Loans and asset quality - Credit Risk Indicators Narrative (Details) - USD ($)	9 Months Ended
Sep. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of exposure to investment grade securities	96.00%
Percentage of financial institution exposure secured	68.00%
Percent of financial institution exposure expiring within one year	84.00%
Loans	$ 64,328,000,000	$ 56,469,000,000
Reverse repurchase agreements, allowance for credit loss	$ 0
Wealth management loans and mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan to value ratio at origination (percent)	62.00%
Percentage of past due mortgages (less than)	1.00%
Wealth management loans and mortgages \| California \| Geographic Concentration Risk \| Revenue Benchmark
Accounts, Notes, Loans and Financing Receivable [Line Items]
Geographic concentrations (percent)	22.00%
Wealth management loans and mortgages \| New York \| Geographic Concentration Risk \| Revenue Benchmark
Accounts, Notes, Loans and Financing Receivable [Line Items]
Geographic concentrations (percent)	16.00%
Wealth management loans and mortgages \| Florida \| Geographic Concentration Risk \| Revenue Benchmark
Accounts, Notes, Loans and Financing Receivable [Line Items]
Geographic concentrations (percent)	9.00%
Wealth management loans and mortgages \| Massachusetts \| Geographic Concentration Risk \| Revenue Benchmark
Accounts, Notes, Loans and Financing Receivable [Line Items]
Geographic concentrations (percent)	9.00%
Wealth management loans and mortgages \| Other \| Geographic Concentration Risk \| Revenue Benchmark
Accounts, Notes, Loans and Financing Receivable [Line Items]
Geographic concentrations (percent)	44.00%
Other residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 317,000,000	389,000,000
Purchased mortgages	54,000,000
Overdrafts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 4,600,000,000	2,700,000,000
Overdraft repayment period	2 days
Margin loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 20,800,000,000	$ 15,400,000,000

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Overdraft Repayment Period

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Percentage of Exposure to Investment Grade Securities

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Percentage of Financial Institution Exposure Secured

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Percent of Financial Institution Exposure Expiring Within One Year

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Purchased residential mortgages.

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Percentage of financing receivable balance that is past due.

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Amount, before allowance for credit loss, of financing receivable.

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Amount of allowance for credit loss on security purchased under agreement to resell.

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