Revenue Recognition - Impact on Statement of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended							9 Months Ended		12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Operating revenues:
Total operating revenues	$ 904,330	$ 851,771	$ 704,138	$ 666,182	$ 687,227	$ 700,185	$ 589,957	$ 2,460,239	$ 1,977,369	$ 2,643,552	$ 2,320,021
Operating expenses:
Aircraft fuel	258,818			175,205	158,300	142,294	139,782	709,644	440,376	615,581	447,553
Salaries, wages and benefits	185,043			136,815	134,114	129,892	127,138	527,895	391,144	527,959	472,471
Aircraft rent	42,682			42,820	53,396	52,566	57,070	134,618	163,032	205,852	201,675
Landing fees and other rents	54,542			46,117	48,498	45,592	40,448	162,774	134,538	180,655	151,679
Depreciation and amortization	43,773			36,472	36,840	35,331	31,509	128,764	103,680	140,152	101,136
Maintenance, materials and repairs	37,778			28,966	26,176	28,985	26,312	99,141	81,473	110,439	98,587
Distribution	37,868			28,170	29,695	29,835	25,772	103,496	85,302	113,472	96,895
Special charges (credits)	(686)			0	7,853	0	4,776	88,656	12,629	12,629	37,189
Loss on disposal of assets	1,069			1,054	516	1,493	1,105	6,561	3,114	4,168	4,187
Other operating	98,318			79,267	87,965	102,885	77,703	283,841	268,553	347,820	267,191
Total operating expenses	759,205			574,886	583,353	568,873	531,615	2,245,390	1,683,841	2,258,727	1,878,563
Operating income	145,125			91,296	103,874	131,312	58,342	214,849	293,528	384,825	441,458
Other (income) expense:
Interest expense	21,925			16,065	15,018	13,746	12,473	60,272	41,237	57,302	41,654
Capitalized interest	(2,657)			(3,668)	(3,203)	(3,342)	(3,580)	(7,205)	(10,125)	(13,793)	(12,705)
Interest income	(4,776)			(2,990)	(2,605)	(1,828)	(1,313)	(13,272)	(5,746)	(8,736)	(5,276)
Other expense	302			145	114	104	3	623	221	366	528
Total other (income) expense	16,538			9,552	9,324	8,680	7,583	130,775	25,587	35,139	24,201
Income before income taxes	128,587			81,744	94,550	122,632	50,759	84,074	267,941	349,686	417,257
Provision (benefit) for income taxes	31,107			(165,231)	34,506	45,391	19,498	20,262	99,395	(65,836)	153,774
Net income	$ 97,480			$ 246,975	$ 60,044	$ 77,241	$ 31,261	$ 63,812	$ 168,546	$ 415,522	$ 263,483
Basic earnings per share	$ 1.43			$ 3.59	$ 0.87	$ 1.11	$ 0.45	$ 0.94	$ 2.43	$ 6.00	$ 3.75
Diluted earnings per share	$ 1.42			$ 3.58	$ 0.86	$ 1.11	$ 0.45	$ 0.93	$ 2.42	$ 5.99	$ 3.74
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 (credits)										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										$ 6.08	$ 3.77
Diluted earnings per share										$ 6.06	$ 3.76
Accounting Standards Update 2014-09 [Member] | 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 (credits)				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				$ (0.05)	$ 0	$ (0.01)	$ (0.01)			$ (0.07)	$ (0.02)
Diluted earnings per share				$ (0.05)	$ 0	$ (0.01)	$ (0.01)			$ (0.07)	$ (0.02)
Passenger [Member]
Operating revenues:
Total operating revenues	$ 887,956	836,350	689,141	$ 650,647	$ 669,072	$ 680,880	$ 572,287	$ 2,413,447	$ 1,922,239	$ 2,572,887	$ 2,257,801
Passenger [Member] | As Reported [Member]
Operating revenues:
Total operating revenues										1,366,034	1,200,621
Passenger [Member] | Accounting Standards Update 2014-09 [Member] | Restatement Adjustment [Member]
Operating revenues:
Total operating revenues				312,504	312,865	308,959	272,525			1,206,853	1,057,180
Other [Member]
Operating revenues:
Total operating revenues	$ 16,374	$ 15,421	$ 14,997	15,535	18,155	19,305	17,670	$ 46,792	$ 55,130	70,665	62,220
Other [Member] | As Reported [Member]
Operating revenues:
Total operating revenues										1,281,632	1,121,335
Other [Member] | Accounting Standards Update 2014-09 [Member] | Restatement Adjustment [Member]
Operating revenues:
Total operating revenues				$ (313,329)	$ (312,869)	$ (310,455)	$ (274,314)			$ (1,210,967)	$ (1,059,115)