Equity investees - Summarized Aggregate Financial Information for Equity Investees (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity Method Investments and Joint Ventures [Abstract]
Revenues	$ 24,274	$ 28,843	$ 31,562
Net income	6,542	8,830	7,914
Total assets	32,324	33,555	25,533
Notes payable	20,895	20,200	12,727
Stockholders’ equity	$ 9,483	$ 7,213	$ 7,665