Summary of Significant Accounting Policies - Cash Proceeds Received from the Sale of Mileage Credits (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accounting Policies [Abstract]
Consideration received from credit card mile programs	$ 33,819	$ 28,496	$ 24,938
Portion of proceeds recognized immediately as marketing component	$ 28,140	$ 23,124	$ 20,998