Equity investees - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsidiary, Sale of Stock [Line Items]
Net premium revenues from policies issued by equity investees	$ 25,351,000	$ 24,274,000	$ 28,843,000
Earnings related to equity investees	4,997,000	6,542,000	8,830,000
Goodwill related to equity investees	231,428,000	217,094,000	217,722,000
Impairment of goodwill	0	0	35,749,000
Equity securities
Subsidiary, Sale of Stock [Line Items]
Goodwill related to equity investees	7,300,000	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	2,100,000	2,500,000
Earnings related to equity investees	$ 2,200,000	$ 2,800,000	$ 3,600,000