Annual report pursuant to Section 13 and 15(d)

Debt Instruments - Schedule of long-term debt (Details)

v3.8.0.1
Debt Instruments - Schedule of long-term debt (Details) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Dec. 09, 2015
Debt Instrument [Line Items]      
Unamortized discount and deferred issuance costs $ (71,986,000) $ (92,753,000)  
Long-term debt, net 273,014,000 342,247,000  
Line of credit      
Debt Instrument [Line Items]      
Credit facility 0 90,000,000  
Convertible debt | 1.75% convertible senior notes due 2020      
Debt Instrument [Line Items]      
1.75% convertible senior notes due 2020 $ 345,000,000 $ 345,000,000  
Interest rate 1.75%   1.75%