Segment information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) segment	Mar. 31, 2019 USD ($)	Jul. 02, 2019
Segment Reporting Information [Line Items]
Number of operating segments | segment	1
Gross premiums written	$ 234,902	$ 186,689
Net premiums written	186,700	145,387
Net premiums earned	140,039	146,094
United States
Segment Reporting Information [Line Items]
Net premiums written	36,303	18,402
Europe
Segment Reporting Information [Line Items]
Net premiums written	21,203	23,258
Bermuda
Segment Reporting Information [Line Items]
Net premiums written	129,194	103,727
Casualty reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	83,818	75,601
Net premiums written	83,667	75,065
Net premiums earned	52,765	63,313
Other specialty reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	36,880	24,298
Net premiums written	35,484	23,182
Net premiums earned	35,364	44,561
Property catastrophe reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	9,832	5,992
Net premiums written	9,832	5,982
Net premiums earned	4,884	2,971
Insurance programs and coinsurance
Segment Reporting Information [Line Items]
Gross premiums written	104,372	80,798
Net premiums written	57,717	41,158
Net premiums earned	$ 47,026	$ 35,249
Senior notes
Segment Reporting Information [Line Items]
Stated interest rate (percent)	6.50%		6.50%