Revenue Recognition - Contract Liabilities - Customer Loyalty Programs (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Change in Contract with Customer, Liability [Roll Forward]
Outstanding at December 31, 2017	$ 1,126
Increase (decrease) attributed to:
Customer loyalty net deferrals	1,790
Customer loyalty redemptions	(1,445)
Outstanding at March 31, 2018	$ 1,471