Equity investees - Summarized Aggregate Financial Information for Equity Investees (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Equity Method Investments and Joint Ventures [Abstract]
Revenues	$ 28,843	$ 31,562	$ 42,105
Net income	8,830	7,914	8,064
Total assets	33,555	25,533	25,470
Notes payable	20,200	12,727	12,390
Stockholders’ equity	$ 7,213	$ 7,665	$ 5,561