v3.8.0.1
Document and Entity Information - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Feb. 14, 2018
Jun. 30, 2017
Document Information [Line Items]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2017    
Document Fiscal Year Focus 2017    
Document Fiscal Period Focus FY    
Trading Symbol ROKU    
Entity Registrant Name Roku, Inc.    
Entity Central Index Key 0001428439    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer No    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Filer Category Non-accelerated Filer    
Entity Public Float     $ 0
Class A Common Stock      
Document Information [Line Items]      
Entity Common Stock, Shares Outstanding   19,746,394  
Class B Common Stock      
Document Information [Line Items]      
Entity Common Stock, Shares Outstanding   79,823,487