Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables and Accruals [Abstract]
Salaries and wages	$ 54,338	$ 54,578
Airport obligations	56,299	43,989
Federal excise and other passenger taxes and fees payable	42,036	42,064
Aircraft maintenance	33,033	30,233
Fuel	25,171	14,828
Aircraft and facility lease obligations	16,992	10,378
Interest payable	11,384	8,499
Other	23,117	21,442
Other current liabilities	$ 262,370	$ 226,011