Segment reporting - Breakdown of net premiums written by location and reportable segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Net premiums written	$ 1,734.2	$ 542.2	$ 657.0
Client Location: United States and Canada
Segment Reporting Information [Line Items]
Net premiums written	1,141.0	313.9	354.6
Client Location: Europe
Segment Reporting Information [Line Items]
Net premiums written	300.1	114.5	161.6
Client Location: Bermuda, the Caribbean and Latin America
Segment Reporting Information [Line Items]
Net premiums written	128.0	108.6	137.3
Client Location: Asia and Other
Segment Reporting Information [Line Items]
Net premiums written	165.1	5.2	3.5
Underwriting Location: United States and Canada
Segment Reporting Information [Line Items]
Net premiums written	857.6	241.2	272.1
Underwriting Location: Europe
Segment Reporting Information [Line Items]
Net premiums written	455.3
Underwriting Location: Bermuda, the Caribbean and Latin America
Segment Reporting Information [Line Items]
Net premiums written	369.1	301.0	384.9
Underwriting Location: Asia and Other
Segment Reporting Information [Line Items]
Net premiums written	52.2
Operating Segments
Segment Reporting Information [Line Items]
Net premiums written	1,777.7	513.3	569.4
Operating Segments | Reinsurance
Segment Reporting Information [Line Items]
Net premiums written	1,124.9	497.3	563.9
Operating Segments | Reinsurance | Client Location: United States and Canada
Segment Reporting Information [Line Items]
Net premiums written	579.1	300.1	349.3
Operating Segments | Reinsurance | Client Location: Europe
Segment Reporting Information [Line Items]
Net premiums written	309.5	83.7	73.9
Operating Segments | Reinsurance | Client Location: Bermuda, the Caribbean and Latin America
Segment Reporting Information [Line Items]
Net premiums written	114.3	108.6	137.3
Operating Segments | Reinsurance | Client Location: Asia and Other
Segment Reporting Information [Line Items]
Net premiums written	122.0	4.9	3.4
Operating Segments | Reinsurance | Underwriting Location: United States and Canada
Segment Reporting Information [Line Items]
Net premiums written	447.1	236.3	267.0
Operating Segments | Reinsurance | Underwriting Location: Europe
Segment Reporting Information [Line Items]
Net premiums written	379.8
Operating Segments | Reinsurance | Underwriting Location: Bermuda, the Caribbean and Latin America
Segment Reporting Information [Line Items]
Net premiums written	246.1	261.0	296.9
Operating Segments | Reinsurance | Underwriting Location: Asia and Other
Segment Reporting Information [Line Items]
Net premiums written	51.9
Operating Segments | Insurance & Services
Segment Reporting Information [Line Items]
Net premiums written	652.8	16.0	5.5
Operating Segments | Insurance & Services | Client Location: United States and Canada
Segment Reporting Information [Line Items]
Net premiums written	560.3	13.8	5.3
Operating Segments | Insurance & Services | Client Location: Europe
Segment Reporting Information [Line Items]
Net premiums written	36.4	1.9	0.1
Operating Segments | Insurance & Services | Client Location: Bermuda, the Caribbean and Latin America
Segment Reporting Information [Line Items]
Net premiums written	13.7	0.0	0.0
Operating Segments | Insurance & Services | Client Location: Asia and Other
Segment Reporting Information [Line Items]
Net premiums written	42.4	0.3	0.1
Operating Segments | Insurance & Services | Underwriting Location: United States and Canada
Segment Reporting Information [Line Items]
Net premiums written	408.9	4.9	5.1
Operating Segments | Insurance & Services | Underwriting Location: Europe
Segment Reporting Information [Line Items]
Net premiums written	93.0
Operating Segments | Insurance & Services | Underwriting Location: Bermuda, the Caribbean and Latin America
Segment Reporting Information [Line Items]
Net premiums written	150.9	11.1	0.4
Operating Segments | Insurance & Services | Underwriting Location: Asia and Other
Segment Reporting Information [Line Items]
Net premiums written	0.0
Corporate
Segment Reporting Information [Line Items]
Net premiums written	(43.5)	28.9	87.6
Corporate | Client Location: United States and Canada
Segment Reporting Information [Line Items]
Net premiums written	1.6	0.0	0.0
Corporate | Client Location: Europe
Segment Reporting Information [Line Items]
Net premiums written	(45.8)	28.9	87.6
Corporate | Client Location: Bermuda, the Caribbean and Latin America
Segment Reporting Information [Line Items]
Net premiums written	0.0	0.0	0.0
Corporate | Client Location: Asia and Other
Segment Reporting Information [Line Items]
Net premiums written	0.7	0.0	0.0
Corporate | Underwriting Location: United States and Canada
Segment Reporting Information [Line Items]
Net premiums written	1.6	0.0	0.0
Corporate | Underwriting Location: Europe
Segment Reporting Information [Line Items]
Net premiums written	(17.5)
Corporate | Underwriting Location: Bermuda, the Caribbean and Latin America
Segment Reporting Information [Line Items]
Net premiums written	(27.9)	$ 28.9	$ 87.6
Corporate | Underwriting Location: Asia and Other
Segment Reporting Information [Line Items]
Net premiums written	$ 0.3