Fair Value Measurements - Quantitative Information about Nonrecurring Fair Value Measurement of Assets and Liabilities Classified as Level 3 (Detail) $ in Millions		Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure		$ 86,653	$ 88,988
Gross Balance, Liabilities		53,201	68,270
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure		2,288	2,359
Gross Balance, Liabilities		1,603	1,690
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure		94	380
Fair Value, Measurements, Nonrecurring [Member] | Residential mortgages [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	[1]	2	2
Fair Value, Measurements, Nonrecurring [Member] | Impaired commercial loans [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	[2]	36	289
Fair Value, Measurements, Nonrecurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure		38	291
Fair Value, Measurements, Nonrecurring [Member] | Level 3 [Member] | Residential mortgages [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	[1]	2	2
Fair Value, Measurements, Nonrecurring [Member] | Level 3 [Member] | Impaired commercial loans [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	[2]	36	289
Derivative financial assets [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	[3]	31,279	45,297
Derivative financial assets [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	[3]	156	301
Other assets [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	[4]	4	15
Other assets [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	[4]	$ 4	$ 15
Measurement Input, Loss Severity Rates [Member] | Fair Value, Measurements, Nonrecurring [Member] | Level 3 [Member] | Residential mortgages [Member] | Minimum [Member] | Third Party Appraisal [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Loans Held-for-sale, Measurement Input		0.30	0.30
Measurement Input, Loss Severity Rates [Member] | Fair Value, Measurements, Nonrecurring [Member] | Level 3 [Member] | Residential mortgages [Member] | Maximum [Member] | Third Party Appraisal [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Loans Held-for-sale, Measurement Input		1.00	1.00
Measurement Input, Loss Severity Rates [Member] | Fair Value, Measurements, Nonrecurring [Member] | Level 3 [Member] | Impaired commercial loans [Member] | Minimum [Member] | Third Party Appraisal [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Loans Held-for-sale, Measurement Input		0.13	0.09
Measurement Input, Loss Severity Rates [Member] | Fair Value, Measurements, Nonrecurring [Member] | Level 3 [Member] | Impaired commercial loans [Member] | Maximum [Member] | Third Party Appraisal [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Loans Held-for-sale, Measurement Input		1.00	0.61

[1] At September 30, 2018 and December 31, 2017, the fair value of the loans held for sale was below cost. Certain residential mortgage loans held for sale have been classified as Level 3 fair value measurements within the fair value hierarchy as the underlying real estate properties used to determine fair value are illiquid assets as a result of market conditions. Additionally, the fair value of these properties is affected by, among other things, the location, the payment history and the completeness of the loan documentation.

[2] Certain commercial loans have undergone troubled debt restructurings and are considered impaired. As a matter of practical expedient, we measure the credit impairment of a collateral-dependent loan based on the fair value of the collateral asset. The collateral often involves real estate properties that are illiquid due to market conditions. As a result, these loans are classified as a Level 3 fair value measurement within the fair value hierarchy.

[3] Includes trading derivative assets of $3,144 million and $3,725 million and trading derivative liabilities of $2,343 million and $2,633 million at September 30, 2018 and December 31, 2017, respectively, as well as derivatives held for hedging and commitments accounted for as derivatives. See Note 9, "Derivatives," for additional information. Excluding changes in fair value of a derivative instrument associated with the effective portion of a qualifying cash flow hedge, which is recognized initially in other comprehensive income (loss), derivative assets and liabilities are recorded at fair value through net income.

[4] Represents contingent consideration receivable associated with the sale of a portion of our Private Banking business.