Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Passenger Revenues (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]
Credit Shell, Term of Expiration									60 days
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
Baggage
Disaggregation of Revenue [Line Items]
Total operating revenues									620,154	488,434	434,269
Passenger usage fee
Disaggregation of Revenue [Line Items]
Total operating revenues									531,459	411,742	358,920
Advance seat selection
Disaggregation of Revenue [Line Items]
Total operating revenues									180,012	131,821	110,966
Other
Disaggregation of Revenue [Line Items]
Total operating revenues									224,283	177,495	155,539
Non-fare revenue
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