Quarterly report pursuant to Section 13 or 15(d)

Debt Instruments - Summary of the Effective Interest Rate, Contractually Stated Interest Expense and Costs Related to Amortization of Discount for the Notes (Details)

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Debt Instruments - Summary of the Effective Interest Rate, Contractually Stated Interest Expense and Costs Related to Amortization of Discount for the Notes (Details) - Convertible debt - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Convertible senior notes due 2020        
Debt Instrument [Line Items]        
Effective interest rate 10.18% 10.18% 10.18% 10.18%
Contractually stated interest expense $ 691 $ 1,509 $ 3,320 $ 4,511
Amortization of discount $ 2,593 $ 5,135 $ 12,031 $ 14,934
Convertible senior notes due 2023        
Debt Instrument [Line Items]        
Effective interest rate 10.38%   10.38%  
Contractually stated interest expense $ 844   $ 1,191  
Amortization of discount $ 2,381   $ 3,360