Loans and Leases (Narrative) (Details)	12 Months Ended
	Dec. 31, 2021 USD ($) loan contract	Dec. 31, 2020 USD ($) loan	Dec. 31, 2019 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Draw period for credit line products	10 years
Total loans and leases	$ 40,480,600,000	$ 33,759,700,000
Other U.S. loans and non-U.S. loans, short duration advances	1,600,000,000	1,100,000,000
Demand deposits	8,000,000	26,400,000
Held for Sale	12,300,000	0
Financing receivable, nonaccrual, interest income	4,600,000	4,600,000	$ 7,300,000
Financing receivable, nonaccrual troubled debt restructuring	76,700,000	38,900,000
Financing receivable, troubled debt restructuring	16,500,000	29,300,000
Financing receivable, troubled debt restructuring, commitment to lend	$ 200,000	$ 10,400,000
Financing receivable, troubled debt restructuring, subsequent default, number of contracts | loan	2	0
Recorded investment	$ 56,900,000	$ 40,500,000
Unpaid principal balance	63,500,000	41,200,000
Other real estate, foreclosed assets, and repossessed assets	3,000,000.0	700,000
Consumer loans collateralized by residential properties in foreclosure	$ 2,600,000	$ 7,900,000
Percentage of loans granted payment deferral in past due or nonaccrual status (less than)	10.00%
Paycheck Protection Program
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans approved for forgiveness | loan	1,301	36
Loans Approved For Forgiveness	$ 231,500,000	$ 6,700,000
Loan processing fee	100,000	2,600,000
Personal
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases	21,993,200,000	18,497,700,000
Recorded investment	1,300,000	16,200,000
Unpaid principal balance	1,500,000	16,700,000
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases	18,487,400,000	15,262,000,000
Recorded investment	55,600,000	24,300,000
Unpaid principal balance	$ 62,000,000.0	24,500,000
CARES Act
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, non-TDR modifications, number of contracts paid off | contract	148
Financing receivable, non-TDR modifications, recorded investment paid off	$ 209,400,000
Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investment	5,600,000
Unpaid principal balance	$ 5,600,000
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	$ 6,319,900,000	6,035,700,000
Recorded investment	1,300,000	16,200,000
Unpaid principal balance	1,500,000	16,700,000
Home Equity Loan | Personal
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases	$ 258,200,000	$ 304,400,000
Loans held by Northern trust	97.00%	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	11,489,200,000	10,058,300,000
Recorded investment	19,600,000	24,300,000
Unpaid principal balance	19,600,000	$ 24,500,000
Commercial and Institutional | Commercial | Paycheck Protection Program
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases	$ 56,900,000
Financing receivable, number of contracts | loan	213,000,000
Financing receivable, average balance per loan	$ 300,000