Statutory financial information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statutory net income and statutory capital surplus [Abstract]
Statutory net income	$ 23,841	$ 22,850	$ 21,972
Statutory capital and surplus	$ 213,833	$ 193,387	$ 127,675
Percentage of policyholder surplus as of end of prior year used in determining maximum dividend distribution	10.00%
Maximum dividend distribution that can be paid without prior approval	$ 23,700