Equity investees - Summarized Aggregate Financial Information for Equity Investees (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Equity Method Investments and Joint Ventures [Abstract]
Revenues	$ 31,562	$ 42,105	$ 66,145
Net earnings	7,914	8,064	11,037
Total assets	25,533	25,470	52,500
Notes payable	12,727	12,390	23,681
Stockholders’ equity	$ 7,665	$ 5,561	$ 12,758