Reporting Segments (Results of Reporting Segments) (Details) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2024 USD ($)	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Sep. 30, 2023 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Sep. 30, 2024 USD ($) segment	Sep. 30, 2023 USD ($)
Segment Reporting Information [Line Items]
Number of reporting segments | segment							2
Noninterest Income
Trust, Investment and Other Servicing Fees	$ 1,196.6			$ 1,111.9			$ 3,505.6	$ 3,271.8
Foreign Exchange Trading Income	54.1			51.8			169.5	154.9
Other Noninterest Income (Loss)	155.5			106.6			1,042.4	302.6
Total Noninterest Income	1,406.2			1,270.3			4,717.5	3,729.3
Net Interest Income	562.3			456.2			1,613.3	1,498.9
Revenue	1,968.5			1,726.5			6,330.8	5,228.2
Provision for Credit Losses	8.0			14.0			7.5	13.5
Noninterest Expense	1,359.4			1,278.2			4,258.0	3,895.7
Income before Income Taxes	601.1			434.3			2,065.3	1,319.0
Provision for Income Taxes	136.2			106.5			489.6	324.8
Net Income	$ 464.9	$ 896.1	$ 214.7	$ 327.8	$ 331.8	$ 334.6	$ 1,575.7	$ 994.2
Percentage of Consolidated Net Income	100.00%			100.00%			100.00%	100.00%
Average Assets	$ 146,842.9			$ 140,201.6			$ 146,654.8	$ 144,691.6
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)	52.5			2.4			737.9	(9.2)
Total Noninterest Income	52.5			2.4			737.9	(9.2)
Net Interest Income	0.0			0.0			0.0	0.0
Revenue	52.5			2.4			737.9	(9.2)
Provision for Credit Losses	(3.8)			0.0			(6.3)	0.0
Noninterest Expense	7.4			12.0			119.1	33.6
Income before Income Taxes	48.9			(9.6)			625.1	(42.8)
Provision for Income Taxes	11.1			(2.3)			141.8	(10.6)
Net Income	$ 37.8			$ (7.3)			$ 483.3	$ (32.2)
Percentage of Consolidated Net Income	8.00%			(2.00%)			31.00%	(3.00%)
Average Assets	$ 0.0			$ 0.0			$ 0.0	$ 0.0
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	(7.1)			(13.2)			(21.3)	(39.5)
Revenue	(7.1)			(13.2)			(21.3)	(39.5)
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	(7.1)			(13.2)			(21.3)	(39.5)
Provision for Income Taxes	(7.1)			(13.2)			(21.3)	(39.5)
Net Income	0.0			0.0			0.0	0.0
ASSET SERVICING | Operating Segments
Noninterest Income
Trust, Investment and Other Servicing Fees	667.1			626.0			1,957.3	1,850.2
Foreign Exchange Trading Income	59.5			54.3			181.6	161.2
Other Noninterest Income (Loss)	67.3			66.1			199.2	199.0
Total Noninterest Income	793.9			746.4			2,338.1	2,210.4
Net Interest Income	321.4			281.2			900.5	902.6
Revenue	1,115.3			1,027.6			3,238.6	3,113.0
Provision for Credit Losses	(0.6)			3.1			(1.9)	(3.3)
Noninterest Expense	860.7			809.3			2,634.0	2,459.7
Income before Income Taxes	255.2			215.2			606.5	656.6
Provision for Income Taxes	57.1			54.3			146.1	167.4
Net Income	$ 198.1			$ 160.9			$ 460.4	$ 489.2
Percentage of Consolidated Net Income	43.00%			49.00%			29.00%	49.00%
Average Assets	$ 108,108.0			$ 108,656.7			$ 108,025.6	$ 110,380.4
WEALTH MANAGEMENT | Operating Segments
Noninterest Income
Trust, Investment and Other Servicing Fees	529.5			485.9			1,548.3	1,421.6
Foreign Exchange Trading Income	(5.4)			(2.5)			(12.1)	(6.3)
Other Noninterest Income (Loss)	35.7			38.1			105.3	112.8
Total Noninterest Income	559.8			521.5			1,641.5	1,528.1
Net Interest Income	248.0			188.2			734.1	635.8
Revenue	807.8			709.7			2,375.6	2,163.9
Provision for Credit Losses	12.4			10.9			15.7	16.8
Noninterest Expense	491.3			456.9			1,504.9	1,402.4
Income before Income Taxes	304.1			241.9			855.0	744.7
Provision for Income Taxes	75.1			67.7			223.0	207.5
Net Income	$ 229.0			$ 174.2			$ 632.0	$ 537.2
Percentage of Consolidated Net Income	49.00%			53.00%			40.00%	54.00%
Average Assets	$ 38,734.9			$ 31,544.9			$ 38,629.2	$ 34,311.2