Long-term Debt (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($) aircraft	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Current portion long-term debt	$ 349,018	$ 354,072
Less current portion of unamortized debt issue cost, net	(3,765)	(3,866)
Current portion of long-term debt, net of debt issue costs	345,253	350,206
Long-term debt, net of current maturities	2,739,621	2,831,366
Less long-term portion of unamortized debt issue cost, net	(20,580)	(21,598)
Long-term debt, net of current maturities and debt issue costs	2,719,041	2,809,768
Total long-term debt (including current portion)	3,088,639	3,185,438
Total unamortized debt issue cost, net	(24,345)	(25,464)
Total	$ 3,064,294	3,159,974
Number of aircraft leased | aircraft	92
Debt issued to purchase of aircraft	$ 14,500
Letter of credit
Debt Instrument [Line Items]
Amount outstanding		$ 78,700
Line of credit
Debt Instrument [Line Items]
Amount outstanding	$ 77,300
E 175
Debt Instrument [Line Items]
Number of aircraft delivered | aircraft	5
Purchase of aircraft financed through long-term debt	$ 99,900
Debt instrument, term	12 years
Purchased, previously-leased aircraft
Debt Instrument [Line Items]
Number of aircraft purchased | aircraft	2
Debt instrument, term	18 months
Interest rate (as a percent)	2.00%
Minimum | E 175
Debt Instrument [Line Items]
Interest rate (as a percent)	3.80%
Maximum | E 175
Debt Instrument [Line Items]
Interest rate (as a percent)	4.10%