Revenue Recognition - Impact on Statement of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						6 Months Ended		12 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Operating revenues:
Total operating revenues	$ 851,771	$ 704,138	$ 666,182	$ 687,227	$ 700,185	$ 589,957	$ 1,555,909	$ 1,290,142	$ 2,643,552	$ 2,320,021
Operating expenses:
Aircraft fuel	246,180		175,205	158,300	142,294	139,782	450,826	282,076	615,581	447,553
Salaries, wages and benefits	187,756		136,815	134,114	129,892	127,138	342,852	257,030	527,959	472,471
Aircraft rent	41,745		42,820	53,396	52,566	57,070	91,936	109,636	205,852	201,675
Landing fees and other rents	58,602		46,117	48,498	45,592	40,448	108,232	86,040	180,655	151,679
Depreciation and amortization	45,618		36,472	36,840	35,331	31,509	84,991	66,840	140,152	101,136
Maintenance, materials and repairs	31,653		28,966	26,176	28,985	26,312	61,363	55,297	110,439	98,587
Distribution	34,997		28,170	29,695	29,835	25,772	65,628	55,607	113,472	96,895
Special charges	174		0	7,853	0	4,776	89,342	4,776	12,629	37,189
Loss on disposal of assets	4,644		1,054	516	1,493	1,105	5,492	2,598	4,168	4,187
Other operating	91,881		79,267	87,965	102,885	77,703	185,523	180,588	347,820	267,191
Total operating expenses	743,250		574,886	583,353	568,873	531,615	1,486,185	1,100,488	2,258,727	1,878,563
Operating income	108,521		91,296	103,874	131,312	58,342	69,724	189,654	384,825	441,458
Other (income) expense:
Interest expense	20,498		16,065	15,018	13,746	12,473	38,347	26,219	57,302	41,654
Capitalized interest	(2,296)		(3,668)	(3,203)	(3,342)	(3,580)	(4,548)	(6,922)	(13,793)	(12,705)
Interest income	(4,430)		(2,990)	(2,605)	(1,828)	(1,313)	(8,496)	(3,141)	(8,736)	(5,276)
Other expense	188		145	114	104	3	321	107	366	528
Total other (income) expense	93,372		9,552	9,324	8,680	7,583	114,237	16,263	35,139	24,201
Income (loss) before income taxes	15,149		81,744	94,550	122,632	50,759	(44,513)	173,391	349,686	417,257
Provision (benefit) for income taxes	3,895		(165,231)	34,506	45,391	19,498	(10,845)	64,889	(65,836)	153,774
Net income (loss)	$ 11,254		$ 246,975	$ 60,044	$ 77,241	$ 31,261	$ (33,668)	$ 108,502	$ 415,522	$ 263,483
Basic earnings (loss) per share	$ 0.16		$ 3.59	$ 0.87	$ 1.11	$ 0.45	$ (0.49)	$ 1.56	$ 6.00	$ 3.75
Diluted earnings (loss) per share	$ 0.16		$ 3.58	$ 0.86	$ 1.11	$ 0.45	$ (0.49)	$ 1.56	$ 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									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 (loss) before income taxes									353,652	419,460
Provision (benefit) for income taxes									(66,954)	154,581
Net income (loss)									$ 420,606	$ 264,879
Basic earnings (loss) per share									$ 6.08	$ 3.77
Diluted earnings (loss) 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			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 (loss) 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 (loss)			$ (3,363)	$ (146)	$ (901)	$ (674)			$ (5,084)	$ (1,396)
Basic earnings (loss) per share			$ (0.05)	$ 0.00	$ (0.01)	$ (0.01)			$ (0.07)	$ (0.02)
Diluted earnings (loss) per share			$ (0.05)	$ 0.00	$ (0.01)	$ (0.01)			$ (0.07)	$ (0.02)
Passenger [Member]
Operating revenues:
Total operating revenues	$ 836,350	689,141	$ 650,647	$ 669,072	$ 680,880	$ 572,287	$ 1,525,491	$ 1,253,167	$ 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	$ 15,421	$ 14,997	15,535	18,155	19,305	17,670	$ 30,418	$ 36,975	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)