Name:	roku_DebtInstrumentCovenantRequiredMinimumCurrentRatio
Namespace Prefix:	roku_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt instrument floor rate.

+ References

No definition available.

+ Details

Name:	roku_DebtInstrumentFloorRate
Namespace Prefix:	roku_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Letter of credit maximum borrowing capacity.

+ References

No definition available.

+ Details

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

- Definition

Line of credit facility, final payment fees, percent.

+ References

No definition available.

+ Details

Name:	roku_LineOfCreditFacilityFinalPaymentFeesPercent
Namespace Prefix:	roku_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line of credit facility remaining amount available for drawing in increments.

+ References

No definition available.

+ Details

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

- Definition

Line of credit facility remaining amount available term.

+ References

No definition available.

+ Details

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

- Definition

Line of credit facility, used capacity, fee percentage.

+ References

No definition available.

+ Details

Name:	roku_LineOfCreditFacilityUsedCapacityFeePercentage
Namespace Prefix:	roku_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Payment in kind interest fees, percent.

+ References

No definition available.

+ Details

Name:	roku_PaymentInKindInterestFeesPercent
Namespace Prefix:	roku_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Subordinated loan agreement maximum borrowing capacity.

+ References

No definition available.

+ Details

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

- Definition

Subordinated loan agreement minimum borrowing capacity.

+ References

No definition available.

+ Details

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

- Definition

Exercise price per share or per unit of warrants or rights outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(i)(4))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of warrants or rights outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

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

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: 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(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_LineOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Date the credit facility terminates, in CCYY-MM-DD format.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityExpirationDate1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

The effective interest rate at the end of the reporting period.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_DebtInstrumentAxis=roku_SecondAmendmentToRestatedTwentyFourteenLoanAndSecurityAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_PrimeRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=roku_TwoThousandSeventeenSubordinatedLoanAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=roku_TermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ClassOfWarrantOrRightAxis=roku_SeriesHPreferredStockWarrantsMember
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_LineOfCreditFacilityAxis=roku_SiliconValleyBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=roku_AmendedAndRestatedTwentyFourteenLoanAndSecurityAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=roku_ThirdAmendmentToRestatedTwentyFourteenLoanAndSecurityAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Debt - Additional Information (Details) - USD ($)	1 Months Ended
Debt - Additional Information (Details) - USD ($)	Jun. 30, 2017	Sep. 30, 2018	Jul. 18, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Oct. 31, 2017
Debt Instrument [Line Items]
Outstanding debt		$ 0				$ 0
Letters of credit outstanding		$ 23,000,000				1,500,000
Warrants outstanding		0
Second Amendment To Restated 2014 Loan and Security Agreement
Debt Instrument [Line Items]
Minimum current ratio required under financial covenant	1.25%
Line of credit, termination date	Jun. 30, 2019
Letters of credit outstanding		$ 23,000,000				$ 1,500,000
Interest rate on line of credit		5.01%				4.31%
Second Amendment To Restated 2014 Loan and Security Agreement \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	2.75%
Second Amendment To Restated 2014 Loan and Security Agreement \| Prime Rate
Debt Instrument [Line Items]
Basis spread on variable rate	1.00%
2017 Subordinated Loan Agreement \| Term Loan
Debt Instrument [Line Items]
Subordinated loan agreement maximum borrowing capacity	$ 40,000,000
Subordinated loan agreement minimum borrowing capacity	$ 25,000,000
Duration of remaining amount available	24 months
Facility fee, percentage	1.00%
LIBOR Floor rate	1.00%
Payment in kind interest fees, percent	2.50%
Warrants outstanding		0
Warrants cancelled							100,000
Warrants exercised					200,000	100,000
2017 Subordinated Loan Agreement \| Term Loan \| Series H Convertible Preferred Stock Warrants
Debt Instrument [Line Items]
Warrants outstanding	400,000
Warrants, exercise price	$ 9.17340
Warrants, exercisable period	10 years
2017 Subordinated Loan Agreement \| Term Loan \| Minimum
Debt Instrument [Line Items]
Remaining amount to be drawn as installment	$ 5,000,000
Line of credit facility final payment fees, percent	1.00%
2017 Subordinated Loan Agreement \| Term Loan \| Maximum
Debt Instrument [Line Items]
Line of credit facility final payment fees, percent	4.00%
2017 Subordinated Loan Agreement \| LIBOR \| Term Loan
Debt Instrument [Line Items]
Basis spread on variable rate	6.50%
2017 Subordinated Loan Agreement \| Prime Rate \| Term Loan
Debt Instrument [Line Items]
Basis spread on variable rate	3.50%
Silicon Valley Bank \| Restated 2014 LSA
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 5,000,000
Silicon Valley Bank \| Third Amendment To Restated 2014 Loan and Security Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 5,000,000
Maximum borrowing capacity			$ 30,000,000
Revolving Credit Facility \| Second Amendment To Restated 2014 Loan and Security Agreement
Debt Instrument [Line Items]
Line of credit outstanding		0				$ 0
Revolving Credit Facility \| Silicon Valley Bank \| Restated 2014 LSA
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 30,000,000