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Fair Value Measurements - Carrying Value and Fair Value of Long-Term Debt and Other Financial Instruments (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
5.00% Convertible Senior Notes due 2021    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, interest rate 5.00%  
7.75% Senior Secured Note due 2025    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, interest rate 7.75%  
Carrying Value | Warrant    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Warrants not settleable in cash, fair value $ 6,063 $ 6,808
Carrying Value | 5.00% Convertible Senior Notes due 2021    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt 96,674 95,486
Carrying Value | 7.75% Senior Secured Note due 2025    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt 289,826 289,326
Fair Value | Warrant    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Warrants not settleable in cash, fair value 6,063 6,808
Fair Value | 5.00% Convertible Senior Notes due 2021    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt 136,951 149,007
Fair Value | 7.75% Senior Secured Note due 2025    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt $ 303,855 $ 300,423