Fair Value Option - Level 3 Rollforward (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Other Financial Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Beginning Balance	$ 2	$ 7	$ 6	$ 4
Net Realized Gains / (Losses)	5		5	2
Net unrealized gains/(losses)	(2)	4	(6)	6
Settlements	(5)	(1)	(5)	(2)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Ending Balance		10		10
Other Financial Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value, Beginning Balance	(20,819)	(17,583)	(19,397)	(15,462)
Net Realized Gains / (Losses)	(135)	(47)	(307)	(161)
Net unrealized gains/(losses)	(185)	(165)	(1,839)	619
Sales				3
Issuances	(3,964)	(4,359)	(8,693)	(11,545)
Settlements	3,910	2,927	9,509	6,583
Transfers Into Level 3	(570)	(402)	(1,112)	(517)
Transfers Out Of Level 3	670	408	746	1,259
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value, Ending Balance	(21,093)	(19,221)	(21,093)	(19,221)
Other Financial Liabilities [Member] | Deposits at Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value, Beginning Balance	(3,622)	(3,271)	(3,168)	(2,968)
Net Realized Gains / (Losses)		(18)	(3)	(24)
Net unrealized gains/(losses)	(129)	6	(398)	94
Issuances	(267)	(185)	(661)	(630)
Settlements	150	242	272	293
Transfers Into Level 3	(136)		(59)	(16)
Transfers Out Of Level 3	58	32	71	57
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value, Ending Balance	(3,946)	(3,194)	(3,946)	(3,194)
Other Financial Liabilities [Member] | Repurchase Agreements [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value, Beginning Balance	(28)	(33)	(29)	(37)
Net unrealized gains/(losses)	1		(2)
Settlements	2		6	4
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value, Ending Balance	(25)	(33)	(25)	(33)
Other Financial Liabilities [Member] | Other Secured Financings at Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value, Beginning Balance	(202)	(270)	(170)	(389)
Net Realized Gains / (Losses)	4	3	19	6
Net unrealized gains/(losses)	(5)	(4)	(24)	(8)
Issuances	(1)	(3)	(20)	(8)
Settlements	3	8	14	95
Transfers Into Level 3	(133)	(80)	(153)	(86)
Transfers Out Of Level 3		25		69
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value, Ending Balance	(334)	(321)	(334)	(321)
Other Financial Liabilities [Member] | Unsecured Short-Term Borrowings Including Current Portion of Unsecured Long-Term Borrowings at Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value, Beginning Balance	(5,026)	(5,120)	(4,076)	(4,594)
Net Realized Gains / (Losses)	(50)	(25)	(99)	(133)
Net unrealized gains/(losses)	34	(7)	(238)	186
Issuances	(2,147)	(2,066)	(4,085)	(5,596)
Settlements	1,982	1,765	3,270	4,288
Transfers Into Level 3	(159)	(208)	(217)	(277)
Transfers Out Of Level 3	383	189	462	654
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value, Ending Balance	(4,983)	(5,472)	(4,983)	(5,472)
Other Financial Liabilities [Member] | Unsecured Long-Term Borrowings at Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value, Beginning Balance	(11,769)	(8,821)	(11,823)	(7,434)
Net Realized Gains / (Losses)	(96)	(13)	(244)	(27)
Net unrealized gains/(losses)	(60)	(166)	(1,110)	369
Sales				3
Issuances	(1,542)	(2,099)	(3,907)	(5,294)
Settlements	1,773	912	5,947	1,903
Transfers Into Level 3	(142)	(114)	(683)	(138)
Transfers Out Of Level 3	229	162	213	479
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value, Ending Balance	(11,607)	(10,139)	(11,607)	(10,139)
Other Financial Liabilities [Member] | Other Liabilities at Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value, Beginning Balance	(172)	(68)	(131)	(40)
Net Realized Gains / (Losses)	7	6	20	17
Net unrealized gains/(losses)	(26)	6	(67)	(22)
Issuances	(7)	(6)	(20)	(17)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value, Ending Balance	$ (198)	$ (62)	$ (198)	$ (62)