CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 340,099	$ 280,372	$ 428,907
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	368,098	334,589	292,768
Stock based compensation expense	10,274	13,105	10,580
Gain on sale of subsidiary	(46,525)
Special items	21,869
Net increase (decrease) in deferred income taxes	109,654	99,139	(145,517)
Changes in operating assets and liabilities:
Decrease in restricted cash			8,243
Decrease (increase) in receivables	(40,700)	(21,464)	4,201
Decrease (increase) in income tax receivable	14,795	(20,620)	1,673
Increase in inventories	(4,303)	(7,935)	(1,246)
Decrease in other current assets	2,707	77,484	26,017
Increase in operating lease right-of-use asset	(895)
Decrease in deferred aircraft credits		(14,243)	(8,520)
Increase (decrease) in accounts payable and other current liabilities	(54,043)	62,107	67,018
NET CASH PROVIDED BY OPERATING ACTIVITIES	721,030	802,534	684,124
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of marketable securities	(1,938,750)	(2,308,768)	(1,533,867)
Sales of marketable securities	1,866,761	2,451,344	1,440,283
Net cash received from sale of subsidiary	53,200
Acquisition of property and equipment:
Aircraft and rotable spare parts	(581,329)	(1,062,380)	(661,176)
Buildings and ground equipment	(80,586)	(34,397)	(27,467)
Proceeds from the sale of aircraft, property and equipment	26,008		51,994
Deposits on aircraft	(52,817)	(41,937)	(46,733)
Return of deposits applied towards acquired aircraft	46,346	49,550	36,533
Decrease (increase) in other assets	4,133	(36,816)	(10,904)
NET CASH USED IN INVESTING ACTIVITIES	(657,034)	(983,404)	(751,337)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of long-term debt	200,040	784,665	471,677
Principal payments on long-term debt	(382,442)	(370,775)	(330,258)
Proceeds from issuance of common stock	6,271	5,212	4,660
Purchase of treasury stock	(78,476)	(54,448)	(20,008)
Purchase of common stock	(16,145)
Employee income tax paid on vested equity awards	(9,313)	(13,556)	(5,080)
Increase in debt issuance cost	(1,642)	(3,892)	(3,737)
Payment of cash dividends	(23,467)	(19,744)	(15,015)
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	(305,174)	327,462	102,239
Increase (decrease) in cash and cash equivalents	(241,178)	146,592	35,026
Cash and cash equivalents at beginning of period	328,384	181,792	146,766
CASH AND CASH EQUIVALENTS AT END OF PERIOD	87,206	328,384	181,792
Non-cash investing activities:
Acquisition of rotable spare parts	16,514	367	755
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
Cash paid during the period for:
Interest, net of capitalized amounts	131,733	118,268	105,639
Income taxes	2,490	$ 2,661	$ 5,010
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