Debt and Other Obligations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term debt	$ 1,354,700		$ 1,354,700		$ 1,012,400
Less current maturities	105,958		105,958		84,354
Less unamortized discounts	34,600		34,600		30,600
Total	1,214,138		1,214,138		897,359
Repayments of debt obligations	13,400	$ 10,000	63,400	$ 29,600
Senior Loans [Member]
Debt Instrument [Line Items]
Long-term debt	$ 426,400		$ 426,400		451,900
Weighted-average interest rate	4.10%	4.10%	4.10%	4.10%
Junior Loans [Member]
Debt Instrument [Line Items]
Long-term debt	$ 41,300		$ 41,300		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	$ 363,400		$ 363,400		0
Weighted-average interest rate	3.76%		3.76%
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%