Segment information (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)	Jul. 02, 2019
Segment Reporting Information [Line Items]
Number of operating segments | segment			1
Gross premiums written	$ 157,927	$ 161,978	$ 392,829	$ 348,667
Net premiums written	105,856	119,370	292,556	264,757
Net premiums earned	131,535	151,318	271,574	297,412
United States
Segment Reporting Information [Line Items]
Net premiums written	27,798	22,385	64,101	40,787
Europe
Segment Reporting Information [Line Items]
Net premiums written	22,921	23,927	44,124	47,185
Bermuda
Segment Reporting Information [Line Items]
Net premiums written	55,137	73,058	184,331	176,785
Casualty reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	25,125	32,557	108,943	108,158
Net premiums written	24,774	32,077	108,441	107,142
Net premiums earned	48,146	67,506	100,911	130,819
Other specialty reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	21,080	37,836	57,960	62,134
Net premiums written	19,843	36,523	55,327	59,705
Net premiums earned	29,876	42,635	65,240	87,196
Property catastrophe reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	11,253	5,929	21,085	11,921
Net premiums written	10,506	5,621	20,338	11,603
Net premiums earned	5,824	3,119	10,708	6,090
Insurance programs and coinsurance
Segment Reporting Information [Line Items]
Gross premiums written	100,469	85,656	204,841	166,454
Net premiums written	50,733	45,149	108,450	86,307
Net premiums earned	$ 47,689	$ 38,058	$ 94,715	$ 73,307
Senior notes
Segment Reporting Information [Line Items]
Stated interest rate (percent)	6.50%		6.50%		6.50%