Fair Value Measurements - Quantitative Information about Recurring Fair Value Measurement of Assets and Liabilities Classified as Level 3 (Detail) - Recurring [Member] $ in Millions		6 Months Ended	12 Months Ended
		Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure		$ 87,757	$ 88,988
Liabilities, Fair Value Disclosure		(55,931)	(68,270)
Level 3 [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure		2,306	2,359
Liabilities, Fair Value Disclosure		(1,626)	(1,690)
Borrowings [Member] | Level 3 [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Liabilities, Fair Value Disclosure	[1],[2]	(557)	(641)
Deposits [Member] | Level 3 [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Liabilities, Fair Value Disclosure	[1],[2]	(876)	(897)
Credit contracts [Member] | Level 3 [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure		110	114
Equity contracts [Member] | Level 3 [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure	[2]	2	81
Foreign exchange contracts [Member] | Level 3 [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure	[2]	(1)	0
Interest rate contracts [Member] | Level 3 [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure			0
Other Contracts [Member] | Level 3 [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure		(51)	(46)
Trading assets, excluding derivatives [Member] | Corporate and other domestic debt securities [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure		1,803	1,803
Trading assets, excluding derivatives [Member] | Corporate and other domestic debt securities [Member] | Level 3 [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure		1,803	1,803
Trading assets, excluding derivatives [Member] | Collateralized debt obligations [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure		131	129
Trading assets, excluding derivatives [Member] | Collateralized debt obligations [Member] | Level 3 [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure		131	129
Available-for-sale securities [Member] | Other asset-backed securities available-for-sale [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure	[3]	107	510
Available-for-sale securities [Member] | Other asset-backed securities available-for-sale [Member] | Level 3 [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure	[3]	107	111
Other assets [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure	[4]	12	15
Other assets [Member] | Level 3 [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure	[4]	$ 12	$ 15
Measurement Input, Prepayment Rates [Member] | Collateralized debt obligations [Member] | Minimum [Member] | Valuation Technique, Broker Quotes or Consensus Pricing [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Debt Securities, Trading, Measurement Input		0.00	0.00
Measurement Input, Prepayment Rates [Member] | Collateralized debt obligations [Member] | Maximum [Member] | Valuation Technique, Broker Quotes or Consensus Pricing [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Debt Securities, Trading, Measurement Input		0.00	0.06
Measurement Input, Default Rates [Member] | Collateralized debt obligations [Member] | Minimum [Member] | Valuation Technique, Broker Quotes or Consensus Pricing [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Debt Securities, Trading, Measurement Input		0.00	0.04
Measurement Input, Default Rates [Member] | Collateralized debt obligations [Member] | Maximum [Member] | Valuation Technique, Broker Quotes or Consensus Pricing [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Debt Securities, Trading, Measurement Input		0.01	0.06
Measurement Input, Default Rates [Member] | Credit contracts [Member] | Minimum [Member] | Valuation Technique, Option Pricing Model [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input			0.82
Measurement Input, Default Rates [Member] | Credit contracts [Member] | Maximum [Member] | Valuation Technique, Option Pricing Model [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input			0.83
Measurement Input, Loss Severity Rates [Member] | Collateralized debt obligations [Member] | Minimum [Member] | Valuation Technique, Broker Quotes or Consensus Pricing [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Debt Securities, Trading, Measurement Input		0.90	0.55
Measurement Input, Loss Severity Rates [Member] | Collateralized debt obligations [Member] | Maximum [Member] | Valuation Technique, Broker Quotes or Consensus Pricing [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Debt Securities, Trading, Measurement Input		0.95	0.60
Measurement Input, Spread Volatility on Collateral Assets [Member] | Corporate and other domestic debt securities [Member] | Minimum [Member] | Valuation Technique, Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Debt Securities, Trading, Measurement Input		0.02	0.02
Measurement Input, Spread Volatility on Collateral Assets [Member] | Corporate and other domestic debt securities [Member] | Maximum [Member] | Valuation Technique, Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Debt Securities, Trading, Measurement Input		0.04	0.04
Measurement Input, Correlation between Insurance Claim Shortfall and Collateral Value [Member] | Corporate and other domestic debt securities [Member] | Valuation Technique, Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Debt Securities, Trading, Measurement Input		0.80	0.80
