Reserves for unpaid losses and loss adjustment expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ 194,602	$ 124,126	$ 91,970
Commutation of 2016 MLQS	0	9,109	0
Commutation of 2015 MLQS	27,929	0	0
Commutation of 2014 MLQS	0	24,296	0
Commutation of 2012 MLQS	0	0	8,587
Commutations of MLQS	27,929	33,405	8,587
Adjusted net reserves for losses and loss adjustment expenses, beginning of year	222,531	157,531	100,557
Incurred losses and loss adjustment expenses [Abstract]
Current year	114,960	83,675	51,434
Prior year losses and loss adjustment expense	(11,280)	(12,714)	(9,196)
Total net losses and loss adjustment expenses incurred	103,680	70,961	42,238
Payments [Abstract]
Current year	13,792	5,494	2,226
Prior years	44,926	28,396	16,443
Total payments	58,718	33,890	18,669
Net reserves for unpaid losses and loss adjustment expenses, end of period	267,493	194,602	124,126
Reinsurance recoverable for unpaid losses	48,224	70,199	95,503
Gross reserves for unpaid losses and loss adjustment expenses, end of period	$ 315,717	$ 264,801	$ 219,629