Condensed Statements Of Operations (unaudited) - USD ($) $ in Thousands	3 Months Ended					12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Operating revenues:
Total operating revenues	$ 704,138	$ 666,182	$ 687,227	$ 700,185	$ 589,957	$ 2,643,552	$ 2,320,021
Operating expenses:
Aircraft fuel	204,646	175,205	158,300	142,294	139,782	615,581	447,553
Salaries, wages and benefits	155,096	136,815	134,114	129,892	127,138	527,959	472,471
Aircraft rent	50,191	42,820	53,396	52,566	57,070	205,852	201,675
Landing fees and other rents	49,630	46,117	48,498	45,592	40,448	180,655	151,679
Depreciation and amortization	39,373	36,472	36,840	35,331	31,509	140,152	101,136
Maintenance, materials and repairs	29,710	28,966	26,176	28,985	26,312	110,439	98,587
Distribution	30,631	28,170	29,695	29,835	25,772	113,472	96,895
Special charges	89,168	0	7,853	0	4,776	12,629	37,189
Loss on disposal of assets	848	1,054	516	1,493	1,105	4,168	4,187
Other operating	93,642	79,267	87,965	102,885	77,703	347,820	267,191
Total operating expenses	742,935	574,886	583,353	568,873	531,615	2,258,727	1,878,563
Operating income (loss)	(38,797)	91,296	103,874	131,312	58,342	384,825	441,458
Other (income) expense:
Interest expense	17,849	16,065	15,018	13,746	12,473	57,302	41,654
Capitalized interest	(2,252)	(3,668)	(3,203)	(3,342)	(3,580)	(13,793)	(12,705)
Interest income	(4,066)	(2,990)	(2,605)	(1,828)	(1,313)	(8,736)	(5,276)
Other expense	133	145	114	104	3	366	528
Special charges, non-operating	9,201				0
Total other (income) expense	20,865	9,552	9,324	8,680	7,583	35,139	24,201
Income (loss) before income taxes	(59,662)	81,744	94,550	122,632	50,759	349,686	417,257
Provision (benefit) for income taxes	(14,740)	(165,231)	34,506	45,391	19,498	(65,836)	153,774
Net income (loss)	$ (44,922)	$ 246,975	$ 60,044	$ 77,241	$ 31,261	$ 415,522	$ 263,483
Basic earnings per share (in dollars per share)	$ (0.66)	$ 3.59	$ 0.87	$ 1.11	$ 0.45	$ 6.00	$ 3.75
Diluted earnings per share (in dollars per share)	$ (0.66)	$ 3.58	$ 0.86	$ 1.11	$ 0.45	$ 5.99	$ 3.74
Passenger [Member]
Operating revenues:
Total operating revenues	$ 689,141	$ 650,647	$ 669,072	$ 680,880	$ 572,287	$ 2,572,887	$ 2,257,801
Other [Member]
Operating revenues:
Total operating revenues	$ 14,997	$ 15,535	$ 18,155	$ 19,305	$ 17,670	$ 70,665	$ 62,220