Commitments and Contingencies (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) aircraft	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Future minimum rental payments required under operating leases
2019	$ 87,256
2020	101,741
2021	90,787
2022	72,593
2023	65,749
Thereafter	59,820
Total future lease obligations	$ 477,946
Number of aircraft leased | aircraft	260
Total rental expense for non-cancelable aircraft operating leases	$ 154,900	$ 215,800	$ 262,600
Minimum rental expense for airport station rents	$ 19,600	$ 30,300	$ 31,400