Summary of Significant Accounting Policies - Passenger Revenues (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Air traffic liability	$ 315,408				$ 291,981				$ 315,408	$ 291,981
Credit shell term of expiration									60 days
Total operating revenues	$ 969,816	$ 991,968	$ 1,012,956	$ 855,796	$ 862,795	$ 904,330	$ 851,771	$ 704,138	$ 3,830,536	3,323,034	$ 2,643,552
Baggage
Disaggregation of Revenue [Line Items]
Total operating revenues									734,243	620,154	488,434
Passenger usage fee
Disaggregation of Revenue [Line Items]
Total operating revenues									669,177	531,459	411,742
Advance seat selection
Disaggregation of Revenue [Line Items]
Total operating revenues									228,876	180,012	131,821
Other
Disaggregation of Revenue [Line Items]
Total operating revenues									238,454	224,283	177,495
Non-fare revenue
Disaggregation of Revenue [Line Items]
Total operating revenues									$ 1,870,750	$ 1,555,908	$ 1,209,492