Loss and loss adjustment expense reserves - Schedule of activity in the reserve for losses and loss adjustment expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves for loss and loss adjustment expenses, beginning of year	$ 466,047	$ 277,362	$ 134,331
Less: loss and loss adjustment expenses recoverable, beginning of year	(1)	(125)	(814)	$ (9,277)
Net reserves for loss and loss adjustment expenses, beginning of year	465,922	276,548	125,054
Increase (decrease) in net loss and loss adjustment expenses incurred in respect of losses occurring in:
Current year	373,048	419,377	286,787
Prior years	(23,930)	3,330	3,559
Amortization of deferred gains on retroactive reinsurance contracts	(1,046)	(856)	(81)
Total incurred loss and loss adjustment expenses	395,932	415,191	283,147
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Current year	(105,921)	(100,403)	(70,562)
Prior years	(133,241)	(121,665)	(61,091)
Total net paid losses	(239,162)	(222,068)	(131,653)
Foreign currency translation	(17,564)	(3,749)	0
Net reserve for loss and loss adjustment expenses, end of year	605,128	465,922	276,548
Plus: loss and loss adjustment expenses recoverable, end of year	(1)	(125)	(814)
Gross reserve for loss and loss adjustment expenses, end of year	605,129	466,047	277,362
Change In Policy Acquisition Costs	6,400
Prior Year Claims and Claims Adjustment Expense, Increase (Decrease) in Premium Estimates	19,500
Prior Year Claims and Claims Adjustment Expense, Increase (Decrease) in Net Underwriting Loss	(12,800)
Impact On Underwriting Income [Member]
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Change in Accounting Estimate, Financial Impact	300
Premium Adjustments on Certain Contracts [Member]
Increase (decrease) in net loss and loss adjustment expenses incurred in respect of losses occurring in:
Prior years	(13,400)	2,100	2,900
Favorable Reserve Development [Member]
Increase (decrease) in net loss and loss adjustment expenses incurred in respect of losses occurring in:
Prior years	(10,500)	5,400	700
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Change in policy acquisition costs		13,200	300
Impact On Underwriting Income [Member]
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Change in Accounting Estimate, Financial Impact	12,500
Paid Losses on Commuted Contracts	1,000	79,100	10,800
Impact On Underwriting Income [Member] | Multi Line Contract [Member]
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Change in Accounting Estimate, Financial Impact	4,800
Impact On Underwriting Income [Member] | Non-Standard Auto Contract [Member]
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Change in Accounting Estimate, Financial Impact	4,000
Impact On Underwriting Income [Member] | Reinsurance Contracts [Member]
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Change in Accounting Estimate, Financial Impact	3,700
Impact On Underwriting Income [Member] | Worker Compensation Contracts [Member]
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Change in Accounting Estimate, Financial Impact	3,300
Impact On Underwriting Income [Member] | Other Contracts [Member]
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Change in Accounting Estimate, Financial Impact	2,100
Impact On Underwriting Income [Member] | Favorable Reserve Development [Member]
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Prior Year Claims and Claims Adjustment Expense, Increase (Decrease) in Net Underwriting Loss		7,800	(400)
Ratio Split [Member]
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Change In Policy Acquisition Costs	2,000
Premium Adjustments on Certain Contracts [Member]
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Increase (decrease) in earned premium related to decrease in premium estimates		7,500	3,700
Change in policy acquisition costs		$ 300	(400)
Prior Year Claims and Claims Adjustment Expense, Increase (Decrease) in Net Underwriting Loss			(400)
Premium Adjustments on Certain Contracts [Member] | Impact On Underwriting Income [Member]
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Change in Accounting Estimate, Financial Impact	$ 19,800		$ 3,300