Securitization Activities - Firms Continuing Involvement in Securitization Entities to Which Firm Sold Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Qualitative and Quantitative Information, Transferor's Continuing Involvement [Line Items]
Outstanding principal amount	$ 85,982	$ 67,694
Retained interests	3,848	3,372
Purchased interests	69	33
U.S. government agency-issued CMOs [Member]
Qualitative and Quantitative Information, Transferor's Continuing Involvement [Line Items]
Outstanding principal amount	21,836	14,328
Retained interests	1,606	1,530
Purchased interests	4	3
Other Residential Mortgage-backed [Member]
Qualitative and Quantitative Information, Transferor's Continuing Involvement [Line Items]
Outstanding principal amount	24,610	24,166
Retained interests	1,316	1,078
Purchased interests	25	24
Other Commercial Mortgage-backed [Member]
Qualitative and Quantitative Information, Transferor's Continuing Involvement [Line Items]
Outstanding principal amount	35,442	25,588
Retained interests	767	615
Purchased interests	30	6
Corporate debt and other asset-backed [Member]
Qualitative and Quantitative Information, Transferor's Continuing Involvement [Line Items]
Outstanding principal amount	4,094	3,612
Retained interests	159	$ 149
Purchased interests	$ 10