Investments Schedule of Financial Instrument and Derivative Offsetting (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Offsetting Derivative Assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	$ 73.0	$ 69.7
Net Derivative Assets Reported in Balance Sheet	73.0	69.7
Offsetting Securities Loaned [Abstract]
Gross Securities Loaned	100.8	106.1
Assets Offsetting Securities Loaned in Balance Sheet	0.0	0.0
Net Securities Loaned Reported in Balance Sheet	100.8	106.1
Securities Given as Collateral on Securities Loaned	(33.2)	(30.0)
Cash Given as Collateral on Securities Loaned	(67.6)	(76.1)
Securities Loaned Net of Collateral	0.0	0.0
Offsetting Derivative Asset, Securities Purchased under Agreements to Resell, Securities Borrowed [Abstract]
Gross Financial Assets	158.7	158.0
Financial Liabilities Offsetting Financial Assets in Balance Sheet	0.0	0.0
Net Financial Assets Reported in Balance Sheet	158.7	158.0
Securities Received as Collateral on Financial Assets	(90.0)	(81.9)
Cash Received as Collateral on Financial Assets	(67.6)	(76.1)
Financial Assets Net of Collateral	1.1	0.0
Offsetting Derivative Liabilities
Gross Derivative Liability	223.0	269.7
Net Derivative Liabilities Reported in Balance Sheet	223.0	269.7
Offsetting Securities Borrowed [Abstract]
Gross Securities Borrowed	67.6	76.1
Liabilities Offsetting Securities Borrowed in Balance Sheet	0.0	0.0
Net Securities Borrowed Reported in Balance Sheet	67.6	76.1
Securities Received as Collateral on Securities Borrowed	(67.6)	(76.1)
Cash Received as Collateral on Securities Borrowed	0.0	0.0
Securities Borrowed Net of Collateral	0.0	0.0
Offsetting Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned [Abstract]
Gross Financial Liability	290.6	345.8
Financial Assets Offsetting Financial Liabilities in Balance Sheet	0.0	0.0
Net Financial Liabilities Reported in Balance Sheet	290.6	345.8
Securities Given as Collateral on Financial Liabilities	(287.5)	(345.4)
Cash Given as Collateral on Financial Liabilities	0.0	0.0
Financial Liabilities Net of Collateral	3.1	0.4
Over the Counter
Offsetting Derivative Assets [Abstract]
Derivative Asset, Fair Value, Gross Asset	57.9	51.9
Derivative Liabilities Offsetting Derivative Assets in Balance Sheet	0.0	0.0
Net Derivative Assets Reported in Balance Sheet	57.9	51.9
Securities Received as Collateral on Derivative Assets	(56.8)	(51.9)
Cash Collateral from Counterparties	0.0	0.0
Derivative Assets Net of Collateral	1.1	0.0
Offsetting Derivative Liabilities
Gross Derivative Liability	223.0	269.7
Derivative Assets Offsetting Derivative Liabilities in Balance Sheet	0.0	0.0
Net Derivative Liabilities Reported in Balance Sheet	223.0	269.7
Securities Given as Collateral on Derivative Liabilities	(219.9)	(269.3)
Cash Collateral to counterparties	0.0	0.0
Derivative Liabilities Net of Collateral	$ 3.1	$ 0.4