Regulation and Capital Adequacy - Risk-weighted Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Standardized [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	$ 496,039	$ 473,338
Market RWAs	60,560	74,572
Total RWAs	556,599	547,910	$ 555,611
Standardized [Member] | Derivatives [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	123,787	122,511
Standardized [Member] | Commitments Guarantees and Loans [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	172,059	160,305
Standardized [Member] | Securities Financing Transactions [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	70,628	66,363
Standardized [Member] | Equity Investments [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	54,802	53,563
Standardized [Member] | Other [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	74,763	70,596
Standardized [Member] | Regulatory VaR [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	8,600	7,782
Standardized [Member] | Stressed VaR [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	27,828	27,952
Standardized [Member] | Incremental Risk [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	4,630	10,469
Standardized [Member] | Comprehensive Risk [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	1,418	2,770
Standardized [Member] | Specific Risk [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	18,084	25,599
Advanced [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	403,495	368,751
Market RWAs	60,106	74,572
Total Operational RWAs	102,000	114,788
Total RWAs	565,601	558,111	$ 617,646
Advanced [Member] | Derivatives [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	86,759	82,301
Advanced [Member] | Commitments Guarantees and Loans [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	160,057	143,356
Advanced [Member] | Securities Financing Transactions [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	18,635	18,259
Advanced [Member] | Equity Investments [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	57,106	55,154
Advanced [Member] | Other [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit RWAs	80,938	69,681
Advanced [Member] | Regulatory VaR [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	8,600	7,782
Advanced [Member] | Stressed VaR [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	27,828	27,952
Advanced [Member] | Incremental Risk [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	4,630	10,469
Advanced [Member] | Comprehensive Risk [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	964	2,770
Advanced [Member] | Specific Risk [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Market RWAs	$ 18,084	$ 25,599