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, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 11,382.2	$ 10,879.7	$ 11,091.0	$ 10,805.5	$ 10,616.2	$ 10,508.3	$ 11,091.0	$ 10,508.3
Net Change	54.1	325.4	239.7	68.8	185.5	108.4	619.2	362.7
Ending balance	11,579.1	11,382.2	10,879.7	10,811.3	10,805.5	10,616.2	11,579.1	10,811.3
TOTAL
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	370.4	45.0	(194.7)	(159.8)	(345.3)	(453.7)	(194.7)	(453.7)
Net Change	54.1	325.4	239.7	68.8	185.5	108.4
Ending balance	424.5	370.4	45.0	(91.0)	(159.8)	(345.3)	424.5	(91.0)
NET UNREALIZED GAINS (LOSSES) ON DEBT SECURITIES AVAILABLE FOR SALE
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	631.8		114.0	133.1		(114.9)	114.0	(114.9)
Net Change	33.5			57.0			551.3	305.0
Ending balance	665.3	631.8		190.1	133.1		665.3	190.1
NET UNREALIZED (LOSSES) GAINS ON CASH FLOW HEDGES
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	0.3		(3.7)	4.5		4.0	(3.7)	4.0
Net Change	1.3			(2.4)			5.3	(1.9)
Ending balance	1.6	0.3		2.1	4.5		1.6	2.1
NET FOREIGN CURRENCY ADJUSTMENT
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	145.5		117.8	96.8		67.9	117.8	67.9
Net Change	1.8			9.9			29.5	38.8
Ending balance	147.3	145.5		106.7	96.8		147.3	106.7
NET PENSION AND OTHER POSTRETIREMENT BENEFIT ADJUSTMENTS
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(407.2)		$ (422.8)	(394.2)		$ (410.7)	(422.8)	(410.7)
Net Change	17.5			4.3			33.1	20.8
Ending balance	$ (389.7)	$ (407.2)		$ (389.9)	$ (394.2)		$ (389.7)	$ (389.9)