Income Taxes - Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statutory Federal income tax rate (as a percent)		35.00%
Reconciliation between the statutory federal income tax rate of 35% and the effective rate which is derived by dividing the provision for income taxes by income (loss) before income taxes
Computed provision (benefit) for income taxes at the statutory rate		$ 100,864	$ (87,084)	$ 68,013
State income tax provision (benefit), net of federal income tax benefit		7,778	(5,768)	5,416
Non-deductible expenses		3,230	3,552	3,641
Valuation allowance changes affecting the provision for income taxes		505	751	(899)
Excess tax benefits from share based compensation		(5,377)
Revaluation of net deferred tax liabilities for the Tax Act		(246,845)
Other, net		(879)	1,323	334
Provision (benefit) for income taxes		(140,724)	(87,226)	$ 76,505
Deferred tax assets:
Intangible Asset		(206)	4,983
Accrued benefits		31,651	48,482
Net operating loss carryforward		122,648	286,389
AMT credit carryforward		23,443	17,589
Deferred aircraft credits		53,870	60,415
Accrued reserves and other		26,853	47,906
Total deferred tax assets		258,259	465,764
Valuation allowance		(10,642)	(8,877)
Deferred tax liabilities:
Accelerated depreciation		(666,637)	(1,022,291)
Total deferred tax liabilities		(666,637)	(1,022,291)
Net deferred tax liability		$ (419,020)	$ (565,404)
Forecast
Statutory Federal income tax rate (as a percent)	21.00%