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, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2024	Sep. 30, 2023
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 12,655.8	$ 12,101.8	$ 11,897.9	$ 11,635.7	$ 11,587.8	$ 11,259.5	$ 11,897.9	$ 11,259.5
Net Change	85.5	52.6	220.3	42.5	(39.7)	203.0	358.4	205.8
Ending balance	12,749.0	12,655.8	12,101.8	11,847.2	11,635.7	11,587.8	12,749.0	11,847.2
TOTAL
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(865.0)	(917.6)	(1,137.9)	(1,405.9)	(1,366.2)	(1,569.2)	(1,137.9)	(1,569.2)
Net Change	85.5	52.6	220.3	42.5	(39.7)	203.0
Ending balance	(779.5)	(865.0)	(917.6)	(1,363.4)	(1,405.9)	(1,366.2)	(779.5)	(1,363.4)
NET UNREALIZED GAINS (LOSSES) ON AVAILABLE FOR SALE DEBT SECURITIES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(667.1)		(923.9)	(1,229.4)		(1,367.6)	(923.9)	(1,367.6)
Net Change	63.9			32.3			320.7	170.5
Ending balance	(603.2)	(667.1)		(1,197.1)	(1,229.4)		(603.2)	(1,197.1)
NET UNREALIZED GAINS (LOSSES) ON CASH FLOW HEDGES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	0.0		0.8	0.5		1.2	0.8	1.2
Net Change	0.3			(0.4)			(0.5)	(1.1)
Ending balance	0.3	0.0		0.1	0.5		0.3	0.1
NET FOREIGN CURRENCY ADJUSTMENT
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	215.2		203.6	189.2		164.6	203.6	164.6
Net Change	18.9			9.6			30.5	34.2
Ending balance	234.1	215.2		198.8	189.2		234.1	198.8
NET PENSION AND OTHER POSTRETIREMENT BENEFIT ADJUSTMENTS
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(413.1)		$ (418.4)	(366.2)		$ (367.4)	(418.4)	(367.4)
Net Change	2.4			1.0			7.7	2.2
Ending balance	$ (410.7)	$ (413.1)		$ (365.2)	$ (366.2)		$ (410.7)	$ (365.2)