Debt and Other Obligations - Long-term Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,012,400	$ 659,300
Less current maturities	84,354	49,637
Less unamortized discount, net	30,600	13,000
Total	897,359	596,693
Senior loans
Debt Instrument [Line Items]
Long-term debt, gross	$ 451,900	$ 484,200
Weighted-average interest rates	4.10%	4.10%
Junior loans
Debt Instrument [Line Items]
Long-term debt, gross	$ 47,100	$ 54,300
Weighted-average interest rates	6.90%	6.90%
Equipment Notes, Series A
Debt Instrument [Line Items]
Long-term debt, gross	$ 409,800	$ 95,800
Weighted-average interest rates	4.10%	4.03%
Equipment Notes, Series B
Debt Instrument [Line Items]
Long-term debt, gross	$ 103,600	$ 25,000
Weighted-average interest rates	4.45%	4.37%