v3.8.0.1
Document and Entity Information - shares
9 Months Ended
Sep. 30, 2017
Nov. 03, 2017
Document Information [Line Items]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Sep. 30, 2017  
Document Fiscal Year Focus 2017  
Document Fiscal Period Focus Q3  
Trading Symbol ROKU  
Entity Registrant Name Roku, Inc.  
Entity Central Index Key 0001428439  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Class A Common Stock    
Document Information [Line Items]    
Entity Common Stock, Shares Outstanding   18,106,218
Class B Common Stock    
Document Information [Line Items]    
Entity Common Stock, Shares Outstanding   79,718,676