Deferred policy acquisition costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Movement analysis of deferred policy acquisition costs [Roll Forward]
Balance, beginning of period	$ 11,775	$ 10,150
Balance, beginning of period			$ (1,696)
Policy acquisition costs deferred:
Direct commissions deferred	40,546	32,927	28,027
Ceding commissions deferred	(11,239)	(9,984)	(9,213)
Other underwriting and policy acquisition costs deferred	3,141	2,827	2,999
Policy acquisition costs deferred	32,448	25,770	21,813
Amortization of net policy acquisition costs	(29,422)	(24,145)	(9,967)
Balance, end of period	$ 14,801	$ 11,775	$ 10,150