| Document and Entity Information | Jan. 07, 2022 |
| Cover [Abstract] | |
| Entity Registrant Name | ROKU, INC |
| Amendment Flag | false |
| Entity Central Index Key | 0001428439 |
| Document Type | 8-K |
| Document Period End Date | Jan. 7, 2022 |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-38211 |
| Entity Tax Identification Number | 26-2087865 |
| Entity Address, Address Line One | 1155 Coleman Avenue |
| Entity Address, City or Town | San Jose |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 95110 |
| City Area Code | (408) |
| Local Phone Number | 556-9040 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Class A Common Stock, $0.0001 par value |
| Trading Symbol | ROKU |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
