Loans (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Draw period for credit line products	10 years
Total Loans	$ 41,948,300,000	$ 43,390,600,000
Other U.S. loans and non-U.S. loans, short duration advances	4,500,000,000	3,800,000,000
Demand deposits	12,000,000.0	47,600,000
Held for Sale	6,800,000	6,800,000
Financing receivable, nonaccrual, interest income	4,100,000	2,100,000	$ 3,400,000
Loans in process of foreclosure	7,900,000	3,500,000
Loans modified	$ 51,300,000	7,200,000	40,700,000
Payment deferrals excluding material modifications, considered insignificant, less than	90 days
Loans modified, charged off	$ 2,000,000.0	8,500,000	2,000,000.0
30-89 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans modified in previous 12 months	0	0	4,700,000
90 days or greater past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans modified in previous 12 months	18,400,000	1,300,000	$ 16,200,000
Personal
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans	21,517,300,000	23,111,800,000
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans	$ 20,431,000,000	20,278,800,000
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	$ 6,077,300,000	6,109,900,000
Home Equity Loan | Personal
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans	$ 294,000,000.0	$ 250,300,000
Loans held by Northern trust	96.00%	97.00%
Commercial Real Estate | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans	$ 5,272,200,000	$ 5,314,200,000
Loans sold	$ 3,100,000	$ 0