Investments Schedule of Financial Instrument and Derivative Offsetting (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Offsetting Derivative Assets [Abstract]
Net Derivative Assets Reported in Balance Sheet	$ 100.1	$ 99.9
Offsetting Securities Loaned
Gross Securities Loaned	60.2	72.0
Assets Offsetting Securities Loaned in Balance Sheet	0.0	0.0
Net Securities Loaned Reported in Balance Sheet	60.2	72.0
Securities Given as Collateral on Securities Loaned	(13.3)	(8.9)
Cash Given as Collateral on Securities Loaned	(46.9)	(63.1)
Securities Loaned, Amount Offset Against Collateral	0.0	0.0
Offsetting Financial Assets
Gross Financial Assets	155.8	171.9
Financial Liabilities Offsetting Financial Assets in Balance Sheet	0.0	0.0
Net Financial Assets Reported in Balance Sheet	155.8	171.9
Securities Received as Collateral on Financial Assets	(106.5)	(100.8)
Cash Received as Collateral on Financial Assets	(47.8)	(69.5)
Financial Assets Net of Collateral	1.5	1.6
Offsetting Derivative Liabilities
Net Derivative Liabilities Reported in Balance Sheet	147.0	117.7
Offsetting Securities Borrowed
Gross Securities Borrowed	46.9	63.1
Liabilities Offsetting Securities Borrowed in Balance Sheet	0.0	0.0
Net Securities Borrowed Reported in Balance Sheet	46.9	63.1
Securities Received as Collateral on Securities Borrowed	(46.9)	(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	193.9	179.3
Financial Assets Offsetting Financial Liabilities in Balance Sheet	0.0	0.0
Net Financial Liabilities Reported in Balance Sheet	193.9	179.3
Securities Given as Collateral on Financial Liabilities	(193.7)	(172.5)
Cash Given as Collateral on Financial Liabilities	0.0	0.0
Financial Liabilities Net of Collateral	0.2	6.8
Over the Counter
Offsetting Derivative Assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	95.6	99.9
Derivative Liabilities Offsetting Derivative Assets in Balance Sheet	0.0	0.0
Net Derivative Assets Reported in Balance Sheet	95.6	99.9
Securities Received as Collateral on Derivative Assets	(93.2)	(91.9)
Cash Collateral from Counterparties	(0.9)	(6.4)
Derivative Assets Net of Collateral	1.5	1.6
Offsetting Derivative Liabilities
Gross Derivative Liability	147.0	116.2
Derivative Assets Offsetting Derivative Liabilities in Balance Sheet	0.0	0.0
Net Derivative Liabilities Reported in Balance Sheet	147.0	116.2
Securities Given as Collateral on Derivative Liabilities	(146.8)	(109.4)
Cash Collateral to counterparties	0.0	0.0
Derivative Liabilities Net of Collateral	$ 0.2	$ 6.8