Revenue Disaggregation (Details) - USD ($) $ 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
Disaggregation of Revenue [Line Items]
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
As Reported [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues										2,647,666	2,321,956
ASU 2014-09 | Restatement Adjustment [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues					(825)	(4)	(1,496)	(1,789)		(4,114)	(1,935)
Fare
Disaggregation of Revenue [Line Items]
Total operating revenues	445,203	476,660	439,549	342,695	337,324	355,593	371,443	299,035		1,363,395	1,198,107
Fare | As Reported [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues										1,366,034	1,200,621
Fare | ASU 2014-09 | Restatement Adjustment [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues										(2,639)	(2,514)
Non Fare
Disaggregation of Revenue [Line Items]
Total operating revenues	401,365	411,296	396,801	346,446	313,323	313,479	309,437	273,252	1,555,908	1,209,492	1,059,694
Non Fare | As Reported [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues										0	0
Non Fare | ASU 2014-09 | Restatement Adjustment [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues										1,209,492	1,059,694
Passenger
Disaggregation of Revenue [Line Items]
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]
Disaggregation of Revenue [Line Items]
Total operating revenues										1,366,034	1,200,621
Passenger | ASU 2014-09 | 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
Other
Disaggregation of Revenue [Line Items]
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]
Disaggregation of Revenue [Line Items]
Total operating revenues										1,281,632	1,121,335
Other | ASU 2014-09 | 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)