Accumulated Other Comprehensive Income (Loss) (Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 12,016.8	$ 11,688.3
Net Change	(871.4)	(362.1)
Ending balance	11,409.8	11,457.9
TOTAL
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(35.6)	428.0
Net Change	(871.4)	(362.1)
Ending balance	(907.0)	65.9
NET UNREALIZED GAINS (LOSSES) ON DEBT SECURITIES AVAILABLE FOR SALE DEBT SECURITIES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	107.1	641.8
Net Change	(868.4)	(369.7)
Ending balance	(761.3)	272.1
NET UNREALIZED (LOSSES) GAINS ON CASH FLOW HEDGES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(2.4)	(3.2)
Net Change	(1.4)	(5.2)
Ending balance	(3.8)	(8.4)
NET FOREIGN CURRENCY ADJUSTMENT
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	155.2	144.7
Net Change	(7.9)	5.0
Ending balance	147.3	149.7
NET PENSION AND OTHER POSTRETIREMENT BENEFIT ADJUSTMENTS
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(295.5)	(355.3)
Net Change	6.3	7.8
Ending balance	$ (289.2)	$ (347.5)