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Equity Incentive Plans - Summary of option activity (Details) - Stock options - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Stock option activity      
Outstanding at the beginning of the period (in shares) 11,262    
Granted (in shares) 2,388    
Exercised (in shares) (1,710)    
Forfeited (in shares) (206)    
Expired (in shares) (4)    
Outstanding at the end of the period (in shares) 11,730 11,262  
Outstanding, net of expected forfeitures (in shares) 11,614    
Outstanding, exercisable ("vested") (in shares) 6,968    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]      
Outstanding at the beginning of the period (in dollars per share) $ 58.46    
Granted (in dollars per share) 93.04    
Exercised (in dollars per share) 53.57    
Forfeited (in dollars per share) 80.79    
Expired (in dollars per share) 52.21    
Outstanding at the end of the period (in dollars per share) 65.82 $ 58.46  
Outstanding, net of expected forfeitures (in dollars per share) 65.59    
Outstanding, exercisable ("vested") (in dollars per share) $ 54.14    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]      
Aggregate intrinsic value, Outstanding $ 221,999    
Aggregate intrinsic value, net of expected forfeitures, outstanding 221,832    
Aggregate intrinsic value, exercisable ("vested"), outstanding $ 198,747    
Weighted average remaining contractual term, Outstanding 6 years 3 months 21 days    
Weighted average remaining contractual term, net of expected forfeitures, outstanding 6 years 3 months 21 days    
Weighted average remaining contractual term, exercisable ("vested"), outstanding 4 years 11 months 24 days    
Weighted average grant date fair value of options granted (in dollars per share) $ 17.03 $ 14.60 $ 12.25
Intrinsic value of options exercised $ 72,000 $ 199,000 $ 119,000