Reserve for losses and loss adjustment expenses - Prior year development (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year favorable (unfavorable) development	$ 180,000	$ 51,000
Casualty reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year favorable (unfavorable) development	4,000,000	1,800,000
Other specialty reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year favorable (unfavorable) development	100,000	(400,000)
Insurance programs
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year favorable (unfavorable) development	(4,400,000)	(600,000)
Property catastrophe reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year favorable (unfavorable) development	$ 500,000	$ (800,000)