Reporting Segments (Results of Reporting Segments) (Details) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2023 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Sep. 30, 2022 USD ($)	Jun. 30, 2022 USD ($)	Mar. 31, 2022 USD ($)	Sep. 30, 2023 USD ($) segment	Sep. 30, 2022 USD ($)
Segment Reporting Information [Line Items]
Number of reporting segments | segment							2
Noninterest Income
Trust, Investment and Other Servicing Fees	$ 1,111.9			$ 1,078.7			$ 3,271.8	$ 3,390.5
Foreign Exchange Trading Income	51.8			64.7			154.9	223.2
Other Noninterest Income (Loss)	106.6			98.4			302.6	275.8
Total Noninterest Income	1,270.3			1,241.8			3,729.3	3,889.5
Net Interest Income	456.2			513.0			1,498.9	1,352.7
Revenue	1,726.5			1,754.8			5,228.2	5,242.2
Provision for Credit Losses	14.0			0.5			13.5	7.0
Noninterest Expense	1,278.2			1,229.8			3,895.7	3,659.3
Income before Income Taxes	434.3			524.5			1,319.0	1,575.9
Provision for Income Taxes	106.5			129.7			324.8	395.6
Net Income	$ 327.8	$ 331.8	$ 334.6	$ 394.8	$ 396.2	$ 389.3	$ 994.2	$ 1,180.3
Percentage of Consolidated Net Income	100.00%			100.00%			100.00%	100.00%
Average Assets	$ 140,201.6			$ 146,402.2			$ 144,691.6	$ 154,152.1
RECONCILING ITEMS
Noninterest Income
Trust, Investment and Other Servicing Fees	0.0			0.0			0.0	0.0
Foreign Exchange Trading Income	0.0			0.0			0.0	0.0
Other Noninterest Income (Loss)	0.0			0.0			0.0	0.0
Total Noninterest Income	0.0			0.0			0.0	0.0
Net Interest Income	(13.2)			(12.3)			(39.5)	(30.1)
Revenue	(13.2)			(12.3)			(39.5)	(30.1)
Provision for Credit Losses	0.0			0.0			0.0	0.0
Noninterest Expense	0.0			0.0			0.0	0.0
Income before Income Taxes	(13.2)			(12.3)			(39.5)	(30.1)
Provision for Income Taxes	(13.2)			(12.3)			(39.5)	(30.1)
Net Income	0.0			0.0			0.0	0.0
OTHER
Noninterest Income
Trust, Investment and Other Servicing Fees	0.0			0.0			0.0	0.0
Foreign Exchange Trading Income	0.0			0.0			0.0	0.0
Other Noninterest Income (Loss)	2.4			(1.1)			(9.2)	(11.2)
Total Noninterest Income	2.4			(1.1)			(9.2)	(11.2)
Net Interest Income	0.0			0.0			0.0	0.0
Revenue	2.4			(1.1)			(9.2)	(11.2)
Provision for Credit Losses	0.0			0.0			0.0	0.0
Noninterest Expense	12.0			20.4			33.6	56.7
Income before Income Taxes	(9.6)			(21.5)			(42.8)	(67.9)
Provision for Income Taxes	(2.3)			(5.3)			(10.6)	(16.9)
Net Income	$ (7.3)			$ (16.2)			$ (32.2)	$ (51.0)
Percentage of Consolidated Net Income	(2.00%)			(4.00%)			(3.00%)	(4.00%)
Average Assets	$ 0.0			$ 0.0			$ 0.0	$ 0.0
ASSET SERVICING | Operating Segments
Noninterest Income
Trust, Investment and Other Servicing Fees	626.0			603.2			1,850.2	1,908.4
Foreign Exchange Trading Income	54.3			63.7			161.2	215.9
Other Noninterest Income (Loss)	66.1			62.5			199.0	185.4
Total Noninterest Income	746.4			729.4			2,210.4	2,309.7
Net Interest Income	281.2			303.3			902.6	748.5
Revenue	1,027.6			1,032.7			3,113.0	3,058.2
Provision for Credit Losses	3.1			(5.1)			(3.3)	3.8
Noninterest Expense	809.3			760.4			2,459.7	2,269.4
Income before Income Taxes	215.2			277.4			656.6	785.0
Provision for Income Taxes	54.3			67.7			167.4	194.0
Net Income	$ 160.9			$ 209.7			$ 489.2	$ 591.0
Percentage of Consolidated Net Income	49.00%			53.00%			49.00%	50.00%
Average Assets	$ 108,656.7			$ 109,677.2			$ 110,380.4	$ 117,251.0
WEALTH MANAGEMENT | Operating Segments
Noninterest Income
Trust, Investment and Other Servicing Fees	485.9			475.5			1,421.6	1,482.1
Foreign Exchange Trading Income	(2.5)			1.0			(6.3)	7.3
Other Noninterest Income (Loss)	38.1			37.0			112.8	101.6
Total Noninterest Income	521.5			513.5			1,528.1	1,591.0
Net Interest Income	188.2			222.0			635.8	634.3
Revenue	709.7			735.5			2,163.9	2,225.3
Provision for Credit Losses	10.9			5.6			16.8	3.2
Noninterest Expense	456.9			449.0			1,402.4	1,333.2
Income before Income Taxes	241.9			280.9			744.7	888.9
Provision for Income Taxes	67.7			79.6			207.5	248.6
Net Income	$ 174.2			$ 201.3			$ 537.2	$ 640.3
Percentage of Consolidated Net Income	53.00%			51.00%			54.00%	54.00%
Average Assets	$ 31,544.9			$ 36,725.0			$ 34,311.2	$ 36,901.1