Loans - Summary of Financing receivable, past due (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	$ 700	$ 627
Past due loans divided by gross loans at amortized cost	0.60%	0.70%
Corporate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	$ 238	$ 239
Wealth management [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	36	28
Commercial real estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	218	177
Residential real estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	34	37
Installment [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	88	100
Credit Card Receivable [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	74	39
Other Loans [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	12	7
30-89 days [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	335	395
30-89 days [Member] | Corporate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	66	197
30-89 days [Member] | Wealth management [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	20	13
30-89 days [Member] | Commercial real estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	127	54
30-89 days [Member] | Residential real estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	11	19
30-89 days [Member] | Installment [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	65	71
30-89 days [Member] | Credit Card Receivable [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	35	35
30-89 days [Member] | Other Loans [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	11	6
90 days or more [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	365	232
90 days or more [Member] | Corporate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	172	42
90 days or more [Member] | Wealth management [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	16	15
90 days or more [Member] | Commercial real estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	91	123
90 days or more [Member] | Residential real estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	23	18
90 days or more [Member] | Installment [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	23	29
90 days or more [Member] | Credit Card Receivable [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	39	4
90 days or more [Member] | Other Loans [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	$ 1	$ 1