v3.10.0.1
Document and Entity Information - shares
6 Months Ended
Jun. 30, 2018
Jul. 31, 2018
Document Information [Line Items]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Jun. 30, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q2  
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   68,400,971
Class B Common Stock    
Document Information [Line Items]    
Entity Common Stock, Shares Outstanding   37,677,499