Investments Schedule of Financial Instrument and Derivative Offsetting (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Offsetting Derivative Assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	$ 69.0	$ 69.7
Net Derivative Assets Reported in Balance Sheet	69.0	69.7
Offsetting Securities Loaned [Abstract]
Gross Securities Loaned	99.2	106.1
Assets Offsetting Securities Loaned in Balance Sheet	0.0	0.0
Net Securities Loaned Reported in Balance Sheet	99.2	106.1
Securities Given as Collateral on Securities Loaned	(36.3)	(30.0)
Cash Given as Collateral on Securities Loaned	(62.9)	(76.1)
Securities Loaned Net of Collateral	0.0	0.0
Offsetting Financial Assets
Gross Financial Assets	149.5	158.0
Financial Liabilities Offsetting Financial Assets in Balance Sheet	0.0	0.0
Net Financial Assets Reported in Balance Sheet	149.5	158.0
Securities Received as Collateral on Financial Assets	(84.4)	(81.9)
Cash Received as Collateral on Financial Assets	(62.9)	(76.1)
Financial Assets Net of Collateral	2.2	0.0
Offsetting Derivative Liabilities
Gross Derivative Liability	421.5	269.7
Net Derivative Liabilities Reported in Balance Sheet	421.5	269.7
Offsetting Securities Borrowed
Gross Securities Borrowed	62.9	76.1
Liabilities Offsetting Securities Borrowed in Balance Sheet	0.0	0.0
Securities Borrowed	(62.9)	(76.1)
Securities Received as Collateral on Securities Borrowed	(62.9)	(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	484.4	345.8
Financial Assets Offsetting Financial Liabilities in Balance Sheet	0.0	0.0
Net Financial Liabilities Reported in Balance Sheet	484.4	345.8
Securities Given as Collateral on Financial Liabilities	(483.2)	(345.4)
Cash Given as Collateral on Financial Liabilities	0.0	0.0
Financial Liabilities Net of Collateral	1.2	0.4
Over the Counter
Offsetting Derivative Assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	50.3	51.9
Derivative Liabilities Offsetting Derivative Assets in Balance Sheet	0.0	0.0
Net Derivative Assets Reported in Balance Sheet	50.3	51.9
Securities Received as Collateral on Derivative Assets	(48.1)	(51.9)
Cash Collateral from Counterparties	0.0	0.0
Derivative Assets Net of Collateral	2.2	0.0
Offsetting Derivative Liabilities
Gross Derivative Liability	421.5	269.7
Derivative Assets Offsetting Derivative Liabilities in Balance Sheet	0.0	0.0
Net Derivative Liabilities Reported in Balance Sheet		269.7
Securities Given as Collateral on Derivative Liabilities	(420.3)	(269.3)
Derivative Liability, Subject to Master Netting Arrangement, Collateral, Right to Reclaim Cash Not Offset	0.0	0.0
Derivative Liabilities Net of Collateral	$ 1.2	$ 0.4