Accrued Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Payables and Accruals [Abstract]
Salaries and wages	$ 145,995	$ 54,338
Federal excise and other passenger taxes and fees payable	55,723	42,036
Airport obligations	54,685	56,299
Aircraft maintenance	47,221	33,033
Interest payable	20,035	11,384
Fuel	19,971	25,171
Aircraft and facility lease obligations	11,864	16,992
Other	22,684	23,117
Other current liabilities	$ 378,178	$ 262,370	$ 226,011