Accrued Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Payables and Accruals [Abstract]
Salaries and wages	$ 81,278	$ 54,338
Federal excise and other passenger taxes and fees payable	55,743	42,036
Airport obligations	47,476	56,299
Aircraft maintenance	26,438	33,033
Fuel	21,637	25,171
Interest payable	18,965	11,384
Aircraft and facility lease obligations	12,236	16,992
Other	23,369	23,117
Other current liabilities	$ 287,142	$ 262,370	$ 226,011