Loans (Narrative) (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 47,342,600,000		$ 47,617,000,000
Other U.S. loans and non-U.S. loans, short duration advances	9,000,000,000		8,400,000,000
Demand deposits	10,700,000		12,100,000
Financing receivable, nonaccrual, interest income	700,000	$ 700,000
Loans in process of foreclosure	3,700,000		3,500,000
Modified loans, charged off	8,200,000
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	500,000
90 DAYS OR MORE PAST DUE
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified, after 12 Months	7,800,000
Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, held-for-sale, not part of disposal group, after valuation allowance	0		0
Personal
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 21,925,500,000		22,204,200,000
Personal | Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collateral required on Residential real estate loan, lower limit	65.00%
Collateral required on Residential real estate loan, upper limit	80.00%
Loans and leases	$ 6,315,200,000		6,327,100,000
Personal | Home Equity Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Draw period for credit line products	10 years
Loans and leases	$ 229,900,000		$ 228,700,000
Percentage of total equity credit lines	98.00%		96.00%
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 25,417,100,000		$ 25,412,800,000