Debt - Convertible Notes (Details)
9 Months Ended
May 31, 2014
USD ($)
$ / shares
Sep. 30, 2019
USD ($)
$ / shares
Dec. 31, 2018
USD ($)
Debt Instrument [Line Items]      
Unamortized deferred financing costs   $ 18,569,000 $ 14,932,000
Convertible Senior Notes      
Debt Instrument [Line Items]      
Debt discount, value of the embedded conversion premium $ 56,700,000 7,700,000 13,300,000
Unamortized deferred financing costs 19,600,000 $ 2,400,000 $ 4,300,000
Convertible Senior Notes | Convertible Senior Notes Due 2019      
Debt Instrument [Line Items]      
Aggregate principal amount of debt $ 402,500,000    
Stated interest rate 2.875%    
Debt instrument, maturity date May 15, 2019    
Convertible Senior Notes | Convertible Senior Notes Due 2021      
Debt Instrument [Line Items]      
Aggregate principal amount of debt $ 345,000,000.0    
Stated interest rate 3.75%    
Debt instrument, maturity date May 15, 2021    
Debt conversion ratio 15.2727 17.4458  
Conversion price (in USD per share) | $ / shares $ 65.48    
Premium above public offering price 22.50%    
Anti-dilutive cash dividends, exceeding (in USD per share) | $ / shares   $ 0.73026