Accrued Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Payables and Accruals [Abstract]
Federal excise and other passenger taxes and fees payable	$ 78,393	$ 42,036
Salaries and wages	77,363	54,338
Airport obligations	59,981	56,299
Aircraft maintenance	44,065	33,033
Fuel	24,543	25,171
Interest payable	19,620	11,384
Aircraft and facility lease obligations	14,020	16,992
Other	28,422	23,117
Other current liabilities	$ 346,407	$ 262,370	$ 226,011