Accumulated Other Comprehensive Income (Loss) (Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2026	Mar. 31, 2026	Jun. 30, 2025	Mar. 31, 2025	Jun. 30, 2026	Jun. 30, 2025
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 12,987.3	$ 12,957.9	$ 12,878.5	$ 12,788.4	$ 12,957.9	$ 12,788.4
Net Change	84.9	(46.1)	40.3	74.5	38.8	114.8
Ending balance	13,401.6	12,987.3	12,866.5	12,878.5	13,401.6	12,866.5
TOTAL
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(636.6)	(590.5)	(739.5)	(814.0)	(590.5)	(814.0)
Net Change	84.9	(46.1)	40.3	74.5
Ending balance	(551.7)	(636.6)	(699.2)	(739.5)	(551.7)	(699.2)
NET UNREALIZED GAINS (LOSSES) ON AVAILABLE FOR SALE DEBT SECURITIES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(445.9)	(401.3)	(527.1)	(598.1)	(401.3)	(598.1)
Net Change	72.7		45.9		28.1	116.9
Ending balance	(373.2)	(445.9)	(481.2)	(527.1)	(373.2)	(481.2)
NET UNREALIZED GAINS (LOSSES) ON CASH FLOW HEDGES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(4.4)	0.9	0.1	0.6	0.9	0.6
Net Change	(1.7)		(0.1)		(7.0)	(0.6)
Ending balance	(6.1)	(4.4)	0.0	0.1	(6.1)	0.0
NET FOREIGN CURRENCY ADJUSTMENTS
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	250.5	248.9	237.8	233.1	248.9	233.1
Net Change	9.0		(7.7)		10.6	(3.0)
Ending balance	259.5	250.5	230.1	237.8	259.5	230.1
NET PENSION AND OTHER POSTRETIREMENT BENEFIT ADJUSTMENTS
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(436.8)	(439.0)	(450.3)	(449.6)	(439.0)	(449.6)
Net Change	4.9		2.2		7.1	1.5
Ending balance	$ (431.9)	$ (436.8)	$ (448.1)	$ (450.3)	$ (431.9)	$ (448.1)