Accrued Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Federal excise and other passenger taxes and fees payable	$ 97,375	$ 60,604
Salaries and wages	83,379	82,900
Airport obligations	64,065	52,029
Aircraft maintenance	41,563	59,805
Fuel	27,204	25,368
Aircraft and facility lease obligations	20,277	15,149
Interest payable	17,558	18,086
Other	27,620	25,736
Other current liabilities	$ 379,041	$ 339,677