Regulation and Capital Adequacy - Risk-weighted Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Standardized [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	$ 454,239	$ 498,987
Market RWAs	80,318	64,588
Total Operational RWAs	0
Total RWAs	534,557	563,575	$ 547,910
Standardized [Member] | Derivatives [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	119,176	120,906
Standardized [Member] | Commitments Guarantees and Loans [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	163,098	179,740
Standardized [Member] | Securities Financing Transactions [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	56,441	65,867
Standardized [Member] | Equity Investments [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	45,142	56,814
Standardized [Member] | Other [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	70,382	75,660
Standardized [Member] | Regulatory VaR [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	18,322	8,933
Standardized [Member] | Stressed VaR [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	38,885	30,911
Standardized [Member] | Incremental Risk [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	5,385	4,308
Standardized [Member] | Comprehensive Risk [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	1,648	1,393
Standardized [Member] | Specific Risk [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	16,078	19,043
Advanced [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	390,758	361,079
Market RWAs	80,318	64,386
Total Operational RWAs	128,550	119,188
Total RWAs	599,626	544,653	$ 558,111
Advanced [Member] | Derivatives [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	112,940	72,631
Advanced [Member] | Commitments Guarantees and Loans [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	136,659	134,456
Advanced [Member] | Securities Financing Transactions [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	9,883	13,834
Advanced [Member] | Equity Investments [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	47,305	61,892
Advanced [Member] | Other [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	83,971	78,266
Advanced [Member] | Regulatory VaR [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	18,322	8,933
Advanced [Member] | Stressed VaR [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	38,885	30,911
Advanced [Member] | Incremental Risk [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	5,385	4,308
Advanced [Member] | Comprehensive Risk [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	1,648	1,191
Advanced [Member] | Specific Risk [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	$ 16,078	$ 19,043