Long-term Debt (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 723,500		$ 659,300
Less current maturities	53,012		49,637
Less unamortized discounts, net	15,400		13,000
Total	655,103		596,693
Repayments of debt obligations	9,700	$ 2,600
Senior Loans [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 476,200		484,200
Long-term Debt, Weighted Average Interest Rate	4.10%	4.10%
Junior Loans [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 52,600		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	$ 154,200		95,800
Long-term Debt, Weighted Average Interest Rate	4.03%
Equipment Notes, Series B [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 40,500		$ 25,000
Long-term Debt, Weighted Average Interest Rate	4.37%