Accumulated Other Comprehensive Income (Loss) (Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 11,897.9	$ 11,259.5
Net Change	220.3	203.0
Ending balance	12,101.8	11,587.8
TOTAL
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(1,137.9)	(1,569.2)
Net Change	220.3	203.0
Ending balance	(917.6)	(1,366.2)
NET UNREALIZED GAINS (LOSSES) ON AVAILABLE FOR SALE DEBT SECURITIES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(923.9)	(1,367.6)
Net Change	213.4	180.2
Ending balance	(710.5)	(1,187.4)
NET UNREALIZED GAINS (LOSSES) ON CASH FLOW HEDGES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	0.8	1.2
Net Change	(0.7)	(0.1)
Ending balance	0.1	1.1
NET FOREIGN CURRENCY ADJUSTMENT
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	203.6	164.6
Net Change	4.6	22.7
Ending balance	208.2	187.3
NET PENSION AND OTHER POSTRETIREMENT BENEFIT ADJUSTMENTS
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(418.4)	(367.4)
Net Change	3.0	0.2
Ending balance	$ (415.4)	$ (367.2)