Accrued Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Salaries and wages	$ 82,572	$ 82,900
Federal excise and other passenger taxes and fees payable	68,716	60,604
Airport obligations	65,351	52,029
Aircraft maintenance	30,113	59,805
Aircraft and facility lease obligations	21,807	15,149
Fuel	21,126	25,368
Interest payable	18,753	18,086
Other	29,247	25,736
Other current liabilities	$ 337,685	$ 339,677