Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities:
Net income	$ 155,749	$ 415,522	$ 263,483
Adjustments to reconcile net income to net cash provided by operations:
Losses reclassified from other comprehensive income	315	335	354
Stock-based compensation	11,021	8,522	7,105
Allowance for doubtful accounts (recoveries)	(11)	(53)	80
Amortization of deferred gains and losses and debt issuance costs	8,819	7,944	5,732
Depreciation and amortization	176,727	140,152	101,136
Deferred income tax expense (benefit)	46,303	(492)	85,339
Loss on disposal of assets	9,580	4,168	4,187
Lease termination costs	0	12,629	37,189
Special charges, non-operating	90,357	0	0
Changes in operating assets and liabilities:
Accounts receivable	1,674	(8,134)	(12,951)
Aircraft maintenance deposits, net	14,019	(37,930)	(45,869)
Long-term deposits and other assets	(4,803)	(50,951)	(47,204)
Deferred heavy maintenance, net	(190,381)	(78,237)	(30,222)
Income tax receivable	69,844	(69,844)	0
Prepaid income taxes	0	0	72,278
Accounts payable	15,317	6,030	(6,823)
Air traffic liability	28,270	43,527	(9,455)
Other liabilities	74,038	31,672	47,200
Other	(375)	380	206
Net cash provided by operating activities	506,463	425,240	471,765
Investing activities:
Purchase of available-for-sale investment securities	(124,430)	(107,246)	(103,258)
Proceeds from the maturity of available-for-sale investment securities	122,947	105,906	2,842
Proceeds from sale of property and equipment	11,400	0	50
Pre-delivery deposits on flight equipment, net of refunds	(177,424)	(149,477)	(173,947)
Capitalized interest	(8,729)	(12,305)	(10,834)
Assets under construction for others	(501)	0	0
Purchase of property and equipment	(606,971)	(628,881)	(539,209)
Net cash used in investing activities	(783,708)	(792,003)	(824,356)
Financing activities:
Proceeds from issuance of long-term debt	832,099	629,725	417,275
Proceeds from stock options exercised	51	45	92
Payments on debt obligations	(137,275)	(102,313)	(64,371)
Payments on capital lease obligations	(205,720)	(425)	(50)
Reimbursement for assets under construction for others	501	0	0
Excess tax (deficiency) benefit from equity-based compensation	0	0	(470)
Repurchase of common stock	(1,162)	(46,580)	(102,510)
Debt issuance costs	(7,365)	(13,740)	(107)
Net cash provided by financing activities	481,129	466,712	249,859
Net increase (decrease) in cash and cash equivalents	203,884	99,949	(102,732)
Cash and cash equivalents at beginning of period	800,849	700,900	803,632
Cash and cash equivalents at end of period	1,004,733	800,849	700,900
Cash payments for:
Interest, net of capitalized interest	65,123	37,902	39,963
Income taxes paid, net of refunds	(73,489)	5,826	(5,579)
Non-cash transactions:
Capital expenditures funded by capital lease borrowings	$ (987)	$ (1,370)	$ (31)