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, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves for loss and loss adjustment expenses, beginning of year	$ 937,157	$ 720,570	$ 605,129
Less: loss and loss adjustment expenses recoverable, beginning of year	(5,520)	(2,031)	(1,113)	$ (5,520)	$ (2,031)	$ (1,113)	$ (1)
Net reserves for loss and loss adjustment expenses, beginning of year				1,099,434	931,279	719,457	605,128
Increase (decrease) in net loss and loss adjustment expenses incurred in respect of losses occurring in:
Current year	489,994	434,276	422,801
Prior years	86,495	(4,138)	52,743
Total incurred loss and loss adjustment expenses	403,499	438,414	370,058
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Current year	(63,638)	(85,173)	(110,799)
Prior years	(188,392)	(132,336)	(162,447)
Total net paid losses	(252,030)	(217,509)	(273,246)
Foreign currency translation	16,686	(9,083)	17,517
Net reserves for loss and loss adjustment expenses, beginning of year				1,099,434	931,279	719,457	605,128
Plus: loss and loss adjustment expenses recoverable, end of year	(5,520)	(2,031)	(1,113)
Deferred Costs				$ (6,738)	$ (3,847)	$ 0	$ 0
Gross reserves for loss and loss adjustment expenses, end of year	1,111,692	937,157	720,570
Prior Year Claims and Claims Adjustment Expense, Including Amortization of Deferred Gains	86,500	(4,100)	52,700
Impact On Underwriting Income [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	3,700	6,200	4,600
Ratio Split [Member]
Increase (decrease) in net loss and loss adjustment expenses incurred in respect of losses occurring in:
Prior years	98,500	12,900	22,300
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Change In Policy Acquisition Costs	(100,900)	(7,700)	(19,800)
Prior Year Claims and Claims Adjustment Expense, Increase (Decrease) in Earned Premium	(7,800)
Prior Year Claims and Claims Adjustment Expense, Increase (Decrease) in Net Underwriting Loss	5,400	5,200	2,500
Premium Adjustments on Certain Contracts [Member]
Increase (decrease) in net loss and loss adjustment expenses incurred in respect of losses occurring in:
Prior years	(12,000)	(17,000)	30,400
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Change in Accounting Estimate, Financial Impact	(4,800)	(22,400)	52,100
Change In Policy Acquisition Costs	7,200	(5,400)	21,700
Prior Year Claims and Claims Adjustment Expense, Increase (Decrease) in Premium Estimates	3,100	23,400	(50,000)
Prior Year Claims and Claims Adjustment Expense, Increase (Decrease) in Net Underwriting Loss	(1,700)	1,000	2,100
Auto contracts [Member] | Ratio Split [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	3,500		(4,600)
Retroactive reinsurance contracts | Ratio Split [Member]
Increase (decrease) in net loss and loss adjustment expenses incurred in respect of losses occurring in:
Prior years	69,400
Net loss and loss adjustment expenses paid in respect of losses occurring in:
Change In Policy Acquisition Costs	(68,900)
Prior Year Claims and Claims Adjustment Expense, Increase (Decrease) in Net Underwriting Loss	500
Other Contracts [Member] | Ratio Split [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		(10,500)	1,300
Multi-line | Ratio Split [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	(8,100)
Workers’ Compensation | Ratio Split [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	14,900	$ 15,800	$ 5,800
General Liability [Member] | Ratio Split [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	$ (8,800)