Reserves for unpaid losses and loss adjustment expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of consolidated beginning and ending reserves for unpaid losses and loss adjustment expenses [Roll Forward]
Net reserves for unpaid losses and loss adjustment expenses, beginning of period	$ 313,763	$ 267,493	$ 194,602
Commutation of 2015 MLQS	0	0	27,929
Adjusted net reserves for losses and loss adjustment expenses, beginning of year	313,763	267,493	222,531
Incurred losses and loss adjustment expenses [Abstract]
Current year	178,986	135,078	114,960
Prior year losses and loss adjustment expense	(9,423)	(7,037)	(11,280)
Total net losses and loss adjustment expenses incurred	169,563	128,041	103,680
Payments [Abstract]
Current year	19,054	14,118	13,792
Prior years	74,006	67,653	44,926
Total payments	93,060	81,771	58,718
Net reserves for unpaid losses and loss adjustment expenses, end of period	390,266	313,763	267,493
Reinsurance recoverable for unpaid losses	69,792	55,389	48,224
Gross reserves for unpaid losses and loss adjustment expenses, end of period	$ 460,058	$ 369,152	$ 315,717