Loans (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Draw period for credit line products	10 years
Total Loans	$ 43,390.6	$ 47,617.0
Other U.S. loans and non-U.S. loans, short duration advances	3,800.0	8,400.0
Demand deposits	47.6	12.1
Loans held for sale	0.0	0.0
Financing receivable, nonaccrual, interest income	2.1	3.4	$ 4.1
Loans in process of foreclosure	3.5
Loans modified	7.2	40.7
Loans modified, charged off	8.5	2.0
90 days or greater past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans modified in previous 12 months	1.3	16.2
Personal
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans	23,111.8	22,204.2
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans	$ 20,278.8	25,412.8
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	$ 6,109.9	6,327.1
Home Equity Loan | Personal
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans	$ 250.3	$ 228.7
Loans held by Northern trust	97.00%	96.00%
Commercial Real Estate | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans	$ 5,314.2	$ 5,134.2
Loans sold	0.0	2.5
Commercial and Institutional | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans	10,537.1	11,555.3
Commercial and Institutional | Commercial | 30-89 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans modified in previous 12 months	$ 0.0	$ 4.7