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Debt - 2015 Convertible Notes (Details)
1 Months Ended 12 Months Ended
Aug. 31, 2015
USD ($)
day
$ / shares
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Long-term debt      
Additional paid-in capital   $ 1,288,137,000 $ 966,534,000
3.00% Convertible senior notes due 2022 | Convertible debt      
Long-term debt      
Debt instrument $ 150,000,000 150,000,000 150,000,000
Interest rate 3.00%    
Net proceeds from issuance of convertible notes $ 145,400,000    
Trading days, number | day 20    
Consecutive trading days, period | day 30    
Stock price trigger 130.00%    
Business days, period 5 days    
Consecutive trading-day period 5 days    
Maximum product of the closing sale price of shares of the Company's common stock and the applicable conversion rate for such trading day 98.00%    
Conversion ratio 0.0177    
Conversion price per share | $ / shares $ 56.34    
Minimum percentage of principal held by convertible debt instrument holders required to issue notice for declaration of principal and unpaid interest payable upon events of default 25.00%    
Convertible instruments principal and unpaid interest payable upon events of default 100.00%    
Term of the convertible notes 7 years    
Additional paid-in capital   57,500,000  
Net deferred tax liability in connection with convertible notes $ 22,300,000    
Debt instrument   $ 146,600,000 $ 115,700,000
Remaining amortization period   3 years 7 months 13 days  
3.00% Convertible senior notes due 2022 | Convertible debt | Redemption on or after August 20, 2018      
Long-term debt      
Trading days, number | day 19    
Consecutive trading days, period | day 30    
Stock price trigger 130.00%    
Redemption price 100.00%