Investments Schedule of Financial Instrument and Derivative Offsetting (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Offsetting Derivative Assets
Derivative Asset, Fair Value, Gross Asset	$ 95.1	$ 89.1
Derivative Liabilities Offsetting Derivative Assets in Balance Sheet	0.0	0.0
Net Derivative Assets Reported in Balance Sheet	95.1	89.1
Securities Received as Collateral on Derivative Assets	(36.0)	(38.0)
Cash Collateral from Counterparties	(51.2)	(49.4)
Derivative Assets Net of Collateral	7.9	1.7
Offsetting Securities Borrowed
Gross Securities Borrowed	56.6	88.5
Liabilities Offsetting Securities Borrowed in Balance Sheet	0.0	0.0
Net Securities Borrowed Reported in Balance Sheet	56.6	88.5
Securities Received as Collateral on Securities Borrowed	(56.6)	(88.5)
Cash Received as Collateral on Securities Borrowed	0.0	0.0
Securities Borrowed Net of Collateral	0.0	0.0
Offsetting Financial Assets
Gross Financial Assets	184.4	241.5
Financial Liabilities Offsetting Financial Assets in Balance Sheet	0.0	0.0
Net Financial Assets Reported in Balance Sheet	184.4	241.5
Securities Received as Collateral on Financial Assets	(68.7)	(101.9)
Cash Received as Collateral on Financial Assets	(107.8)	(137.9)
Financial Assets Net of Collateral	7.9	1.7
Offsetting Securities Loaned
Gross Securities Loaned	89.3	152.4
Assets Offsetting Securities Loaned in Balance Sheet	0.0	0.0
Net Securities Loaned Reported in Balance Sheet	89.3	152.4
Securities Given as Collateral on Securities Loaned	(32.7)	(63.9)
Cash Given as Collateral on Securities Loaned	(56.6)	(88.5)
Securities Loaned, Amount Offset Against Collateral	0.0	0.0
Offsetting Financial Liabilities
Gross Financial Liability	105.9	162.5
Financial Assets Offsetting Financial Liabilities in Balance Sheet	0.0	0.0
Net Financial Liabilities Reported in Balance Sheet	105.9	162.5
Securities Given as Collateral on Financial Liabilities	(105.7)	(161.7)
Cash Given as Collateral on Financial Liabilities	0.0	0.0
Financial Liabilities Net of Collateral	0.2	0.8
Over the Counter
Offsetting Derivative Liabilities
Gross Derivative Liability	49.3	74.0
Derivative Assets Offsetting Derivative Liabilities in Balance Sheet	0.0	0.0
Net Derivative Liabilities Reported in Balance Sheet	49.3	74.0
Securities Given as Collateral on Derivative Liabilities	(49.1)	(73.2)
Cash Collateral to counterparties	0.0	0.0
Derivative Liabilities Net of Collateral	$ 0.2	$ 0.8