Loans (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($) loan	Dec. 31, 2021 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Draw period for credit line products	10 years
Total loans and leases	$ 47,617.0	$ 42,893.3
Other U.S. loans and non-U.S. loans, short duration advances	8,400.0	2,900.0
Demand deposits	12.1	24.4
Held for Sale	0.0	0.0
Loan processing fee	2.5	11.2
Financing receivable, nonaccrual, interest income	3.4	4.1	$ 4.6
Financing receivable, excluding accrued interest, modified, write-off	2.0
Financing receivable, nonaccrual troubled debt restructuring		35.3
Financing receivable, troubled debt restructuring		39.7
Financing receivable, troubled debt restructuring, commitment to lend		$ 0.2
Financing receivable, troubled debt restructuring, subsequent default, number of contracts | loan		0
Recorded investment		$ 32.8
Unpaid principal balance		33.7
Consumer loans collateralized by residential properties in foreclosure	3.5	1.1
Financial Asset, 30 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified, after 12 Months	4.7
90 DAYS OR MORE PAST DUE
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified, after 12 Months	16.2
Personal
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases	22,204.2	21,257.7
Recorded investment		0.6
Unpaid principal balance		0.6
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases	$ 25,412.8	21,635.6
Recorded investment		32.2
Unpaid principal balance		33.1
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	$ 6,327.1	6,413.5
Recorded investment		0.2
Unpaid principal balance		0.2
Home Equity Loan | Personal
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases	$ 228.7	$ 248.6
Loans held by Northern trust	96.00%	98.00%
Commercial and Institutional | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases	$ 11,555.3	$ 12,415.0
Recorded investment		0.6
Unpaid principal balance		$ 0.6