Loans - Summary of Financing receivable, past due (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	$ 704	$ 627
Past due loans divided by gross loans at amortized cost	0.70%	0.70%
Corporate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	$ 295	$ 239
Wealth management [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	35	28
Commercial real estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	222	177
Residential real estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	20	37
Installment [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	58	100
Credit Card Receivable [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	51	39
Other Loans [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	23	7
30-89 days [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	113	395
30-89 days [Member] | Corporate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due		197
30-89 days [Member] | Wealth management [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	1	13
30-89 days [Member] | Commercial real estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	17	54
30-89 days [Member] | Residential real estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	4	19
30-89 days [Member] | Installment [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	44	71
30-89 days [Member] | Credit Card Receivable [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	27	35
30-89 days [Member] | Other Loans [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	20	6
90 days or more [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	591	232
90 days or more [Member] | Corporate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	295	42
90 days or more [Member] | Wealth management [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	34	15
90 days or more [Member] | Commercial real estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	205	123
90 days or more [Member] | Residential real estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	16	18
90 days or more [Member] | Installment [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	14	29
90 days or more [Member] | Credit Card Receivable [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	24	4
90 days or more [Member] | Other Loans [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	$ 3	$ 1