Summary of Significant Accounting Policies - Cash Proceeds Received from the Sale of Mileage Credits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Consideration received from credit card mile programs	$ 49,453	$ 48,882	$ 58,005
Portion of proceeds recognized immediately as marketing component	$ 37,960	$ 36,640	$ 35,938