Acquisition of Sirius Group - Identifiable intangible assets (Details) - USD ($) $ in Millions			12 Months Ended
	Aug. 05, 2021	Feb. 26, 2021	Dec. 31, 2022	Dec. 31, 2021
Business Acquisition [Line Items]
Finite lived intangible assets net balance			$ 116.0
Identifiable intangible assets net balance			163.8	$ 171.9
Intangible asset impairments			0.0
Sirius Group
Business Acquisition [Line Items]
Identifiable intangible assets gross balance		$ 178.8
Accumulated amortization			(14.0)	(5.9)
Identifiable intangible assets net balance			164.8	172.9
Sirius Group | Lloyd’s Capacity - Syndicate 1945
Business Acquisition [Line Items]
Indefinite lived intangible assets net balance		41.8	41.8	41.8
Sirius Group | Insurance licenses
Business Acquisition [Line Items]
Indefinite lived intangible assets net balance		$ 7.0	7.0	7.0
Insurance licenses sold	$ (1.0)
Sirius Group | Distribution relationships
Business Acquisition [Line Items]
Economic Useful Life		17 years
Finite lived intangible assets gross balance		$ 75.0
Accumulated amortization			(1.9)	0.0
Finite lived intangible assets net balance			73.1	75.0
Sirius Group | MGA relationships
Business Acquisition [Line Items]
Economic Useful Life		13 years
Finite lived intangible assets gross balance		$ 34.0
Accumulated amortization			(9.4)	(4.9)
Finite lived intangible assets net balance			24.6	29.1
Sirius Group | Trade name
Business Acquisition [Line Items]
Economic Useful Life		16 years
Finite lived intangible assets gross balance		$ 16.0
Accumulated amortization			(0.8)	(0.2)
Finite lived intangible assets net balance			15.2	15.8
Sirius Group | Internally developed computer software
Business Acquisition [Line Items]
Economic Useful Life		5 years
Finite lived intangible assets gross balance		$ 5.0
Accumulated amortization			(1.9)	(0.8)
Finite lived intangible assets net balance			$ 3.1	$ 4.2