Long-term Debt (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) aircraft	Dec. 31, 2016 USD ($)
Long-term Debt
Current portion long-term debt	$ 307,032	$ 308,945
Current portion of unamortized debt issue cost, net	(3,609)	(3,485)
Current portion of long-term debt, net of debt issue costs	303,423	305,460
Long-term debt, net of current maturities	2,402,996	2,261,959
Long-term portion of unamortized debt issue cost, net	(22,204)	(21,908)
Long-term debt, net of current maturities and debt issue costs	2,380,792	2,240,051
Total long-term debt (including current portion)	2,710,028	2,570,904
Total unamortized debt issue cost, net	(25,813)	(25,393)
Total long-term debt, net of debt issue costs	2,684,215	2,545,511
Letter of credit
Long-term Debt
Amount outstanding	$ 85,200	$ 85,200
E 175
Long-term Debt
Number of aircraft delivered | aircraft	18
Debt issued to purchase of aircraft	$ 384,800
Debt instrument, term	12 years
Minimum | E 175
Long-term Debt
Interest rate (as a percent)	4.30%
Maximum | E 175
Long-term Debt
Interest rate (as a percent)	4.70%