Accumulated Other Comprehensive Income (Loss) (Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 11,823.0	$ 11,457.9	$ 11,688.3	$ 11,382.2	$ 10,879.7	$ 11,091.0	$ 11,688.3	$ 11,091.0
Net Change	(80.7)	128.8	(362.1)	54.1	325.4	239.7	(314.0)	619.2
Ending balance	11,898.8	11,823.0	11,457.9	11,579.1	11,382.2	10,879.7	11,898.8	11,579.1
TOTAL
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	194.7	65.9	428.0	370.4	45.0	(194.7)	428.0	(194.7)
Net Change	(80.7)	128.8	(362.1)	54.1	325.4	239.7
Ending balance	114.0	194.7	65.9	424.5	370.4	45.0	114.0	424.5
NET UNREALIZED GAINS (LOSSES) ON DEBT SECURITIES AVAILABLE FOR SALE
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	373.0		641.8	631.8		114.0	641.8	114.0
Net Change	(89.6)			33.5			(358.4)	551.3
Ending balance	283.4	373.0		665.3	631.8		283.4	665.3
OCI, debt securities, available-for-sale, transfer to held-to-maturity, adjustment from AOCI for amortization of gain (loss), after tax	91.7
NET UNREALIZED (LOSSES) GAINS ON CASH FLOW HEDGES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(5.4)		(3.2)	0.3		(3.7)	(3.2)	(3.7)
Net Change	4.6			1.3			2.4	5.3
Ending balance	(0.8)	(5.4)		1.6	0.3		(0.8)	1.6
NET FOREIGN CURRENCY ADJUSTMENT
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	151.8		144.7	145.5		117.8	144.7	117.8
Net Change	3.8			1.8			10.9	29.5
Ending balance	155.6	151.8		147.3	145.5		155.6	147.3
NET PENSION AND OTHER POSTRETIREMENT BENEFIT ADJUSTMENTS
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(324.7)		$ (355.3)	(407.2)		$ (422.8)	(355.3)	(422.8)
Net Change	0.5			17.5			31.1	33.1
Ending balance	$ (324.2)	$ (324.7)		$ (389.7)	$ (407.2)		$ (324.2)	$ (389.7)