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Stock-Based Compensation - Stock Plans (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Jul. 01, 2019
Jan. 01, 2019
Dec. 31, 2018
Stock Plans [Abstract]        
Shares of common stock added 92,635,374     89,345,978
Unrecognized compensation expense related to non-vested stock options $ 3.1      
2015 Equity Incentive Plan [Member]        
Stock Plans [Abstract]        
Number of shares authorized for issuance pursuant to stock awards (in shares) 18,500,000      
Number of shares available for future grant (in shares) 3,854,948      
2015 Equity Incentive Plan [Member] | Stock Options [Member]        
Stock Plans [Abstract]        
Options outstanding (in shares) 13,337,602      
Options exercisable (in shares) 4,680,540      
2015 Equity Incentive Plan [Member] | RSUs [Member]        
Stock Plans [Abstract]        
Restricted stock units outstanding (in shares) 36,455      
2017 Employee Stock Purchase Plan [Member]        
Stock Plans [Abstract]        
Shares of common stock added     500,000  
Number of shares of common stock reserved for issuance (in shares) 1,500,000      
Shares issued under ESPP (in shares) 17,740      
Accrued liability for contributions to employee stock purchase plan $ 0.3      
2017 Employee Stock Purchase Plan [Member] | Subsequent Event [Member]        
Stock Plans [Abstract]        
Number of shares of common stock reserved for issuance (in shares)   1,450,709