Acquisition of Sirius Group - Identifiable intangible assets (Details) - USD ($)			12 Months Ended
	Aug. 05, 2021	Feb. 26, 2021	Dec. 31, 2021	Dec. 31, 2020
Business Acquisition [Line Items]
Finite lived intangible assets net balance			$ 124,100,000
Identifiable intangible assets net balance			171,900,000	$ 0
Intangible asset impairments			0
Sirius Group
Business Acquisition [Line Items]
Identifiable intangible assets gross balance		$ 178,800,000
Accumulated amortization			(5,900,000)
Identifiable intangible assets net balance			172,900,000
Sirius Group | Lloyd’s Capacity - Syndicate 1945
Business Acquisition [Line Items]
Indefinite lived intangible assets net balance		41,800,000	41,800,000
Sirius Group | Insurance licenses
Business Acquisition [Line Items]
Indefinite lived intangible assets net balance		$ 7,000,000.0	7,000,000.0
Insurance licenses sold	$ (1,000,000.0)
Sirius Group | Distribution relationships
Business Acquisition [Line Items]
Economic Useful Life		17 years
Finite lived intangible assets gross balance		$ 75,000,000.0
Accumulated amortization			0
Finite lived intangible assets net balance			75,000,000.0
Sirius Group | MGA relationships
Business Acquisition [Line Items]
Economic Useful Life		13 years
Finite lived intangible assets gross balance		$ 34,000,000.0
Accumulated amortization			(4,900,000)
Finite lived intangible assets net balance			29,100,000
Sirius Group | Trade name
Business Acquisition [Line Items]
Economic Useful Life		16 years
Finite lived intangible assets gross balance		$ 16,000,000.0
Accumulated amortization			(200,000)
Finite lived intangible assets net balance			15,800,000
Sirius Group | Internally developed computer software
Business Acquisition [Line Items]
Economic Useful Life		5 years
Finite lived intangible assets gross balance		$ 5,000,000.0
Accumulated amortization			(800,000)
Finite lived intangible assets net balance			$ 4,200,000