Accumulated Other Comprehensive Income (Loss) (Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 11,259.5	$ 12,016.8
Net Change	203.0	(871.4)
Ending balance	11,587.8	11,409.8
TOTAL
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(1,569.2)	(35.6)
Net Change	203.0	(871.4)
Ending balance	(1,366.2)	(907.0)
NET UNREALIZED GAINS (LOSSES) ON AVAILABLE FOR SALE DEBT SECURITIES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(1,367.6)	107.1
Net Change	180.2	(868.4)
Ending balance	(1,187.4)	(761.3)
NET UNREALIZED (LOSSES) GAINS ON CASH FLOW HEDGES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	1.2	(2.4)
Net Change	(0.1)	(1.4)
Ending balance	1.1	(3.8)
NET FOREIGN CURRENCY ADJUSTMENT
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	164.6	155.2
Net Change	22.7	(7.9)
Ending balance	187.3	147.3
NET PENSION AND OTHER POSTRETIREMENT BENEFIT ADJUSTMENTS
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(367.4)	(295.5)
Net Change	0.2	6.3
Ending balance	$ (367.2)	$ (289.2)