Long-term Debt (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2016 USD ($) aircraft	Sep. 30, 2016 USD ($) aircraft	Dec. 31, 2015 USD ($)
Long-term Debt
Current portion long-term debt	$ 303,604	$ 303,604	$ 272,027
Long-term debt net of current maturities	1,944,701	1,944,701	1,676,776
Total long-term debt (including current portion)	2,248,305	2,248,305	1,948,803
Unamortized debt issue cost, net	(23,172)	(23,172)	(20,902)
Total long-term debt, net of debt issue costs	2,225,133	2,225,133	1,927,901
Letter of Credit
Long-term Debt
Amount outstanding	6,500	6,500
Line of Credit
Long-term Debt
Maximum borrowing capacity	75,000	75,000	$ 25,000
Amount outstanding	0	0
Current borrowing capacity	$ 68,500	$ 68,500
Interest rate (as a percent)	3.00%	3.00%
Line of Credit | LIBOR
Long-term Debt
Basis spread on variable rate		2.50%
E 175
Long-term Debt
Number of aircraft delivered | aircraft	11	22
Debt issued to purchase of aircraft	$ 248,500	$ 497,500
Debt instrument, term		12 years
Minimum | E 175
Long-term Debt
Interest rate (as a percent)	3.40%	3.40%
Maximum | E 175
Long-term Debt
Interest rate (as a percent)	3.80%	3.80%