Summary of Significant Accounting Policies - Cash Proceeds Received from the Sale of Mileage Credits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Consideration received from credit card mile programs	$ 33,201	$ 48,136	$ 39,194
Portion of proceeds recognized immediately as marketing component	$ 25,918	$ 37,151	$ 30,353