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Debt - Carrying Value - Convertible Senior Notes (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Jan. 10, 2017
Dec. 31, 2016
Net carrying value $ 275,630   $ 826
Convertible Senior Notes      
Principal amount 345,000 $ 345,000  
Unamortized debt discount (63,022)    
Unamortized transaction costs $ (6,348)