Accumulated Other Comprehensive Income (Loss) (Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2025	Jun. 30, 2025	Mar. 31, 2025	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2025	Sep. 30, 2024
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 12,866.5	$ 12,878.5	$ 12,788.4	$ 12,655.8	$ 12,101.8	$ 11,897.9	$ 12,788.4	$ 11,897.9
Net Change	64.1	40.3	74.5	85.5	52.6	220.3	178.9	358.4
Ending balance	12,956.0	12,866.5	12,878.5	12,749.0	12,655.8	12,101.8	12,956.0	12,749.0
TOTAL
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(699.2)	(739.5)	(814.0)	(865.0)	(917.6)	(1,137.9)	(814.0)	(1,137.9)
Net Change	64.1	40.3	74.5	85.5	52.6	220.3
Ending balance	(635.1)	(699.2)	(739.5)	(779.5)	(865.0)	(917.6)	(635.1)	(779.5)
NET UNREALIZED GAINS (LOSSES) ON AVAILABLE FOR SALE DEBT SECURITIES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(481.2)		(598.1)	(667.1)		(923.9)	(598.1)	(923.9)
Net Change	44.3			63.9			161.2	320.7
Ending balance	(436.9)	(481.2)		(603.2)	(667.1)		(436.9)	(603.2)
NET UNREALIZED GAINS (LOSSES) ON CASH FLOW HEDGES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	0.0		0.6	0.0		0.8	0.6	0.8
Net Change	0.0			0.3			(0.6)	(0.5)
Ending balance	0.0	0.0		0.3	0.0		0.0	0.3
NET FOREIGN CURRENCY ADJUSTMENT
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	230.1		233.1	215.2		203.6	233.1	203.6
Net Change	17.6			18.9			14.6	30.5
Ending balance	247.7	230.1		234.1	215.2		247.7	234.1
NET PENSION AND OTHER POSTRETIREMENT BENEFIT ADJUSTMENTS
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(448.1)		$ (449.6)	(413.1)		$ (418.4)	(449.6)	(418.4)
Net Change	2.2			2.4			3.7	7.7
Ending balance	$ (445.9)	$ (448.1)		$ (410.7)	$ (413.1)		$ (445.9)	$ (410.7)