Revenue Recognition - Impact on Statement of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Common stock, par value (in dollars per share)	$ 0.0001				$ 0.0001				$ 0.0001	$ 0.0001	$ 0.0001
Operating revenues:
Total operating revenues	$ 862,795	$ 904,330	$ 851,771	$ 704,138	$ 666,182	$ 687,227	$ 700,185	$ 589,957	$ 3,323,034	$ 2,643,552	$ 2,320,021
Operating expenses:
Aircraft fuel					175,205	158,300	142,294	139,782	939,324	615,581	447,553
Salaries, wages and benefits					136,815	134,114	129,892	127,138	719,635	527,959	472,471
Aircraft rent					42,820	53,396	52,566	57,070	177,641	205,852	201,675
Landing fees and other rents					46,117	48,498	45,592	40,448	214,677	180,655	151,679
Depreciation and amortization					36,472	36,840	35,331	31,509	176,727	140,152	101,136
Maintenance, materials and repairs					28,966	26,176	28,985	26,312	129,078	110,439	98,587
Distribution					28,170	29,695	29,835	25,772	137,001	113,472	96,895
Special charges					0	7,853	0	4,776	88,921	12,629	37,189
Loss on disposal of assets					1,054	516	1,493	1,105	9,580	4,168	4,187
Other operating					79,267	87,965	102,885	77,703	379,536	347,820	267,191
Total operating expenses					574,886	583,353	568,873	531,615	2,972,120	2,258,727	1,878,563
Operating income	136,065	145,125	108,521	(38,797)	91,296	103,874	131,312	58,342	350,914	384,825	441,458
Other (income) expense:
Interest expense					16,065	15,018	13,746	12,473	83,777	57,302	41,654
Capitalized interest					(3,668)	(3,203)	(3,342)	(3,580)	(9,841)	(13,793)	(12,705)
Interest income					(2,990)	(2,605)	(1,828)	(1,313)	(19,107)	(8,736)	(5,276)
Other expense					145	114	104	3	752	366	528
Total other (income) expense					9,552	9,324	8,680	7,583	145,938	35,139	24,201
Income before income taxes					81,744	94,550	122,632	50,759	204,976	349,686	417,257
Provision (benefit) for income taxes					(165,231)	34,506	45,391	19,498	49,227	(65,836)	153,774
Net income	$ 91,937	$ 97,480	$ 11,254	$ (44,922)	$ 246,975	$ 60,044	$ 77,241	$ 31,261	$ 155,749	$ 415,522	$ 263,483
Basic earnings per share (in dollars per share)	$ 1.35	$ 1.43	$ 0.16	$ (0.66)	$ 3.59	$ 0.87	$ 1.11	$ 0.45	$ 2.28	$ 6.00	$ 3.75
Diluted earnings per share (in dollars per share)	$ 1.34	$ 1.42	$ 0.16	$ (0.66)	$ 3.58	$ 0.86	$ 1.11	$ 0.45	$ 2.28	$ 5.99	$ 3.74
Common stock, shares authorized (in shares)	240,000,000				240,000,000				240,000,000	240,000,000	240,000,000
Common stock, shares issued (in shares)	69,871,535				69,770,795				69,871,535	69,770,795	73,549,872
Common stock, shares outstanding (in shares)	68,269,567				68,196,964				68,269,567	68,196,964	69,326,202
Treasury stock (in shares)	1,601,968				1,573,831				1,601,968	1,573,831	4,223,670
As Reported [Member]
Operating revenues:
Total operating revenues										$ 2,647,666	$ 2,321,956
Operating expenses:
Aircraft fuel										615,581	447,553
Salaries, wages and benefits										527,959	472,471
Aircraft rent										205,852	201,675
Landing fees and other rents										180,655	151,679
Depreciation and amortization										140,152	101,136
Maintenance, materials and repairs										110,439	98,587
Distribution										113,620	96,627
Special charges										12,629	37,189
Loss on disposal of assets										4,168	4,187
Other operating										347,820	267,191
Total operating expenses										2,258,875	1,878,295
Operating income										388,791	443,661
Other (income) expense:
Interest expense										57,302	41,654
Capitalized interest										(13,793)	(12,705)
Interest income										(8,736)	(5,276)
Other expense										366	528
Total other (income) expense										35,139	24,201
Income before income taxes										353,652	419,460
Provision (benefit) for income taxes										(66,954)	154,581
Net income										$ 420,606	$ 264,879
Basic earnings per share (in dollars per share)										$ 6.08	$ 3.77
Diluted earnings per share (in dollars per share)										$ 6.06	$ 3.76
ASU 2014-09 | Restatement Adjustment [Member]
Operating revenues:
Total operating revenues					$ (825)	$ (4)	$ (1,496)	$ (1,789)		$ (4,114)	$ (1,935)
Operating expenses:
Aircraft fuel					0	0	0	0		0	0
Salaries, wages and benefits					0	0	0	0		0	0
Aircraft rent					0	0	0	0		0	0
Landing fees and other rents					0	0	0	0		0	0
Depreciation and amortization					0	0	0	0		0	0
Maintenance, materials and repairs					0	0	0	0		0	0
Distribution					425	226	(73)	(726)		(148)	268
Special charges					0	0	0	0		0	0
Loss on disposal of assets					0	0	0	0		0	0
Other operating					0	0	0	0		0	0
Total operating expenses					425	226	(73)	(726)		(148)	268
Operating income					(1,250)	(230)	(1,423)	(1,063)		(3,966)	(2,203)
Other (income) expense:
Interest expense					0	0	0	0		0	0
Capitalized interest					0	0	0	0		0	0
Interest income					0	0	0	0		0	0
Other expense					0	0	0	0		0	0
Total other (income) expense					0	0	0	0		0	0
Income before income taxes					(1,250)	(230)	(1,423)	(1,063)		(3,966)	(2,203)
Provision (benefit) for income taxes					2,113	(84)	(522)	(389)		1,118	(807)
Net income					$ (3,363)	$ (146)	$ (901)	$ (674)		$ (5,084)	$ (1,396)
Basic earnings per share (in dollars per share)					$ (0.05)	$ 0	$ (0.01)	$ (0.01)		$ (0.07)	$ (0.02)
Diluted earnings per share (in dollars per share)					$ (0.05)	$ 0	$ (0.01)	$ (0.01)		$ (0.07)	$ (0.02)
Passenger
Operating revenues:
Total operating revenues	$ 846,568	$ 887,956	$ 836,350	$ 689,141	$ 650,647	$ 669,072	$ 680,880	$ 572,287	$ 3,260,015	$ 2,572,887	$ 2,257,801
Passenger | As Reported [Member]
Operating revenues:
Total operating revenues										1,366,034	1,200,621
Passenger | ASU 2014-09 | Restatement Adjustment [Member]
Operating revenues:
Total operating revenues					312,504	312,865	308,959	272,525		1,206,853	1,057,180
Other
Operating revenues:
Total operating revenues	$ 16,227	$ 16,374	$ 15,421	$ 14,997	15,535	18,155	19,305	17,670	$ 63,019	70,665	62,220
Other | As Reported [Member]
Operating revenues:
Total operating revenues										1,281,632	1,121,335
Other | ASU 2014-09 | Restatement Adjustment [Member]
Operating revenues:
Total operating revenues					$ (313,329)	$ (312,869)	$ (310,455)	$ (274,314)		$ (1,210,967)	$ (1,059,115)