Accumulated Other Comprehensive Income (Loss) (Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 11,091.0	$ 10,508.3
Net Change	239.7	108.4
Ending balance	10,879.7	10,616.2
TOTAL
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(194.7)	(453.7)
Ending balance	45.0	(345.3)
NET UNREALIZED GAINS (LOSSES) ON DEBT SECURITIES AVAILABLE FOR SALE (1)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	114.0	(114.9)
Net Change	199.7	85.6
Ending balance	313.7	(29.3)
NET UNREALIZED (LOSSES) GAINS ON CASH FLOW HEDGES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(3.7)	4.0
Net Change	7.3	(4.3)
Ending balance	3.6	(0.3)
NET FOREIGN CURRENCY ADJUSTMENT
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	117.8	67.9
Net Change	25.7	14.8
Ending balance	143.5	82.7
NET PENSION AND OTHER POSTRETIREMENT BENEFIT ADJUSTMENTS
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(422.8)	(410.7)
Net Change	7.0	12.3
Ending balance	$ (415.8)	$ (398.4)