CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income (loss)	$ 428,907	$ (161,586)	$ 117,817
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	292,768	284,969	264,507
Stock based compensation expense	10,580	7,568	5,368
Gain from early extinguishment of debt		(1,279)	(33,660)
Special items		465,649
Net increase (decrease) in deferred income taxes	(145,517)	(83,441)	73,844
Changes in operating assets and liabilities:
Decrease (increase) in restricted cash	8,243	(27)	3,366
Decrease in receivables	4,201	15,260	21,076
Decrease (increase) in income tax receivable	1,673	(4,118)	(92)
Increase in inventories	(1,246)	(1,986)	(2,860)
Decrease (increase) in other current assets	26,017	37,569	(28,598)
Decrease in deferred aircraft credits	(8,520)	(8,108)	(8,635)
Increase (decrease) in accounts payable and accrued aircraft rents	46,934	(47,563)	9,690
Increase (decrease) in other current liabilities	20,084	3,758	(4,498)
NET CASH PROVIDED BY OPERATING ACTIVITIES	684,124	506,665	417,325
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of marketable securities	(1,533,867)	(2,511,388)	(1,170,439)
Sales of marketable securities	1,440,283	2,388,168	1,299,069
Proceeds from the sale of aircraft, property and equipment	51,994	3,008	10,308
Proceeds from settlement of residual value guarantee aircraft agreements		90,000
Acquisition of property and equipment:
Aircraft and rotable spare parts	(661,176)	(1,138,963)	(710,871)
Deposits on aircraft	(46,733)	(650)
Buildings and ground equipment	(27,467)	(14,350)	(10,405)
Return of deposits applied towards acquired aircraft	36,533		1,850
Decrease (increase) in other assets	(10,904)	7,097	10,772
NET CASH USED IN INVESTING ACTIVITIES	(751,337)	(1,177,078)	(569,716)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of long-term debt	471,677	926,069	591,881
Principal payments on long-term debt	(330,258)	(302,158)	(354,277)
Proceeds from issuance of common stock	4,660	8,142	9,754
Purchase of treasury stock	(20,008)		(18,726)
Employee income tax paid on equity awards	(5,080)
Decrease (increase) in debt issuance cost	(3,737)	(8,653)	2,779
Payment of cash dividends	(15,015)	(9,256)	(8,260)
NET CASH PROVIDED BY FINANCING ACTIVITIES	102,239	614,144	223,151
Increase (decrease) in cash and cash equivalents	35,026	(56,269)	70,760
Cash and cash equivalents at beginning of period	146,766	203,035	132,275
CASH AND CASH EQUIVALENTS AT END OF PERIOD	181,792	146,766	203,035
Non-cash investing activities:
Acquisition of rotable spare parts	755	5,688
Cash paid during the period for:
Interest, net of capitalized amounts	105,639	76,589	80,657
Income taxes	$ 5,010	$ 1,212	$ 5,104