Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities:
Net income	$ 420,606	$ 264,879	$ 317,220
Adjustments to reconcile net income to net cash provided by operations:
Unrealized losses on open derivative contracts, net	0	0	2,202
Losses reclassified from other comprehensive income	335	354	132
Stock-based compensation	8,522	7,105	9,222
Allowance for doubtful accounts (recoveries)	(53)	80	12
Amortization of deferred gains and losses and debt issuance costs	7,944	5,732	1,165
Depreciation and amortization	140,152	101,136	73,908
Deferred income tax expense (benefit)	(1,610)	86,146	155,614
Loss on disposal of assets	4,168	4,187	1,604
Lease termination costs	12,629	37,189	0
Changes in operating assets and liabilities:
Accounts receivable	(8,134)	(12,951)	(5,592)
Aircraft maintenance deposits, net	(37,930)	(45,869)	(32,101)
Long-term deposits and other assets	(50,803)	(45,558)	(22,208)
Deferred heavy maintenance	(78,237)	(30,222)	(9,127)
Income tax receivable	(69,844)	0	0
Prepaid income taxes	0	72,278	(72,278)
Accounts payable	6,030	(6,823)	2,706
Air traffic liability	40,011	(11,582)	36,387
Other liabilities	31,074	47,391	14,119
Other	380	206	0
Net cash provided by operating activities	425,240	473,678	472,985
Investing activities:
Purchase of available-for-sale investment securities	(107,246)	(103,258)	0
Proceeds from the maturity of available-for-sale investment securities	105,906	2,842	0
Proceeds from sale of property and equipment	0	50	0
Pre-delivery deposits for flight equipment, net of refunds	(149,477)	(173,947)	(142,323)
Capitalized interest	(12,305)	(10,834)	(10,159)
Purchase of property and equipment	(628,881)	(541,122)	(548,800)
Net cash used in investing activities	(792,003)	(826,269)	(701,282)
Financing activities:
Proceeds from issuance of long-term debt	629,725	417,275	536,780
Proceeds from stock options exercised	45	92	32
Payments on debt and capital lease obligations	(102,738)	(64,421)	(26,364)
Proceeds from sale leaseback transactions	0	0	7,300
Excess tax (deficiency) benefit from equity-based compensation	0	(470)	8,850
Repurchase of common stock	(46,580)	(102,510)	(112,261)
Debt issuance costs	(13,740)	(107)	(15,192)
Net cash provided by financing activities	466,712	249,859	399,145
Net (decrease) increase in cash and cash equivalents	99,949	(102,732)	170,848
Cash and cash equivalents at beginning of period	700,900	803,632	632,784
Cash and cash equivalents at end of period	800,849	700,900	803,632
Cash payments for:
Interest, net of capitalized interest	37,902	39,963	7,061
Income taxes paid, net of refunds	5,826	(5,579)	95,933
Non-cash transactions:
Capital expenditures funded by capital lease borrowings	$ (1,370)	$ (31)	$ 0