Reserve for losses and loss adjustment expenses - Reconciliation of beginning and ending balances of loss reserves (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract]
Gross reserve for losses and loss adjustment expenses at beginning of period	$ 1,263,628,000	$ 1,032,760,000
Unpaid losses and loss adjustment expenses recoverable	165,549,000	81,267,000
Net reserve for losses and loss adjustment expenses at beginning of period	1,098,079,000	951,493,000
Net incurred losses and loss adjustment expenses relating to losses occurring in:
Current period	110,856,000	110,901,000
Prior years	(180,000)	(51,000)
Total net losses and loss adjustment expenses	110,676,000	110,850,000
Foreign exchange (gains) losses	(26,015,000)	4,163,000
Net paid losses and loss adjustment expenses relating to losses occurring in:
Current period	(10,585,000)	(6,330,000)
Prior years	(62,585,000)	(54,598,000)
Total paid losses and loss adjustment expenses	(73,170,000)	(60,928,000)
Net reserve for losses and loss adjustment expenses at end of period	1,109,570,000	1,005,578,000
Unpaid losses and loss adjustment expenses recoverable	190,679,000	98,954,000
Gross reserve for losses and loss adjustment expenses at end of period	$ 1,300,249,000	$ 1,104,532,000