Debt and Other Obligations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term debt	$ 1,218,100		$ 1,218,100		$ 1,012,400
Less current maturities	95,428		95,428		84,354
Less unamortized discounts	33,500		33,500		30,600
Total	1,089,159		1,089,159		897,359
Repayments of debt obligations	39,800	$ 9,900	50,000	$ 19,600
Senior Loans [Member]
Debt Instrument [Line Items]
Long-term debt	$ 435,000		$ 435,000		451,900
Weighted-average interest rate	4.10%	4.10%	4.10%	4.10%
Junior Loans [Member]
Debt Instrument [Line Items]
Long-term debt	$ 43,200		$ 43,200		47,100
Weighted-average interest rate	6.90%	6.90%	6.90%	6.90%
Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Long-term debt	$ 216,300		$ 216,300		0
Weighted-average interest rate	3.82%		3.82%
Equipment Notes, Series A [Member]
Debt Instrument [Line Items]
Long-term debt	$ 423,600		$ 423,600		409,800
Weighted-average interest rate	4.10%	4.03%	4.10%	4.03%
Equipment Notes, Series B [Member]
Debt Instrument [Line Items]
Long-term debt	$ 100,000		$ 100,000		$ 103,600
Weighted-average interest rate	4.45%	4.38%	4.45%	4.38%