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Convertible Senior Secured Notes (Details)
12 Months Ended
Mar. 15, 2018
USD ($)
shares
Mar. 14, 2018
USD ($)
D
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Notes payable      
Total cost of the convertible note hedge transactions     $ 92,897,000
Strike price of the Warrant Transactions (in dollars per share) | $ / shares     $ 80.9063
Proceeds from sale of warrants     $ 65,688,000
Debt financing costs     10,435,000
Liability component of the 2023 Notes      
December 31, 2018 carrying value     329,462,000
2023 Notes      
Notes payable      
Aggregate principal amount     402,500,000
Threshold trading days (whether or not consecutive) | D   20  
Consecutive trading day period (in days) | D   30  
Minimum percentage of the conversion price on trading day   130.00%  
Conversion option stock price trigger | $ / shares   $ 77.13  
Threshold trading days period prior to a conversion date during which principal amount of notes for such trading day was less than 98% of the product of the last reported sale price of common stock | D   5  
Consecutive trading day period prior to a conversion date during which principal amount of notes for such trading day was less than 98% of the product of the last reported sale price of common stock | D   10  
Conversion rate for the Notes (in shares) | shares   16.8545  
Conversion ratio, principal amount   $ 1,000  
Conversion price, per share of Common Stock | $ / shares   $ 59.33  
Amount of principal value to be repaid     $ 402,500,000
Warrants issued | shares     6,783,939
Number of shares per warrant entitled to holder | shares     1
Effective interest rate (as percent)     5.41%
Conversion option reported as debt discount and APIC     $ 2,000,000
Liability component of the 2023 Notes      
Principal amount of the 2023 Notes     402,500,000
Debt discount     (76,434,000)
Deferred financing costs     (8,452,000)
Accretion of debt discount and deferred financing costs     11,848,000
December 31, 2018 carrying value     329,462,000
Shares of common stock issued upon conversion of Notes | shares 6,800,000    
Conversion of debt to equity - principal     $ 0
2023 Notes | Overallotment option      
Notes payable      
Convertible note hedge options issued (in shares) | shares 402,500    
Convertible Senior Notes due 2023 granted to initial purchasers      
Notes payable      
Aggregate principal amount   $ 350,000,000  
Interest rate (as a percent)   0.625%  
Liability component of the 2023 Notes      
Principal amount of the 2023 Notes   $ 350,000,000  
Convertible Senior Notes due 2023 over-allotment option      
Notes payable      
Aggregate principal amount $ 52,500,000    
Over-allotment period for additional principal amount 30 days    
Liability component of the 2023 Notes      
Principal amount of the 2023 Notes $ 52,500,000