Equity investees - Summarized Aggregate Financial Information for Equity Investees (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity Method Investments and Joint Ventures [Abstract]
Revenues	$ 25,351	$ 24,274	$ 28,843
Net income	4,997	6,542	8,830
Total assets	32,171	32,324	33,555
Notes payable	20,902	20,895	20,200
Stockholders’ equity	$ 9,023	$ 9,483	$ 7,213