Accumulated Other Comprehensive Income (Loss) (Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 11,457.9	$ 11,688.3	$ 10,879.7	$ 11,091.0	$ 11,688.3	$ 11,091.0
Net Change	128.8	(362.1)	325.4	239.7	(233.3)	565.1
Ending balance	11,823.0	11,457.9	11,382.2	10,879.7	11,823.0	11,382.2
TOTAL
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	65.9	428.0	45.0	(194.7)	428.0	(194.7)
Net Change	128.8	(362.1)	325.4	239.7
Ending balance	194.7	65.9	370.4	45.0	194.7	370.4
NET UNREALIZED GAINS (LOSSES) ON DEBT SECURITIES AVAILABLE FOR SALE
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	272.1	641.8	313.7	114.0	641.8	114.0
Net Change	100.9		318.1		(268.8)	517.8
Ending balance	373.0	272.1	631.8	313.7	373.0	631.8
OCI, debt securities, available-for-sale, transfer to held-to-maturity, adjustment from AOCI for amortization of gain (loss), after tax	104.4				104.4
NET UNREALIZED (LOSSES) GAINS ON CASH FLOW HEDGES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(8.4)	(3.2)	3.6	(3.7)	(3.2)	(3.7)
Net Change	3.0		(3.3)		(2.2)	4.0
Ending balance	(5.4)	(8.4)	0.3	3.6	(5.4)	0.3
NET FOREIGN CURRENCY ADJUSTMENT
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	149.7	144.7	143.5	117.8	144.7	117.8
Net Change	2.1		2.0		7.1	27.7
Ending balance	151.8	149.7	145.5	143.5	151.8	145.5
NET PENSION AND OTHER POSTRETIREMENT BENEFIT ADJUSTMENTS
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(347.5)	(355.3)	(415.8)	(422.8)	(355.3)	(422.8)
Net Change	22.8		8.6		30.6	15.6
Ending balance	$ (324.7)	$ (347.5)	$ (407.2)	$ (415.8)	$ (324.7)	$ (407.2)