Long-term Debt (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2016 USD ($) aircraft	Jun. 30, 2016 USD ($) aircraft	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Long-term Debt
Current portion long-term debt	$ 280,220	$ 280,220		$ 272,027
Long-term debt net of current maturities	1,781,445	1,781,445		1,676,776
Total long-term debt (including current portion)	2,061,665	2,061,665		1,948,803
Unamortized debt issue cost, net	(21,848)	(21,848)		(20,902)
Total long-term debt, net of debt issue costs	2,039,817	2,039,817		$ 1,927,901
Letter of Credit
Long-term Debt
Amount outstanding	6,800	6,800
Line of Credit
Long-term Debt
Maximum borrowing capacity	75,000	75,000	$ 25,000
Amount outstanding	0	0
Current borrowing capacity	$ 68,200	$ 68,200
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	8	11
Debt issued to purchase of aircraft	$ 180,700	$ 247,800
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%