Loss and loss adjustment expense reserves - Schedule of activity in the reserve for losses and loss adjustment expenses (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves for loss and loss adjustment expenses, beginning of period	$ 720,570	$ 605,129
Less: loss and loss adjustment expenses recoverable, beginning of period	(1,113)	(1)
Net reserves for loss and loss adjustment expenses, beginning of period	719,457	605,128
Increase (decrease) in net loss and loss adjustment expenses incurred in respect of losses occurring in:
Current year	254,265	295,379
Prior years (1)	(11,061)	24,830
Total incurred loss and loss adjustment expenses	265,326	270,549
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Current year	(58,652)	(47,655)
Prior years	(103,572)	(145,550)
Total net paid losses	(162,224)	(193,205)
Foreign currency translation	(5,715)	15,310
Net reserves for loss and loss adjustment expenses, end of period	816,844	697,782
Plus: loss and loss adjustment expenses recoverable, end of period	1,576	1,587
Deferred Costs	3,760	0	$ 0
Gross reserves for loss and loss adjustment expenses, end of period	822,180	699,369
Amortization of deferred gains on retroactive reinsurance contracts		(1,400)
Unfavorable (Favorable) Reserve Development [Member]
Increase (decrease) in net loss and loss adjustment expenses incurred in respect of losses occurring in:
Prior years (1)	7,100	(31,700)
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Change In Policy Acquisition Costs	(2,300)	(31,700)
Increase in net underwriting loss	(4,800)
Premium Adjustments on Certain Contracts [Member]
Increase (decrease) in net loss and loss adjustment expenses incurred in respect of losses occurring in:
Prior years (1)	(18,200)	(6,900)
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Change in Accounting Estimate, Financial Impact	24,300
Prior Year Claims and Claims Adjustment Expense, Increase (Decrease) in Premium Estimates	(25,600)	(8,100)
Change In Policy Acquisition Costs	(6,100)
Increase in net underwriting loss	(1,300)	1,200
Impact On Underwriting Income [Member]
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Increase in net underwriting loss	(6,100)	$ 1,200
Multi Line Contract [Member] | Unfavorable (Favorable) Reserve Development [Member]
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Increase in net underwriting loss	12,700
Property (1) | Unfavorable (Favorable) Reserve Development [Member]
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Increase in net underwriting loss	$ 7,400