Annual report pursuant to Section 13 and 15(d)

Stockholders' Equity - Schedule of Reserve Share of Common Stock for Issuance (Details)

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Stockholders' Equity - Schedule of Reserve Share of Common Stock for Issuance (Details) - shares
Dec. 31, 2017
Dec. 31, 2016
Jul. 31, 2015
Class of Stock [Line Items]      
Shares of common stock reserved for issuance 34,425,000 30,595,000  
Outstanding stock options      
Class of Stock [Line Items]      
Shares of common stock reserved for issuance 16,268,000 12,897,000  
Restricted Stock Units (RSUs)      
Class of Stock [Line Items]      
Shares of common stock reserved for issuance 5,330,000 4,106,000  
2015 Equity Incentive Plan      
Class of Stock [Line Items]      
Shares of common stock reserved for issuance 8,857,000 11,081,000 11,400,000
2015 Employee Stock Purchase Plan      
Class of Stock [Line Items]      
Shares of common stock reserved for issuance 3,970,000 2,511,000