Equity investees - Additional Information (Detail) - USD ($)		12 Months Ended
	Feb. 10, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Subsidiary, Sale of Stock [Line Items]
Net premium revenues from policies issued by equity investees		$ 24,274,000	$ 28,843,000	$ 31,562,000
Earnings related to equity investees		6,542,000	8,830,000	7,914,000
Goodwill related to equity investees		217,094,000	217,722,000	251,868,000
Impairment of goodwill	$ 35,900,000	0	35,749,000	0
Equity securities
Subsidiary, Sale of Stock [Line Items]
Goodwill related to equity investees		7,400,000	7,400,000
Impairment of goodwill		0	0	0
Direct Operations
Subsidiary, Sale of Stock [Line Items]
Net premium revenues from policies issued by equity investees		2,100,000	2,500,000	3,100,000
Earnings related to equity investees		$ 2,800,000	$ 3,600,000	$ 3,400,000