Revenue Disaggregation (Details) - USD ($) $ 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
Disaggregation of Revenue [Line Items]
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
DOT—Domestic [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues	768,300				640,000		1,417,400	1,184,300
DOT—Latin America [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues	83,500				60,200		138,500	105,800
As Reported [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues									2,647,666	2,321,956
Accounting Standards Update 2014-09 [Member] | Restatement Adjustment [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues			(825)	(4)	(1,496)	(1,789)			(4,114)	(1,935)
Passenger [Member]
Disaggregation of Revenue [Line Items]
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]
Disaggregation of Revenue [Line Items]
Total operating revenues									1,366,034	1,200,621
Passenger [Member] | Accounting Standards Update 2014-09 [Member] | Restatement Adjustment [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues			312,504	312,865	308,959	272,525			1,206,853	1,057,180
Fare [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues	439,549	342,695	337,324	355,593	371,443	299,035			1,363,395	1,198,107
Fare [Member] | As Reported [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues									1,366,034	1,200,621
Fare [Member] | Accounting Standards Update 2014-09 [Member] | Restatement Adjustment [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues									(2,639)	(2,514)
Non Fare [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues	396,801	346,446	313,323	313,479	309,437	273,252			1,209,492	1,059,694
Non Fare [Member] | As Reported [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues									0	0
Non Fare [Member] | Accounting Standards Update 2014-09 [Member] | Restatement Adjustment [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues									1,209,492	1,059,694
Other [Member]
Disaggregation of Revenue [Line Items]
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]
Disaggregation of Revenue [Line Items]
Total operating revenues									1,281,632	1,121,335
Other [Member] | Accounting Standards Update 2014-09 [Member] | Restatement Adjustment [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues			$ (313,329)	$ (312,869)	$ (310,455)	$ (274,314)			$ (1,210,967)	$ (1,059,115)