Summary of Significant Accounting Policies (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income taxes
Interest expense and penalties on uncertain income tax matters	$ 0	$ 0	$ 0
Liabilities for uncertain income tax matters	0	0
Point loyalty program
Amount of points redeemable for services and merchandise	1,700,000	1,652,000
Revenue and expense recognition
Goods and services provided without charge to customers as promotional items	18,812,000	18,784,000	18,003,000
Advertising costs
Advertising expenses	$ 2,034,000	$ 2,161,000	$ 2,135,000