Nature of Operations and Summary of Significant Accounting Policies - Long-Lived Assets (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2016 USD ($) aircraft	Dec. 31, 2017 aircraft	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 07, 2005 USD ($)
Impairment of Long Lived and Intangible Assets
Property and equipment and related assets	$ 5,140,005		$ 5,140,005	$ 5,556,728
Intangible assets	8,249		8,249	10,499	$ 12,748
Proceeds from settlement of residual value guarantee aircraft agreements	$ 125,000		$ 90,000
CRJ 200
Impairment of Long Lived and Intangible Assets
Number of aircraft removed from service | aircraft	76
Proceeds from settlement of residual value guarantee aircraft agreements	$ 125,000
ERJ 145
Impairment of Long Lived and Intangible Assets
Number of extension rights | item			2
Term of extension rights			1 year
Atlantic Southeast Airlines Inc.
Impairment of Long Lived and Intangible Assets
Intangible assets related to acquisition of Atlantic Southeast						$ 33,700
Period for amortization of intangible assets			15 years
Accumulated amortization expense	$ 25,500		$ 25,500	$ 23,300
Forecast | CRJ 200
Impairment of Long Lived and Intangible Assets
Number of aircraft removed from service | aircraft		46
Forecast | ERJ 145
Impairment of Long Lived and Intangible Assets
Number of aircraft removed from service | aircraft		45