Reporting Segments (Results of Reporting Segments) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2022 USD ($) segment	Mar. 31, 2021 USD ($)
Segment Reporting Information [Line Items]
Number of reporting segments | segment	2
Noninterest Income
Trust, Investment and Other Servicing Fees	$ 1,168.4	$ 1,063.7
Foreign Exchange Trading Income	80.9	78.7
Other Noninterest Income	88.4	100.9
Total Noninterest Income	1,337.7	1,243.3
Net Interest Income	381.0	340.1
Revenue	1,718.7	1,583.4
Provision for Credit Losses	2.0	(30.0)
Noninterest Expense	1,205.9	1,117.5
Income before Income Taxes	510.8	495.9
Provision for Income Taxes	121.5	120.8
Net Income	$ 389.3	$ 375.1
Percentage of Consolidated Net Income	100.00%	100.00%
Average Assets	$ 162,143.0	$ 153,255.8
RECONCILING ITEMS
Noninterest Income
Trust, Investment and Other Servicing Fees	0.0	0.0
Foreign Exchange Trading Income	0.0	0.0
Other Noninterest Income	0.0	0.0
Total Noninterest Income	0.0	0.0
Net Interest Income	(6.7)	(6.6)
Revenue	(6.7)	(6.6)
Provision for Credit Losses	0.0	0.0
Noninterest Expense	0.0	0.0
Income before Income Taxes	(6.7)	(6.6)
Provision for Income Taxes	(6.7)	(6.6)
Net Income	0.0	0.0
OTHER
Noninterest Income
Trust, Investment and Other Servicing Fees	0.0	0.0
Foreign Exchange Trading Income	0.0	0.0
Other Noninterest Income	(4.5)	(3.6)
Total Noninterest Income	(4.5)	(3.6)
Net Interest Income	0.0	0.0
Revenue	(4.5)	(3.6)
Provision for Credit Losses	0.0	0.0
Noninterest Expense	2.9	1.3
Income before Income Taxes	(7.4)	(4.9)
Provision for Income Taxes	(1.9)	(1.2)
Net Income	$ (5.5)	$ (3.7)
Percentage of Consolidated Net Income	(1.00%)	(1.00%)
Average Assets	$ 0.0	$ 0.0
ASSET SERVICING | Operating Segments
Noninterest Income
Trust, Investment and Other Servicing Fees	662.4	620.5
Foreign Exchange Trading Income	77.4	74.5
Other Noninterest Income	61.1	60.3
Total Noninterest Income	800.9	755.3
Net Interest Income	190.1	160.7
Revenue	991.0	916.0
Provision for Credit Losses	8.4	(5.4)
Noninterest Expense	757.9	713.1
Income before Income Taxes	224.7	208.3
Provision for Income Taxes	51.8	50.1
Net Income	$ 172.9	$ 158.2
Percentage of Consolidated Net Income	44.00%	42.00%
Average Assets	$ 125,234.9	$ 120,157.7
WEALTH MANAGEMENT | Operating Segments
Noninterest Income
Trust, Investment and Other Servicing Fees	506.0	443.2
Foreign Exchange Trading Income	3.5	4.2
Other Noninterest Income	31.8	44.2
Total Noninterest Income	541.3	491.6
Net Interest Income	197.6	186.0
Revenue	738.9	677.6
Provision for Credit Losses	(6.4)	(24.6)
Noninterest Expense	445.1	403.1
Income before Income Taxes	300.2	299.1
Provision for Income Taxes	78.3	78.5
Net Income	$ 221.9	$ 220.6
Percentage of Consolidated Net Income	57.00%	59.00%
Average Assets	$ 36,908.1	$ 33,098.1