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Income Taxes - Narrative (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Income Taxes (Textual) [Abstract]      
Income taxes at a statutory rate 35.00% 35.00% 35.00%
Cumulative loss period 3 years    
Valuation allowance $ 177,405 $ 143,508  
Operating loss carryforwards 260,100    
Operating loss carryforwards subject to expiration (in 2025) 6,900    
Share-based Compensation      
Income Taxes (Textual) [Abstract]      
Operating loss carryforwards 12,600    
Depletion Deduction      
Income Taxes (Textual) [Abstract]      
Depletion deduction carryforward $ 26,800