Investments Schedule of Financial Instrument and Derivative Offsetting (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Offsetting Derivative Assets
Net Derivative Assets Reported in Balance Sheet	$ 99.9	$ 89.1
Offsetting Securities Loaned
Gross Securities Loaned	(72.0)	(152.4)
Assets Offsetting Securities Loaned in Balance Sheet	0.0	0.0
Securities Loaned	(72.0)	(152.4)
Securities Given as Collateral on Securities Loaned	(8.9)	(63.9)
Cash Given as Collateral on Securities Loaned	(63.1)	(88.5)
Securities Loaned Net of Collateral	0.0	0.0
Offsetting Financial Assets
Gross Financial Assets	171.9	241.5
Financial Liabilities Offsetting Financial Assets in Balance Sheet	0.0	0.0
Net Financial Assets Reported in Balance Sheet	171.9	241.5
Securities Received as Collateral on Financial Assets	(100.8)	(101.9)
Cash Received as Collateral on Financial Assets	(69.5)	(137.9)
Financial Assets Net of Collateral	1.6	1.7
Offsetting Derivative Liabilities
Net Derivative Liabilities Reported in Balance Sheet	117.7	87.9
Offsetting Securities Borrowed
Security Borrowed, Subject to Master Netting Arrangement, before Offset	63.1	88.5
Security Borrowed, Subject to Master Netting Arrangement, Liability Offset	0.0	0.0
Securities Borrowed	63.1	88.5
Securities Received as Collateral on Securities Borrowed	(63.1)	(88.5)
Cash Received as Collateral on Securities Borrowed	0.0	0.0
Security Borrowed, Including Not Subject to Master Netting Arrangement, after Offset and Deduction	0.0	0.0
Offsetting Financial Liabilities
Gross Financial Liability	179.3	162.5
Financial Assets Offsetting Financial Liabilities in Balance Sheet	0.0	0.0
Net Financial Liabilities Reported in Balance Sheet	179.3	162.5
Securities Given as Collateral on Financial Liabilities	(172.5)	(161.7)
Cash Given as Collateral on Financial Liabilities	0.0	0.0
Financial Liabilities Net of Collateral	6.8	0.8
Over the Counter
Offsetting Derivative Assets
Derivative Asset, Fair Value, Gross Asset	99.9	89.1
Derivative Liabilities Offsetting Derivative Assets in Balance Sheet	0.0	0.0
Net Derivative Assets Reported in Balance Sheet	99.9	89.1
Securities Received as Collateral on Derivative Assets	(91.9)	(38.0)
Derivative Asset, Subject to Master Netting Arrangement, Collateral, Obligation to Return Cash Not Offset	(6.4)	(49.4)
Derivative Assets Net of Collateral	1.6	1.7
Offsetting Derivative Liabilities
Gross Derivative Liability	116.2	74.0
Derivative Assets Offsetting Derivative Liabilities in Balance Sheet	0.0	0.0
Net Derivative Liabilities Reported in Balance Sheet	116.2	74.0
FMS Collateral Posted to Counterparties	(109.4)	(73.2)
Derivative Liability, Subject to Master Netting Arrangement, Collateral, Right to Reclaim Cash Not Offset	0.0	0.0
Derivative Liabilities Net of Collateral	$ 6.8	$ 0.8