Long-term Debt (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) aircraft	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Current portion long-term debt	$ 361,131	$ 313,243
Less current portion of unamortized debt issue cost, net	(3,804)	(3,565)
Current portion of long-term debt, net of debt issue costs	357,327	309,678	[1]
Long-term debt, net of current maturities	2,773,637	2,399,107
Less long-term portion of unamortized debt issue cost, net	(21,915)	(21,761)
Long-term debt, net of current maturities and debt issue costs	2,751,722	2,377,346	[1]
Total long-term debt (including current portion)	3,134,768	2,712,350
Total unamortized debt issue cost, net	(25,719)	(25,326)
Total	3,109,049	2,687,024
Debt issued to purchase of aircraft	626,200
Letter of credit
Debt Instrument [Line Items]
Amount outstanding	$ 84,800	$ 87,400
E 175
Debt Instrument [Line Items]
Number of aircraft delivered | aircraft	31
Debt instrument, term	12 years
Purchased, previously-leased aircraft
Debt Instrument [Line Items]
Number of aircraft purchased | aircraft	9
Interest rate (as a percent)	6.45%
Minimum | E 175
Debt Instrument [Line Items]
Interest rate (as a percent)	4.60%
Minimum | Purchased, previously-leased aircraft
Debt Instrument [Line Items]
Debt instrument, term	3 years
Maximum | E 175
Debt Instrument [Line Items]
Interest rate (as a percent)	5.00%
Maximum | Purchased, previously-leased aircraft
Debt Instrument [Line Items]
Debt instrument, term	4 years

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part I, Item 1 of this report for additional information.