Reserve for losses and loss adjustment expenses - Reconciliation of beginning and ending balances of loss reserves (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 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	$ 1,032,760
Unpaid losses and loss adjustment expenses recoverable	165,549	81,267
Net reserve for losses and loss adjustment expenses at beginning of period	1,098,079	951,493
Net incurred losses and loss adjustment expenses relating to losses occurring in:
Current period	215,849	222,395
Prior years	(387)	(129)
Total net losses and loss adjustment expenses	215,462	222,266
Foreign exchange (gains) losses	(23,999)	64
Net paid losses and loss adjustment expenses relating to losses occurring in:
Current period	(27,050)	(13,956)
Prior years	(128,021)	(153,229)
Total paid losses and loss adjustment expenses	(155,071)	(167,185)
Net reserve for losses and loss adjustment expenses at end of period	1,134,471	1,006,638
Unpaid losses and loss adjustment expenses recoverable	218,578	119,442
Gross reserve for losses and loss adjustment expenses at end of period	$ 1,353,049	$ 1,126,080