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Long-Term Debt - Carrying Value of 2024 Notes and 2020 Notes (Details) - Convertible Debt - USD ($)
Sep. 30, 2019
Jun. 03, 2019
Dec. 31, 2018
Mar. 11, 2015
Debt Instrument [Line Items]        
Principal $ 494,909,000   $ 316,000,000  
Unamortized interest discount (69,530,000)      
Deferred financing costs (6,243,000)      
Net carrying amount of 2024 and 2020 Notes 419,136,000   298,951,000  
Convertible Notes 2024        
Debt Instrument [Line Items]        
Principal 400,000,000 $ 400,000,000.0    
Unamortized interest discount (67,857,000)      
Deferred financing costs (6,069,000) $ (1,600,000)    
Net carrying amount of 2024 and 2020 Notes 326,074,000      
Convertible Notes 2020        
Debt Instrument [Line Items]        
Principal 94,909,000   316,000,000 $ 316,000,000.0
Unamortized interest discount (1,673,000)   (15,428,000)  
Deferred financing costs (174,000)   (1,621,000)  
Net carrying amount of 2024 and 2020 Notes $ 93,062,000   $ 298,951,000