Revenue Disaggregation (Details) - USD ($) $ in Thousands	3 Months Ended					12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 704,138	$ 666,182	$ 687,227	$ 700,185	$ 589,957	$ 2,643,552	$ 2,320,021
DOT—Domestic [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues	647,800				543,900
DOT—Latin America [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues	56,300				46,000
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	689,141	650,647	669,072	680,880	572,287	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	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	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	$ 14,997	15,535	18,155	19,305	17,670	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)