Reinsurance - Funds Withheld (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	$ 18,857.7		$ 18,857.7		$ 20,703.9
Funds withheld payable	18,557.7		18,557.7		20,436.1
Embedded derivative asset	4,045.7		4,045.7		3,652.8
Change in fair value of funds withheld embedded derivative	925.6	$ 1,237.7	392.9	$ 4,305.0
Fixed maturities, available-for-sale
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	14,779.3		14,779.3		15,693.5
Fixed maturities, trading
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	215.3		215.3		100.8
Equity securities
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	0.3		0.3		11.0
Mortgage loans
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	2,527.9		2,527.9		2,810.8
Other investments
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	434.1		434.1		179.8
Cash and cash equivalents
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	746.9		746.9		1,762.9
Accrued interest income
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net assets	203.7		203.7		178.7
Net other assets (liabilities)
Funds Held under Reinsurance Agreements, Asset [Line Items]
Net asset (liability)	$ (49.8)		$ (49.8)		$ (33.6)