Summary of Significant Accounting Policies - Point loyalty program, revenue recognition and advertising (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Point loyalty program
Interest expense and penalties on uncertain income tax matters	$ 0	$ 0	$ 0
Liabilities for uncertain income tax matters	0	0
Amount of points redeemable for services and merchandise	1,652,000	1,660,000
Revenue and expense recognition
Goods and services provided without charge to customers as promotional items	18,784,000	18,003,000	18,241,000
Advertising costs
Advertising expenses	$ 2,161,000	$ 2,135,000	$ 2,171,000