Accumulated Other Comprehensive Income (Loss) (Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 12,957.9	$ 12,788.4
Net Change	(46.1)	74.5
Ending balance	12,987.3	12,878.5
TOTAL
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(590.5)	(814.0)
Net Change	(46.1)	74.5
Ending balance	(636.6)	(739.5)
NET UNREALIZED GAINS (LOSSES) ON AVAILABLE FOR SALE DEBT SECURITIES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(401.3)	(598.1)
Net Change	(44.6)	71.0
Ending balance	(445.9)	(527.1)
NET UNREALIZED GAINS (LOSSES) ON CASH FLOW HEDGES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	0.9	0.6
Net Change	(5.3)	(0.5)
Ending balance	(4.4)	0.1
NET FOREIGN CURRENCY ADJUSTMENTS
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	248.9	233.1
Net Change	1.6	4.7
Ending balance	250.5	237.8
NET PENSION AND OTHER POSTRETIREMENT BENEFIT ADJUSTMENTS
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(439.0)	(449.6)
Net Change	2.2	(0.7)
Ending balance	$ (436.8)	$ (450.3)