Summary of Significant Accounting Policies - Recent accounting pronouncements (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 01, 2016
Point loyalty program	$ 1,700,000				$ 1,652,000				$ 1,700,000	$ 1,652,000
Retained earnings	110,220,000				111,288,000				110,220,000	111,288,000
Quarterly operating earnings for the years	(35,000)	$ (7,000)	$ 223,000	$ 52,000	(275,000)	$ 1,013,000	$ 1,509,000	$ 34,000	$ 233,000	$ 2,281,000	$ 3,875,000
Expected impact to prior period results | ASU 2014-09
Point loyalty program												$ 500,000
Retained earnings												$ (500,000)
Expected impact to prior period results | ASU 2014-09 | Maximum
Quarterly operating earnings for the years	$ 50,000	$ 50,000	$ 50,000	$ 50,000	$ 50,000	$ 50,000	$ 50,000	$ 50,000