Loans - Summary of Financing receivable, past due (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	$ 483	$ 804
Past due loans divided by gross loans at amortized cost	0.40%	0.80%
Corporate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	$ 121	$ 294
Wealth management [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	50	92
Commercial real estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	134	232
Residential real estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	6	27
Installment [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	28	58
Credit Card Receivable [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	118	77
Other [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	26	24
30-89 days [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	141	219
30-89 days [Member] \| Wealth management [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	4	58
30-89 days [Member] \| Commercial real estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	33	49
30-89 days [Member] \| Residential real estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	1	4
30-89 days [Member] \| Installment [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	20	42
30-89 days [Member] \| Credit Card Receivable [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	66	46
30-89 days [Member] \| Other [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	17	20
90 days or more [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	342	585
90 days or more [Member] \| Corporate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	121	294
90 days or more [Member] \| Wealth management [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	46	34
90 days or more [Member] \| Commercial real estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	101	183
90 days or more [Member] \| Residential real estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	5	23
90 days or more [Member] \| Installment [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	8	16
90 days or more [Member] \| Credit Card Receivable [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	52	31
90 days or more [Member] \| Other [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	$ 9	$ 4