Loans - Summary of Loans and Lending Commitments Accounted for at Amortized Cost by Portfolio (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 146,956	$ 119,989
Lending commitments	198,358	162,513
Corporate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	54,107	48,659
Wealth management [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	41,775	33,023
Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	21,707	20,290
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	13,359	5,750
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	6,308	4,174
Installment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	3,449	3,823
Credit card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	6,251	4,270
Wholesale Portfolio Segment [Member] \| Corporate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	48,943	44,778
Lending commitments	150,464	127,756
Wholesale Portfolio Segment [Member] \| Wealth management [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	35,159	25,151
Lending commitments	4,014	2,314
Wholesale Portfolio Segment [Member] \| Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	17,892	17,096
Lending commitments	4,183	4,154
Wholesale Portfolio Segment [Member] \| Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	12,356	5,236
Lending commitments	2,785	1,804
Wholesale Portfolio Segment [Member] \| Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	5,308	3,211
Lending commitments	5,183	4,841
Consumer Portfolio Segment [Member] \| Installment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	3,449	3,823
Lending commitments	11	4
Consumer Portfolio Segment [Member] \| Credit card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	6,251	4,270
Lending commitments	31,718	21,640
Wholesale and Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	129,358	103,565
Lending commitments	$ 198,358	$ 162,513