Equity investees - Summarized Aggregate Financial Information for Equity Investees (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity Method Investments and Joint Ventures [Abstract]
Revenues	$ 22,286	$ 25,351	$ 24,274
Net income	4,729	4,997	6,542
Total assets	33,268	32,171	32,324
Notes payable	24,833	20,902	20,895
Stockholders’ equity	$ 6,292	$ 9,023	$ 9,483