Loans and Leases (Narrative) (Details)	12 Months Ended
	Dec. 31, 2022 USD ($) loan	Dec. 31, 2021 USD ($) loan	Dec. 31, 2020 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Draw period for credit line products	10 years
Total loans and leases	$ 42,893,300,000	$ 40,480,600,000
Other U.S. loans and non-U.S. loans, short duration advances	2,900,000,000	1,600,000,000
Demand deposits	24,400,000	8,000,000
Held for Sale	0	12,300,000
Loan processing fee	11,200,000	118,800,000
Financing receivable, nonaccrual, interest income	4,100,000	4,600,000	$ 4,600,000
Financing receivable, nonaccrual troubled debt restructuring	35,300,000	76,700,000
Financing receivable, troubled debt restructuring	39,700,000	16,500,000
Financing receivable, troubled debt restructuring, commitment to lend	$ 200,000	$ 200,000
Financing receivable, troubled debt restructuring, subsequent default, number of contracts | loan	0	2
Recorded investment	$ 32,800,000	$ 56,900,000
Unpaid principal balance	33,700,000	63,500,000
Consumer loans collateralized by residential properties in foreclosure	1,100,000	2,600,000
Personal
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases	21,257,700,000	21,993,200,000
Recorded investment	600,000	1,300,000
Unpaid principal balance	600,000	1,500,000
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases	21,635,600,000	18,487,400,000
Recorded investment	32,200,000	55,600,000
Unpaid principal balance	$ 33,100,000	62,000,000.0
Residential Real Estate | Personal
Accounts, Notes, Loans and Financing Receivable [Line Items]
Lower limit of generally required loan to collateral value for residential real estate loans	65.00%
Upper limit of generally required loan to collateral value for residential real estate loans	80.00%
Total loans and leases	$ 6,413,500,000	6,319,900,000
Recorded investment	200,000	1,300,000
Unpaid principal balance	200,000	1,500,000
Home Equity Loan | Personal
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases	$ 248,600,000	$ 258,200,000
Loans held by Northern trust	98.00%	97.00%
Leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Held for Sale	$ 0	$ 0
Commercial and Institutional | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases	12,415,000,000	11,489,200,000
Recorded investment	600,000	19,600,000
Unpaid principal balance	$ 600,000	$ 19,600,000