Investments Schedule of Financial Instrument and Derivative Offsetting (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Offsetting Derivative Assets
Net Derivative Assets Reported in Balance Sheet	$ 99.2	$ 99.9
Offsetting Securities Loaned
Gross Securities Loaned	57.5	72.0
Assets Offsetting Securities Loaned in Balance Sheet	0.0	0.0
Net Securities Loaned Reported in Balance Sheet	57.5	72.0
Securities Given as Collateral on Securities Loaned	(34.8)	(8.9)
Cash Given as Collateral on Securities Loaned	(22.7)	(63.1)
Securities Loaned Net of Collateral	0.0	0.0
Offsetting Financial Assets
Gross Financial Assets	151.3	171.9
Financial Liabilities Offsetting Financial Assets in Balance Sheet	0.0	0.0
Net Financial Assets Reported in Balance Sheet	151.3	171.9
Securities Received as Collateral on Financial Assets	(127.0)	(100.8)
Cash Received as Collateral on Financial Assets	(24.2)	(69.5)
Financial Assets Net of Collateral	0.1	1.6
Offsetting Derivative Liabilities
Net Derivative Liabilities Reported in Balance Sheet	173.6	117.7
Offsetting Securities Borrowed
Gross Securities Borrowed	22.7	63.1
Liabilities Offsetting Securities Borrowed in Balance Sheet	0.0	0.0
Net Securities Borrowed Reported in Balance Sheet	22.7	63.1
Securities Received as Collateral on Securities Borrowed	(22.7)	(63.1)
Cash Received as Collateral on Securities Borrowed	0.0	0.0
Securities Borrowed Net of Collateral	0.0	0.0
Offsetting Financial Liabilities
Gross Financial Liability	196.3	179.3
Financial Assets Offsetting Financial Liabilities in Balance Sheet	0.0	0.0
Net Financial Liabilities Reported in Balance Sheet	196.3	179.3
Securities Given as Collateral on Financial Liabilities	(177.9)	(172.5)
Cash Given as Collateral on Financial Liabilities	0.0	0.0
Financial Liabilities Net of Collateral	18.4	6.8
Over the Counter
Offsetting Derivative Assets
Derivative Asset, Fair Value, Gross Asset	93.8	99.9
Derivative Liabilities Offsetting Derivative Assets in Balance Sheet	0.0	0.0
Net Derivative Assets Reported in Balance Sheet	93.8	99.9
Securities Received as Collateral on Derivative Assets	(92.2)	(91.9)
Cash Collateral from Counterparties	(1.5)	(6.4)
Derivative Assets Net of Collateral	0.1	1.6
Offsetting Derivative Liabilities
Gross Derivative Liability	173.6	116.2
Derivative Assets Offsetting Derivative Liabilities in Balance Sheet	0.0	0.0
Net Derivative Liabilities Reported in Balance Sheet	173.6	116.2
Securities Given as Collateral on Derivative Liabilities	(155.2)	(109.4)
Cash Collateral to counterparties	0.0	0.0
Derivative Liabilities Net of Collateral	$ 18.4	$ 6.8