Accumulated Other Comprehensive Income (Loss) (Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 12,788.4	$ 11,897.9
Net Change	74.5	220.3
Ending balance	12,878.5	12,101.8
TOTAL
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(814.0)	(1,137.9)
Net Change	74.5	220.3
Ending balance	(739.5)	(917.6)
NET UNREALIZED GAINS (LOSSES) ON AVAILABLE FOR SALE DEBT SECURITIES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(598.1)	(923.9)
Net Change	71.0	213.4
Ending balance	(527.1)	(710.5)
NET UNREALIZED GAINS (LOSSES) ON CASH FLOW HEDGES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	0.6	0.8
Net Change	(0.5)	(0.7)
Ending balance	0.1	0.1
NET FOREIGN CURRENCY ADJUSTMENT
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	233.1	203.6
Net Change	4.7	4.6
Ending balance	237.8	208.2
NET PENSION AND OTHER POSTRETIREMENT BENEFIT ADJUSTMENTS
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(449.6)	(418.4)
Net Change	(0.7)	3.0
Ending balance	$ (450.3)	$ (415.4)