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, 2024	Mar. 31, 2024	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2024	Jun. 30, 2023
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 12,101.8	$ 11,897.9	$ 11,587.8	$ 11,259.5	$ 11,897.9	$ 11,259.5
Net Change	52.6	220.3	(39.7)	203.0	272.9	163.3
Ending balance	12,655.8	12,101.8	11,635.7	11,587.8	12,655.8	11,635.7
TOTAL
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(917.6)	(1,137.9)	(1,366.2)	(1,569.2)	(1,137.9)	(1,569.2)
Net Change	52.6	220.3	(39.7)	203.0
Ending balance	(865.0)	(917.6)	(1,405.9)	(1,366.2)	(865.0)	(1,405.9)
NET UNREALIZED GAINS (LOSSES) ON AVAILABLE FOR SALE DEBT SECURITIES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(710.5)	(923.9)	(1,187.4)	(1,367.6)	(923.9)	(1,367.6)
Net Change	43.4		(42.0)		256.8	138.2
Ending balance	(667.1)	(710.5)	(1,229.4)	(1,187.4)	(667.1)	(1,229.4)
NET UNREALIZED GAINS (LOSSES) ON CASH FLOW HEDGES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	0.1	0.8	1.1	1.2	0.8	1.2
Net Change	(0.1)		(0.6)		(0.8)	(0.7)
Ending balance	0.0	0.1	0.5	1.1	0.0	0.5
NET FOREIGN CURRENCY ADJUSTMENT
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	208.2	203.6	187.3	164.6	203.6	164.6
Net Change	7.0		1.9		11.6	24.6
Ending balance	215.2	208.2	189.2	187.3	215.2	189.2
NET PENSION AND OTHER POSTRETIREMENT BENEFIT ADJUSTMENTS
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(415.4)	(418.4)	(367.2)	(367.4)	(418.4)	(367.4)
Net Change	2.3		1.0		5.3	1.2
Ending balance	$ (413.1)	$ (415.4)	$ (366.2)	$ (367.2)	$ (413.1)	$ (366.2)