Equity investees - Summarized Aggregate Financial Information for Equity Investees (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Equity Method Investments and Joint Ventures [Abstract]
Revenues	$ 19,660	$ 22,286	$ 25,351
Net income	6,231	4,729	4,997
Total assets	36,865	33,268	32,171
Notes payable	27,841	24,833	20,902
Stockholders’ equity	$ 6,341	$ 6,292	$ 9,023