Loans and Leases (Narrative) (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2020 USD ($) contract	Dec. 31, 2020 USD ($) loan	Dec. 31, 2019 USD ($) loan	Dec. 31, 2018 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Draw period for credit line products		10 years
Total loans and leases		$ 33,759,700,000	$ 31,409,600,000
Other U.S. loans and non-U.S. loans, short duration advances		1,100,000,000	1,100,000,000
Demand deposits		26,400,000	90,400,000
Financing Receivable, Nonaccrual, Interest Income		4,600,000	7,300,000	$ 8,000,000.0
Financing receivable, nonaccrual troubled debt restructuring		38,900,000	54,900,000
Financing receivable, troubled debt restructuring		29,300,000	27,700,000
Financing receivable, troubled debt restructuring, commitment to lend		$ 10,400,000	$ 8,200,000
Financing receivable, troubled debt restructuring, subsequent default, number of contracts | loan		0	5
Impaired financing receivable, recorded investment			$ 5,800,000
Impaired financing receivable, unpaid principal balance			6,100,000
Other real estate, foreclosed assets, and repossessed assets		$ 700,000	3,200,000
Consumer loans collateralized by residential properties in foreclosure		$ 7,900,000	18,100,000
Paycheck Protection Program
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans approved for forgiveness | loan		41
Loan processing fee		$ 2,600,000
Loans Approved For Forgiveness		$ 6,700,000
Number Of Loans Fully Forgiven | loan		36
Personal
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases		$ 18,497,700,000	17,408,300,000
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases		$ 15,262,000,000.0	14,001,300,000
CARES Act
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, non-TDR modifications, number of contracts paid off | contract	57
Financing receivable, non-TDR modifications, recorded investment paid off	$ 63,000,000.0
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		$ 6,035,700,000	6,095,000,000.0
Home Equity Loan | Personal
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases		$ 304,400,000	$ 448,500,000
Loans held by Northern trust		97.00%	97.00%
Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases		$ 3,053,500,000	$ 2,672,200,000
Loans held for sale		0	0
Leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for sale		0	53,600,000
Commercial and Institutional | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases		10,058,300,000	$ 9,091,100,000
Commercial and Institutional | Commercial | Paycheck Protection Program
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases		$ 207,100,000
Financing receivable, number of contracts | loan		1,087,000,000
Financing receivable, average balance per loan		$ 200,000