Deferred policy acquisition costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Movement analysis of deferred policy acquisition costs [Roll Forward]
Balance, beginning of period	$ 14,801	$ 11,775	$ 10,150
Policy acquisition costs deferred:
Direct commissions deferred	56,841	40,546	32,927
Ceding commissions deferred	(12,373)	(11,239)	(9,984)
Other underwriting and policy acquisition costs deferred	3,727	3,141	2,827
Policy acquisition costs deferred	48,195	32,448	25,770
Amortization of net policy acquisition costs	(39,432)	(29,422)	(24,145)
Balance, end of period	$ 23,564	$ 14,801	$ 11,775