Reserves for unpaid losses and loss adjustment expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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	$ 267,493	$ 194,602	$ 124,126
Commutation of 2016 MLQS	0	0	9,109
Commutation of 2015 MLQS	0	27,929	0
Commutation of 2014 MLQS	0	0	24,296
Commutations of MLQS	0	27,929	33,405
Adjusted net reserves for losses and loss adjustment expenses, beginning of year	267,493	222,531	157,531
Incurred losses and loss adjustment expenses [Abstract]
Current year	135,078	114,960	83,675
Prior year losses and loss adjustment expense	(7,037)	(11,280)	(12,714)
Total net losses and loss adjustment expenses incurred	128,041	103,680	70,961
Payments [Abstract]
Current year	14,118	13,792	5,494
Prior years	67,653	44,926	28,396
Total payments	81,771	58,718	33,890
Net reserves for unpaid losses and loss adjustment expenses, end of period	313,763	267,493	194,602
Reinsurance recoverable for unpaid losses	55,389	48,224	70,199
Gross reserves for unpaid losses and loss adjustment expenses, end of period	$ 369,152	$ 315,717	$ 264,801