Long-term Debt (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 1,008,300		$ 1,008,300		$ 659,300
Less current maturities	84,443		84,443		49,637
Less unamortized discounts, net	29,100		29,100		13,000
Long-term debt, less current maturities	894,809		894,809		596,693
Repayments of debt obligations	10,000	$ 7,600	29,600	$ 15,800
Senior Loans [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 460,100		$ 460,100		484,200
Long-term Debt, Weighted Average Interest Rate	4.10%	4.10%	4.10%	4.10%
Junior Loans [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 48,900		$ 48,900		54,300
Long-term Debt, Weighted Average Interest Rate	6.90%	6.90%	6.90%	6.90%
Equipment Notes, Series A [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 394,800		$ 394,800		95,800
Long-term Debt, Weighted Average Interest Rate	4.03%		4.03%
Equipment Notes, Series B [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 104,500		$ 104,500		$ 25,000
Long-term Debt, Weighted Average Interest Rate	4.38%		4.38%