Reinsurance - Funds Withheld (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	$ 20,772.4		$ 20,703.9
Funds withheld payable	20,524.4		20,436.1
Embedded derivative asset	3,026.2		3,652.8
Change in fair value of funds withheld embedded derivative	(626.6)	$ 0.0
Fixed maturities, available-for-sale
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	16,231.7		15,693.5
Fixed maturities, trading
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	103.2		100.8
Equity securities
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	0.3		11.0
Mortgage loans
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	2,735.1		2,810.8
Other investments
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	203.0		179.8
Cash and cash equivalents
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	1,522.8		1,762.9
Accrued interest income
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	200.0		178.7
Net other assets (liabilities)
Funds Held under Reinsurance Agreements, Asset [Line Items]
Total assets - Net other liabilities	$ (223.7)		$ (33.6)