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Convertible Senior Notes - Convertible Senior Notes (Details)
1 Months Ended 9 Months Ended
Jun. 30, 2018
USD ($)
Apr. 30, 2018
USD ($)
day
$ / shares
shares
Sep. 30, 2018
USD ($)
$ / shares
Sep. 30, 2017
USD ($)
[1]
Dec. 31, 2018
$ / shares
Debt Instrument [Line Items]          
Net proceeds     $ 335,618,000 $ 0  
Senior Notes | 0.25% Convertible Senior Notes Due 2023          
Debt Instrument [Line Items]          
Face value   $ 345,000,000 345,000,000    
Interest rate, stated percentage   0.25%      
Option to purchase additional notes   $ 45,000,000      
Less initial purchasers’ discount   (8,625,000)      
Less other issuance costs   (757,000)      
Net proceeds   $ 335,618,000      
Conversion ratio   0.037051      
Equity component   $ 64,200,000 $ 62,444,000    
Debt issuance costs   (9,400,000)      
Debt instrument, remaining useful life     4 years 7 months 6 days    
Share price (in dollars per share) | $ / shares     $ 28.43    
Principal $ 363,900,000        
Debt instrument, if-converted value in excess of principal   $ (18,900,000)      
Interest rate, effective percentage   4.34%      
Sale Price Is Greater Or Equal 130% | Senior Notes | 0.25% Convertible Senior Notes Due 2023          
Debt Instrument [Line Items]          
Threshold trading days | day   20      
Threshold consecutive trading days | day   30      
Threshold percentage of stock price trigger   130.00%      
Trading Price Per $1,000 Principal Amount Less Than 98% | Senior Notes | 0.25% Convertible Senior Notes Due 2023          
Debt Instrument [Line Items]          
Threshold trading days | day   5      
Threshold consecutive trading days | day   10      
Trading Price Per $1,000 Principal Amount Less Than 98% | Senior Notes | 0.25% Convertible Senior Notes Due 2023 | Maximum          
Debt Instrument [Line Items]          
Threshold percentage of stock price trigger   98.00%      
Liability component | Senior Notes | 0.25% Convertible Senior Notes Due 2023          
Debt Instrument [Line Items]          
Net carrying amount   $ 280,800,000      
Debt issuance costs   (7,600,000)      
Equity component | Senior Notes | 0.25% Convertible Senior Notes Due 2023          
Debt Instrument [Line Items]          
Debt issuance costs   (1,700,000)      
Capped Call | Senior Notes | 0.25% Convertible Senior Notes Due 2023          
Debt Instrument [Line Items]          
Net proceeds   $ 39,200,000      
Conversion price | $ / shares   $ 26.95      
Shares covered by capped call transactions (in shares) | shares   12,801,260      
Scenario, Forecast | Capped Call | Senior Notes | 0.25% Convertible Senior Notes Due 2023          
Debt Instrument [Line Items]          
Conversion price | $ / shares         $ 40.68
[1] Adjusted to reflect the adoption of ASU 2016-18. See Note 1 for more information.