Loans - Summary of Loans and Lending Commitments Accounted for at Amortized Cost by Portfolio (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 115,557	$ 110,345
Lending commitments	146,043	150,100
Corporate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	51,465	46,307
Wealth management [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	29,836	27,940
Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	18,377	17,743
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	4,705	6,958
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	4,154	4,792
Installment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	4,112	4,747
Credit card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,908	1,858
PCI [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		1,622
Wholesale Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	99,170	90,636
Lending commitments	146,043	150,100
Wholesale Portfolio Segment [Member] | Corporate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	47,583	41,129
Lending commitments	116,362	127,226
Wholesale Portfolio Segment [Member] | Wealth management [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	21,799	20,116
Lending commitments	2,278	2,198
Wholesale Portfolio Segment [Member] | Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	15,421	12,803
Lending commitments	3,089	3,207
Wholesale Portfolio Segment [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	4,236	4,965
Lending commitments	1,373	759
Wholesale Portfolio Segment [Member] | Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	3,111	3,396
Lending commitments	4,057	3,029
Consumer Portfolio Segment [Member] | Installment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	4,112	4,747
Lending commitments	4	12
Consumer Portfolio Segment [Member] | Credit card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,908	1,858
Lending commitments	$ 18,880	$ 13,669