Loans - Summary of Credit Concentration by Region (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Product Information [Line Items]
Loans and Leases Receivable, Gross	$ 115,557	$ 110,345
Concentration Risk,Percentage	100.00%	100.00%
Americas [Member]
Product Information [Line Items]
Concentration Risk,Percentage	74.00%	73.00%
Europe, Middle East and Africa [Member]
Product Information [Line Items]
Concentration Risk,Percentage	20.00%	21.00%
Asia [Member]
Product Information [Line Items]
Concentration Risk,Percentage	6.00%	6.00%
Corporate [Member]
Product Information [Line Items]
Loans and Leases Receivable, Gross	$ 51,465	$ 46,307
Concentration Risk,Percentage	100.00%	100.00%
Corporate [Member] | Americas [Member]
Product Information [Line Items]
Concentration Risk,Percentage	61.00%	60.00%
Corporate [Member] | Europe, Middle East and Africa [Member]
Product Information [Line Items]
Concentration Risk,Percentage	31.00%	31.00%
Corporate [Member] | Asia [Member]
Product Information [Line Items]
Concentration Risk,Percentage	8.00%	9.00%
Wealth management [Member]
Product Information [Line Items]
Loans and Leases Receivable, Gross	$ 29,836	$ 27,940
Concentration Risk,Percentage	100.00%	100.00%
Wealth management [Member] | Americas [Member]
Product Information [Line Items]
Concentration Risk,Percentage	90.00%	88.00%
Wealth management [Member] | Europe, Middle East and Africa [Member]
Product Information [Line Items]
Concentration Risk,Percentage	8.00%	9.00%
Wealth management [Member] | Asia [Member]
Product Information [Line Items]
Concentration Risk,Percentage	2.00%	3.00%
Commercial real estate [Member]
Product Information [Line Items]
Loans and Leases Receivable, Gross	$ 18,377	$ 17,743
Concentration Risk,Percentage	100.00%	100.00%
Commercial real estate [Member] | Americas [Member]
Product Information [Line Items]
Concentration Risk,Percentage	68.00%	69.00%
Commercial real estate [Member] | Europe, Middle East and Africa [Member]
Product Information [Line Items]
Concentration Risk,Percentage	20.00%	21.00%
Commercial real estate [Member] | Asia [Member]
Product Information [Line Items]
Concentration Risk,Percentage	12.00%	10.00%
Residential real estate [Member]
Product Information [Line Items]
Loans and Leases Receivable, Gross	$ 4,705	$ 6,958
Concentration Risk,Percentage	100.00%	100.00%
Residential real estate [Member] | Americas [Member]
Product Information [Line Items]
Concentration Risk,Percentage	87.00%	90.00%
Residential real estate [Member] | Europe, Middle East and Africa [Member]
Product Information [Line Items]
Concentration Risk,Percentage	11.00%	9.00%
Residential real estate [Member] | Asia [Member]
Product Information [Line Items]
Concentration Risk,Percentage	2.00%	1.00%
Installment [Member]
Product Information [Line Items]
Loans and Leases Receivable, Gross	$ 4,112	$ 4,747
Concentration Risk,Percentage	100.00%	100.00%
Installment [Member] | Americas [Member]
Product Information [Line Items]
Concentration Risk,Percentage	100.00%	100.00%
Credit card [Member]
Product Information [Line Items]
Loans and Leases Receivable, Gross	$ 2,908	$ 1,858
Concentration Risk,Percentage	100.00%	100.00%
Credit card [Member] | Americas [Member]
Product Information [Line Items]
Concentration Risk,Percentage	100.00%	100.00%
Other [Member]
Product Information [Line Items]
Loans and Leases Receivable, Gross	$ 4,154	$ 4,792
Concentration Risk,Percentage	100.00%	100.00%
Other [Member] | Americas [Member]
Product Information [Line Items]
Concentration Risk,Percentage	82.00%	87.00%
Other [Member] | Europe, Middle East and Africa [Member]
Product Information [Line Items]
Concentration Risk,Percentage	17.00%	12.00%
Other [Member] | Asia [Member]
Product Information [Line Items]
Concentration Risk,Percentage	1.00%	1.00%