Reserve for losses and loss adjustment expenses - Prior year development (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year favorable (unfavorable) development	$ 387	$ 129
Casualty reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year favorable (unfavorable) development	3,700	2,200
Other specialty reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year favorable (unfavorable) development	4,100	(600)
Insurance programs
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year favorable (unfavorable) development	(8,400)	(800)
Property catastrophe reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year favorable (unfavorable) development	$ 1,000	$ (700)