Long-term Debt (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 940,300		$ 659,300
Less current maturities	78,596		49,637
Less unamortized discounts, net	25,300		13,000
Total	836,418		596,693
Repayments of debt obligations	19,600	$ 8,200
Senior Loans [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 468,200		484,200
Long-term Debt, Weighted Average Interest Rate	4.10%	4.04%
Junior Loans [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 50,700		54,300
Long-term Debt, Weighted Average Interest Rate	6.90%	6.89%
Equipment Notes, Series A [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 333,300		95,800
Long-term Debt, Weighted Average Interest Rate	4.03%
Equipment Notes, Series B [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 88,100		$ 25,000
Long-term Debt, Weighted Average Interest Rate	4.38%