Accumulated Other Comprehensive Income (Loss) (Details of Changes In Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2025	Mar. 31, 2024
PRE-TAX
Net Change	$ 72.2	$ 312.5
TAX
Net Change	2.3	(92.2)
AFTER TAX
Other Comprehensive Income (Loss)	74.5	220.3
Unrealized Gains (Losses) on Available for Sale Debt Securities
PRE-TAX
Unrealized Gains (Losses) on Available for Sale Debt Securities	71.2	73.2
Net Change	96.0	287.0
TAX
Unrealized Gains (Losses) on Available for Sale Debt Securities	(18.9)	(19.4)
Net Change	(25.0)	(73.6)
AFTER TAX
Unrealized Gains (Losses) on Available for Sale Debt Securities	52.3	53.8
Other Comprehensive Income (Loss)	71.0	213.4
Interest Income on Debt Securities
PRE-TAX
Reclassification Adjustments for (Gains) Losses Included in Net Income:	24.8	24.4
TAX
Reclassification Adjustments for (Gains) Losses Included in Net Income:	(6.1)	(6.2)
AFTER TAX
Reclassification Adjustments for (Gains) Losses Included in Net Income:	18.7	18.2
Net Losses on Debt Securities
PRE-TAX
Reclassification Adjustments for (Gains) Losses Included in Net Income:	0.0	189.4
TAX
Reclassification Adjustments for (Gains) Losses Included in Net Income:	0.0	(48.0)
AFTER TAX
Reclassification Adjustments for (Gains) Losses Included in Net Income:	0.0	141.4
Unrealized Gains (Losses) on Cash Flow Hedges
PRE-TAX
Unrealized Gains (Losses) on Available for Sale Debt Securities	9.8	4.5
Reclassification Adjustments for (Gains) Losses Included in Net Income:	(10.5)	(5.5)
Net Change	(0.7)	(1.0)
TAX
Unrealized Gains (Losses) on Available for Sale Debt Securities	(2.4)	(1.1)
Reclassification Adjustments for (Gains) Losses Included in Net Income:	2.6	1.4
Net Change	0.2	0.3
AFTER TAX
Unrealized Gains (Losses) on Available for Sale Debt Securities	7.4	3.4
Reclassification Adjustments for (Gains) Losses Included in Net Income:	(7.9)	(4.1)
Other Comprehensive Income (Loss)	(0.5)	(0.7)
Foreign Currency Adjustments
PRE-TAX
Net Change	(22.7)	22.2
TAX
Net Change	27.4	(17.6)
AFTER TAX
Other Comprehensive Income (Loss)	4.7	4.6
Foreign Currency Translation Adjustments
PRE-TAX
Net Change	91.4	(58.5)
TAX
Net Change	(0.6)	2.9
AFTER TAX
Other Comprehensive Income (Loss)	90.8	(55.6)
Long-Term Intra-Entity Foreign Currency Transaction Gains (Losses)
PRE-TAX
Net Change	0.2	(0.6)
TAX
Net Change	0.0	0.1
AFTER TAX
Other Comprehensive Income (Loss)	0.2	(0.5)
Net Investment Hedge Gains (Losses)
PRE-TAX
Net Change	(114.3)	81.3
TAX
Net Change	28.0	(20.6)
AFTER TAX
Other Comprehensive Income (Loss)	(86.3)	60.7
Pension and Other Postretirement Benefit Adjustments
PRE-TAX
Unrealized Gains (Losses) on Available for Sale Debt Securities	(3.4)	1.1
Net Change	(0.4)	4.3
TAX
Unrealized Gains (Losses) on Available for Sale Debt Securities	0.4	(0.5)
Net Change	(0.3)	(1.3)
AFTER TAX
Unrealized Gains (Losses) on Available for Sale Debt Securities	(3.0)	0.6
Other Comprehensive Income (Loss)	(0.7)	3.0
Amortization of Net Actuarial Loss
PRE-TAX
Reclassification Adjustments for (Gains) Losses Included in Net Income:	3.0	3.2
TAX
Reclassification Adjustments for (Gains) Losses Included in Net Income:	(0.7)	(0.8)
AFTER TAX
Reclassification Adjustments for (Gains) Losses Included in Net Income:	$ 2.3	$ 2.4

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