Nature of Operations and Summary of Significant Accounting Policies - Long-Lived Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 07, 2005
Impairment of Long Lived and Intangible Assets
Property and equipment and related assets	$ 5,650,478	$ 5,140,005
Intangible assets	4,896	8,249	$ 10,499
Impairment of long-lived assets	0
Proceeds from settlement of residual value guarantee aircraft agreements		90,000
Atlantic Southeast Airlines Inc.
Impairment of Long Lived and Intangible Assets
Intangible assets related to acquisition of Atlantic Southeast				$ 33,700
Accumulated amortization expense	$ 28,900	$ 25,500