Investments Schedule of Financial Instrument and Derivative Offsetting (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Offsetting Derivative Assets [Abstract]
Net Derivative Assets Reported in Balance Sheet	$ 75.5	$ 69.7
Offsetting Securities Loaned
Gross Securities Loaned	95.7	106.1
Assets Offsetting Securities Loaned in Balance Sheet	0.0	0.0
Net Securities Loaned Reported in Balance Sheet	95.7	106.1
Securities Given as Collateral on Securities Loaned	(34.0)	(30.0)
Cash Given as Collateral on Securities Loaned	(61.7)	(76.1)
Securities Loaned, Amount Offset Against Collateral	0.0	0.0
Offsetting Financial Assets
Gross Financial Assets	156.4	158.0
Financial Liabilities Offsetting Financial Assets in Balance Sheet	0.0	0.0
Net Financial Assets Reported in Balance Sheet	156.4	158.0
Securities Received as Collateral on Financial Assets	(94.1)	(81.9)
Cash Received as Collateral on Financial Assets	(62.2)	(76.1)
Financial Assets Net of Collateral	0.1	0.0
Offsetting Derivative Liabilities
Net Derivative Liabilities Reported in Balance Sheet	261.0	269.7
Offsetting Securities Borrowed
Gross Securities Borrowed	61.7	76.1
Liabilities Offsetting Securities Borrowed in Balance Sheet	0.0	0.0
Net Securities Borrowed Reported in Balance Sheet	61.7	76.1
Securities Received as Collateral on Securities Borrowed	(61.7)	(76.1)
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	322.7	345.8
Financial Assets Offsetting Financial Liabilities in Balance Sheet	0.0	0.0
Net Financial Liabilities Reported in Balance Sheet	322.7	345.8
Securities Given as Collateral on Financial Liabilities	(322.5)	(345.4)
Cash Given as Collateral on Financial Liabilities	0.0	0.0
Financial Liabilities Net of Collateral	0.2	0.4
Over the Counter
Offsetting Derivative Assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	60.7	51.9
Derivative Liabilities Offsetting Derivative Assets in Balance Sheet	0.0	0.0
Net Derivative Assets Reported in Balance Sheet	60.7	51.9
Securities Received as Collateral on Derivative Assets	(60.1)	(51.9)
Cash Collateral from Counterparties	(0.5)	0.0
Derivative Assets Net of Collateral	0.1	0.0
Offsetting Derivative Liabilities
Gross Derivative Liability	261.0	269.7
Derivative Assets Offsetting Derivative Liabilities in Balance Sheet	0.0	0.0
Net Derivative Liabilities Reported in Balance Sheet	261.0	269.7
Securities Given as Collateral on Derivative Liabilities	(260.8)	(269.3)
Cash Collateral to counterparties	0.0	0.0
Derivative Liabilities Net of Collateral	$ 0.2	$ 0.4