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Liabilities - Convertible Senior Notes (Details)
 3 Months Ended 9 Months Ended
Jan. 10, 2017
USD ($)
$ / shares
Sep. 30, 2017
USD ($)
Sep. 30, 2017
USD ($)
Long-term debt      
Proceeds received, net of transaction fees     $ 335,752,000
Convertible Senior Notes      
Long-term debt      
Principal amount $ 345,000,000 $ 345,000,000 345,000,000
Interest rate (as a percent) 2.70%    
Proceeds received, net of transaction fees $ 335,800,000    
Conversion rate of the Notes to common stock, per $1,000 principal amount of Notes 24.9800    
Common stock per principal amount $ 1,000    
Initial conversion price per share of common stock | $ / shares $ 40.03    
Threshold trading days for convertible debt 20 days    
Threshold consecutive trading days for convertible debt 30 days    
Percentage of minimum stock price trigger for conversion 130.00%    
Number of consecutive business days after the measurement period 5 days    
Number of consecutive trading days making up the measurement period 5 days    
Maximum calculated percentage to which trading price of notes is compared in order to trigger conversion feature of notes 98.00%    
Fair value inputs discount rate (as a percent) 7.00%    
Unamortized debt discount $ (72,500,000) (65,570,000) (65,570,000)
Transaction costs 9,200,000    
Transaction costs allocated to the equity component $ 1,900,000    
Unamortized transaction costs   (6,605,000) (6,605,000)
Net carrying value   272,825,000 272,825,000
Cash Interest Expense      
Coupon interest expense   2,329,000 6,573,000
Non-Cash Interest Expense      
Amortization of debt discount   2,502,000 6,942,000
Amortization of transaction costs   252,000 699,000
Total Interest Expense   5,083,000 14,214,000
Convertible Senior Notes | Level 2      
Non-Cash Interest Expense      
Estimated fair value of Convertible Senior Notes   $ 330,900,000 $ 330,900,000