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, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 11,635.7	$ 11,587.8	$ 11,259.5	$ 11,069.7	$ 11,409.8	$ 12,016.8	$ 11,259.5	$ 12,016.8
Net Change	42.5	(39.7)	203.0	(274.4)	(603.6)	(871.4)	205.8	(1,749.4)
Ending balance	11,847.2	11,635.7	11,587.8	11,030.4	11,069.7	11,409.8	11,847.2	11,030.4
TOTAL
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(1,405.9)	(1,366.2)	(1,569.2)	(1,510.6)	(907.0)	(35.6)	(1,569.2)	(35.6)
Net Change	42.5	(39.7)	203.0	(274.4)	(603.6)	(871.4)
Ending balance	(1,363.4)	(1,405.9)	(1,366.2)	(1,785.0)	(1,510.6)	(907.0)	(1,363.4)	(1,785.0)
NET UNREALIZED GAINS (LOSSES) ON AVAILABLE FOR SALE DEBT SECURITIES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(1,229.4)		(1,367.6)	(1,294.7)		107.1	(1,367.6)	107.1
Net Change	32.3			(272.2)			170.5	(1,674.0)
Ending balance	(1,197.1)	(1,229.4)		(1,566.9)	(1,294.7)		(1,197.1)	(1,566.9)
NET UNREALIZED GAINS (LOSSES) ON CASH FLOW HEDGES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	0.5		1.2	0.6		(2.4)	1.2	(2.4)
Net Change	(0.4)			2.1			(1.1)	5.1
Ending balance	0.1	0.5		2.7	0.6		0.1	2.7
NET FOREIGN CURRENCY ADJUSTMENT
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	189.2		164.6	137.6		155.2	164.6	155.2
Net Change	9.6			20.2			34.2	2.6
Ending balance	198.8	189.2		157.8	137.6		198.8	157.8
NET PENSION AND OTHER POSTRETIREMENT BENEFIT ADJUSTMENTS
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(366.2)		$ (367.4)	(354.1)		$ (295.5)	(367.4)	(295.5)
Net Change	1.0			(24.5)			2.2	(83.1)
Ending balance	$ (365.2)	$ (366.2)		$ (378.6)	$ (354.1)		$ (365.2)	$ (378.6)