Investments Schedule of Financial Instrument and Derivative Offsetting (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Offsetting Derivative Assets
Derivative Asset, Fair Value, Gross Asset	$ 89.1	$ 39.5
Derivative Liabilities Offsetting Derivative Assets in Balance Sheet	0.0	0.0
Net Derivative Assets Reported in Balance Sheet	89.1	39.5
Securities Received as Collateral on Derivative Assets	(38.0)	(9.8)
Derivative Asset, Subject to Master Netting Arrangement, Collateral, Obligation to Return Cash Not Offset	(49.4)	(28.4)
Derivative Assets Net of Collateral	1.7	1.3
Offsetting Securities Borrowed
Securities Received as Collateral on Securities Borrowed	(88.5)	(94.8)
Cash Received as Collateral on Securities Borrowed	0.0	0.0
Offsetting Financial Assets
Gross Financial Assets	241.5	323.2
Financial Liabilities Offsetting Financial Assets in Balance Sheet	0.0	0.0
Net Financial Assets Reported in Balance Sheet	241.5	323.2
Securities Received as Collateral on Financial Assets	(101.9)	(198.7)
Cash Received as Collateral on Financial Assets	(137.9)	(123.2)
Financial Assets Net of Collateral	1.7	1.3
Offsetting Derivative Liabilities
Net Derivative Liabilities Reported in Balance Sheet		65.1
Offsetting Securities Loaned
Gross Securities Loaned	(152.4)	(283.7)
Assets Offsetting Securities Loaned in Balance Sheet	0.0	0.0
Securities Loaned	(152.4)	(283.7)
Securities Given as Collateral on Securities Loaned	(63.9)	(188.9)
Cash Given as Collateral on Securities Loaned	(88.5)	(94.8)
Securities Loaned Net of Collateral	0.0	0.0
Offsetting Financial Liabilities
Gross Financial Liability	162.5	129.8
Financial Assets Offsetting Financial Liabilities in Balance Sheet	0.0	0.0
Net Financial Liabilities Reported in Balance Sheet	162.5	129.8
Securities Given as Collateral on Financial Liabilities	(161.7)	(128.8)
Cash Given as Collateral on Financial Liabilities	0.0	0.0
Financial Liabilities Net of Collateral	0.8	1.0
Security Borrowed, Subject to Master Netting Arrangement, before Offset	88.5	94.8
Security Borrowed, Subject to Master Netting Arrangement, Liability Offset	0.0	0.0
Securities Borrowed	88.5	94.8
Security Borrowed, Including Not Subject to Master Netting Arrangement, after Offset and Deduction	0.0	0.0
Miscellaneous Long-term Investments
Offsetting Derivative Assets
Derivative Asset, Fair Value, Gross Asset	89.1	39.5
Over the Counter
Offsetting Derivative Liabilities
Gross Derivative Liability	74.0	35.0
Derivative Assets Offsetting Derivative Liabilities in Balance Sheet	0.0	0.0
Net Derivative Liabilities Reported in Balance Sheet	74.0	35.0
FMS Collateral Posted to Counterparties	(73.2)	(34.0)
Derivative Liability, Subject to Master Netting Arrangement, Collateral, Right to Reclaim Cash Not Offset	0.0	0.0
Derivative Liabilities Net of Collateral	$ 0.8	$ 1.0