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Balance Sheet Components - Narrative (Details) - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Property, Plant and Equipment [Line Items]      
Depreciation expense $ 2,200,000 $ 1,500,000 $ 1,800,000
Capitalized costs 4,000,000 500,000 0
Internal-use software and platform development costs capitalized, amortization expense 100,000 0 1,000,000
Amortization of intangible assets 2,700,000 2,700,000 5,700,000
Carrying amount 6,004,000 8,672,000  
Developed technology and trade names      
Property, Plant and Equipment [Line Items]      
Amortization of intangible assets     2,900,000
Developed technology      
Property, Plant and Equipment [Line Items]      
Carrying amount $ 0 0 2,600,000
User relationships      
Property, Plant and Equipment [Line Items]      
Amortization of intangible assets     $ 2,700,000
Intangible asset, useful life 2 years 3 months 18 days    
Carrying amount $ 6,004,000 $ 8,672,000