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Stock-Based Compensation - Stock Plans (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2019
Jul. 01, 2019
Jan. 01, 2019
Dec. 31, 2018
Stock Plans [Abstract]          
Shares of common stock added 92,921,173 92,921,173     89,345,978
Unrecognized compensation expense related to non-vested stock options $ 82.1 $ 82.1      
Weighted average amortization period   1 year 5 months 15 days      
RSUs [Member]          
Stock Plans [Abstract]          
Unrecognized compensation expense related to non-vested stock options $ 0.9 $ 0.9      
Weighted average amortization period   1 year 5 months 12 days      
2015 Equity Incentive Plan [Member]          
Stock Plans [Abstract]          
Number of shares authorized for issuance pursuant to stock awards (in shares) 18,500,000 18,500,000      
Number of shares available for future grant (in shares) 3,824,563 3,824,563      
2015 Equity Incentive Plan [Member] | Stock Options [Member]          
Stock Plans [Abstract]          
Options outstanding (in shares) 13,068,974 13,068,974      
Options exercisable (in shares) 5,382,040 5,382,040      
2015 Equity Incentive Plan [Member] | RSUs [Member]          
Stock Plans [Abstract]          
Restricted stock units outstanding (in shares) 32,669 32,669      
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 1,500,000      
Shares issued under ESPP (in shares) 0 17,740      
Accrued liability for contributions to employee stock purchase plan $ 0.5 $ 0.5      
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