Equity investees - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Subsidiary, Sale of Stock [Line Items]
Net premium revenues from policies issued by equity investees	$ 31,562	$ 42,105	$ 66,145
Earnings related to equity investees	7,914	8,064	11,037
Goodwill related to equity investees	251,868	231,838	220,955
Direct Operations
Subsidiary, Sale of Stock [Line Items]
Net premium revenues from policies issued by equity investees	3,100	4,400	6,000
Earnings related to equity investees	3,400	4,100	4,300
Equity securities
Subsidiary, Sale of Stock [Line Items]
Goodwill related to equity investees	$ 7,400	$ 7,500