Reinsurance - Funds Withheld (Details) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2022 USD ($)	Jun. 30, 2022 USD ($)
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	$ 22,643.5	$ 22,643.5
Funds withheld payable	22,530.8	22,530.8
Embedded derivative asset	3,067.3	3,067.3
Change in fair value of funds withheld embedded derivative	3,067.3	3,067.3
Fixed maturities, available-for-sale
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	19,373.0	19,373.0
Fixed maturities, trading
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	5.9	5.9
Equity securities
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	10.0	10.0
Mortgage loans
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	2,789.9	2,789.9
Other investments
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	1.8	1.8
Cash and cash equivalents
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	400.3	400.3
Accrued interest income
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	176.8	176.8
Net other assets (liabilities)
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net asset (liability)	$ (114.2)	$ (114.2)