Accrued Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Federal excise and other passenger taxes and fees payable	$ 90,504	$ 60,604
Salaries and wages	79,359	82,900
Airport obligations	65,291	52,029
Aircraft maintenance	65,126	59,805
Fuel	28,378	25,368
Aircraft and facility lease obligations	20,630	15,149
Interest payable	19,344	18,086
Other	26,772	25,736
Other current liabilities	$ 395,404	$ 339,677