CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income (loss)	$ (8,515)	$ 340,099	$ 280,372
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	474,959	368,098	334,589
Stock based compensation expense	6,802	10,274	13,105
Write-off of maintenance assets	3,255
Increase in credit loss reserves	17,087
Loss on disposal of fixed assets	445
Net increase in deferred income taxes	6,100	109,654	99,139
Gain on sale of ExpressJet subsidiary		(46,525)
Special items		21,869
Changes in operating assets and liabilities:
Decrease (increase) in receivables	26,831	(40,700)	(21,464)
Decrease (increase) in income tax receivable	7,225	14,795	(20,620)
Decrease (increase) in inventories	19,307	(4,303)	(7,935)
Decrease in other current assets	8,233	2,707	77,484
Decrease in operating lease right-of-use assets	53,647
Increase in operating lease liabilities	(65,557)	(895)
Increase in deferred revenue	110,728
Decrease in deferred aircraft credits			(14,243)
Increase (decrease) in accounts payable and other current liabilities	(26,984)	(54,043)	62,107
NET CASH PROVIDED BY OPERATING ACTIVITIES	633,563	721,030	802,534
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of marketable securities	(1,347,775)	(1,938,750)	(2,308,768)
Sales of marketable securities	1,170,556	1,866,761	2,451,344
Net cash received from sale of ExpressJet subsidiary		53,200
Acquisition of property and equipment:
Aircraft and rotable spare parts	(425,618)	(581,329)	(1,062,380)
Buildings and ground equipment	(12,678)	(80,586)	(34,397)
Proceeds from the sale of aircraft, property and equipment	3,239	26,008
Deposits on aircraft	(9,625)	(52,817)	(41,937)
Return of deposits applied towards acquired aircraft	27,858	46,346	49,550
Decrease (increase) in other assets	(89,428)	4,133	(36,816)
NET CASH USED IN INVESTING ACTIVITIES	(683,471)	(657,034)	(983,404)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of long-term debt	406,364	200,040	784,665
Principal payments on long-term debt	(187,404)	(382,442)	(370,775)
Net proceeds from issuance of common stock	2,819	6,271	5,212
Purchase of treasury stock	(20,000)	(78,476)	(54,448)
Purchase of common stock		(16,145)
Employee income tax paid on vested equity awards	(6,165)	(9,313)	(13,556)
Payment of debt issuance cost	(4,130)	(1,642)	(3,892)
Payment of cash dividends	(13,059)	(23,467)	(19,744)
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	178,425	(305,174)	327,462
Increase (decrease) in cash and cash equivalents	128,517	(241,178)	146,592
Cash and cash equivalents at beginning of period	87,206	328,384	181,792
CASH AND CASH EQUIVALENTS AT END OF PERIOD	215,723	87,206	328,384
Non-cash investing and financing activities:
Acquisition of rotable spare parts	22,187	16,514	367
Debt assumed on aircraft acquired under operating leases		14,475	59,132
Engines contributed to joint venture		22,313
Non-cash assets used to acquire aircraft under operating leases		153,566
Lease liability arising from the recognition of right-of-use asset		456,472
Warrants issued under CARES Act agreements	8,248
Cash paid during the period for:
Interest, net of capitalized amounts	125,440	131,733	118,268
Income taxes	$ 1,780	2,490	$ 2,661
SUPPLEMENTAL DISCLOSURE OF SALE OF SUBSIDIARY:
Decrease in carrying amount of assets		(101,448)
Decrease in carrying amount of liabilities		68,341
Cash received from buyers		79,632
Gain on sale of subsidiary		$ 46,525