v3.19.3
INDEBTEDNESS - Net Carrying Amount of Convertible Notes (Details) - Convertible Debt - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
May 25, 2018
Mar. 06, 2017
Debt Instrument [Line Items]        
Principal outstanding $ 1,074,226 $ 1,074,228    
Unamortized debt discount (139,332) (166,493)    
Unamortized debt issuance costs (6,025) (8,040)    
Net carrying value 928,869 899,695    
2023 Notes        
Debt Instrument [Line Items]        
Principal outstanding 862,500 862,500    
Unamortized debt discount (117,932) (138,924)    
Unamortized debt issuance costs (3,647) (5,054) $ (5,600)  
Net carrying value 740,921 718,522    
2022 Notes        
Debt Instrument [Line Items]        
Principal outstanding 211,726 211,728    
Unamortized debt discount (21,400) (27,569)    
Unamortized debt issuance costs (2,378) (2,986)   $ (9,400)
Net carrying value $ 187,948 $ 181,173