Other comprehensive income (loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Unrealized gains (losses) arising during the period, before income taxes	$ 9,563	$ (830)	$ (2,347)
Income taxes	(3,094)	290	821
Unrealized gains (losses) arising during the period, net of income tax	6,469	(540)	(1,526)
Less reclassification adjustment [Abstract]
Net realized investment gains (losses)	132	154	57
Income taxes	(47)	(54)	(20)
Reclassification adjustment included in net income	85	100	37
Other comprehensive income (loss)	$ 6,384	$ (640)	$ (1,563)