Accumulated Other Comprehensive Income (Loss) (Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 11,688.3	$ 11,091.0
Net Change	(362.1)	239.7
Ending balance	11,457.9	10,879.7
TOTAL
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	428.0	(194.7)
Net Change	(362.1)	239.7
Ending balance	65.9	45.0
NET UNREALIZED GAINS (LOSSES) ON DEBT SECURITIES AVAILABLE FOR SALE
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	641.8	114.0
Net Change	(369.7)	199.7
Ending balance	272.1	313.7
NET UNREALIZED (LOSSES) GAINS ON CASH FLOW HEDGES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(3.2)	(3.7)
Net Change	(5.2)	7.3
Ending balance	(8.4)	3.6
NET FOREIGN CURRENCY ADJUSTMENT
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	144.7	117.8
Net Change	5.0	25.7
Ending balance	149.7	143.5
NET PENSION AND OTHER POSTRETIREMENT BENEFIT ADJUSTMENTS
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(355.3)	(422.8)
Net Change	7.8	7.0
Ending balance	$ (347.5)	$ (415.8)