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Stock-Based Compensation - Restricted Stock and Restricted Stock Units (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
RSUs      
Stock Based Compensation disclosures      
Number of common shares each award holder is entitled to receive (in shares) 1    
Unrecognized compensation expense related to unvested awards $ 15.0    
Expected weighted average period for recognized and unrecognized compensation expense 2 years 3 months 18 days    
Unvested restricted stock and restricted stock units                
Number of RSUs      
Outstanding, beginning balance (in shares) 500,822 227,826 46,914
Granted (in shares) 559,054 407,655 253,586
Released (in shares) (161,774) (92,145) (51,992)
Forfeited (in shares) (53,711) (42,514) (20,682)
Outstanding, ending balance (in shares) 844,391 500,822 227,826
Weighted Average Grant Price      
Outstanding Weighted Average Grant Price (in dollars per share) $ 28.32 $ 29.14 $ 17.16
Granted (in dollars per share) 23.85 27.89 29.16
Released (in dollars per share) 28.90 28.05 18.46
Forfeited (in dollars per share) 25.43 29.11 29.05
Outstanding Weighted Average Grant Price (in dollars per share) $ 25.43 $ 28.32 $ 29.14
Stock options and RSUs      
Weighted Average Grant Price      
ESPP compensation expense $ 36.2 $ 27.0 $ 26.2
Stock options and RSUs | Research and development expenses      
Weighted Average Grant Price      
ESPP compensation expense 11.8 8.2 9.4
Stock options and RSUs | Selling, general and administrative expenses      
Weighted Average Grant Price      
ESPP compensation expense 24.4 18.8 16.8
Employee Stock      
Weighted Average Grant Price      
ESPP compensation expense $ 1.2 $ 1.6 $ 0.9