Measurement Input, Probability to Fund for Rate Lock Commitments [Member] | Interest rate contracts [Member] | Minimum [Member] | Valuation Technique, Market Comparable Adjusted for Probability to Fund [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input			0.41
Measurement Input, Probability to Fund for Rate Lock Commitments [Member] | Interest rate contracts [Member] | Maximum [Member] | Valuation Technique, Market Comparable Adjusted for Probability to Fund [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input			1.00
Measurement Input, Implied Volatility of Currency Pairs [Member] | Borrowings [Member] | Minimum [Member] | Valuation Technique, Option Adjusted Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Long-term Debt, Measurement Input	[1],[2]	0.09	0.06
Measurement Input, Implied Volatility of Currency Pairs [Member] | Borrowings [Member] | Maximum [Member] | Valuation Technique, Option Adjusted Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Long-term Debt, Measurement Input	[1],[2]	0.11	0.09
Measurement Input, Implied Volatility of Currency Pairs [Member] | Deposits [Member] | Minimum [Member] | Valuation Technique, Option Adjusted Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Domestic Deposits, Measurement Input	[1],[2]	0.09	0.06
Measurement Input, Implied Volatility of Currency Pairs [Member] | Deposits [Member] | Maximum [Member] | Valuation Technique, Option Adjusted Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Domestic Deposits, Measurement Input	[1],[2]	0.11	0.09
Measurement Input, Implied Volatility of Currency Pairs [Member] | Foreign exchange contracts [Member] | Minimum [Member] | Valuation Technique, Option Pricing Model [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	[2]	0.09	0.06
Measurement Input, Implied Volatility of Currency Pairs [Member] | Foreign exchange contracts [Member] | Maximum [Member] | Valuation Technique, Option Pricing Model [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	[2]	0.11	0.09
Measurement Input, Equity / Equity Index Volatility [Member] | Borrowings [Member] | Minimum [Member] | Valuation Technique, Option Adjusted Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Long-term Debt, Measurement Input	[1],[2]	0.07	0.07
Measurement Input, Equity / Equity Index Volatility [Member] | Borrowings [Member] | Maximum [Member] | Valuation Technique, Option Adjusted Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Long-term Debt, Measurement Input	[1],[2]	0.30	0.42
Measurement Input, Equity / Equity Index Volatility [Member] | Deposits [Member] | Minimum [Member] | Valuation Technique, Option Adjusted Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Domestic Deposits, Measurement Input	[1],[2]	0.07	0.07
Measurement Input, Equity / Equity Index Volatility [Member] | Deposits [Member] | Maximum [Member] | Valuation Technique, Option Adjusted Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Domestic Deposits, Measurement Input	[1],[2]	0.30	0.42
Measurement Input, Equity / Equity Index Volatility [Member] | Equity contracts [Member] | Minimum [Member] | Valuation Technique, Option Pricing Model [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	[2]	0.07	0.07
Measurement Input, Equity / Equity Index Volatility [Member] | Equity contracts [Member] | Maximum [Member] | Valuation Technique, Option Pricing Model [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	[2]	0.35	0.42
Measurement Input, Equity/Equity and Equity/Index Correlation [Member] | Borrowings [Member] | Minimum [Member] | Valuation Technique, Option Adjusted Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Long-term Debt, Measurement Input	[1],[2]	0.44	0.42
Measurement Input, Equity/Equity and Equity/Index Correlation [Member] | Borrowings [Member] | Maximum [Member] | Valuation Technique, Option Adjusted Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Long-term Debt, Measurement Input	[1],[2]	0.79	0.80
Measurement Input, Equity/Equity and Equity/Index Correlation [Member] | Deposits [Member] | Minimum [Member] | Valuation Technique, Option Adjusted Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Domestic Deposits, Measurement Input		0.44	0.42
Measurement Input, Equity/Equity and Equity/Index Correlation [Member] | Deposits [Member] | Maximum [Member] | Valuation Technique, Option Adjusted Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Domestic Deposits, Measurement Input		0.51	0.80
Measurement Input, Equity/Equity and Equity/Index Correlation [Member] | Equity contracts [Member] | Minimum [Member] | Valuation Technique, Option Pricing Model [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	[2]	0.44	0.42
Measurement Input, Equity/Equity and Equity/Index Correlation [Member] | Equity contracts [Member] | Maximum [Member] | Valuation Technique, Option Pricing Model [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	[2]	0.79	0.80
Measurement Input, Equity Dividend Yields [Member] | Equity contracts [Member] | Minimum [Member] | Valuation Technique, Option Pricing Model [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	[2]	0.00	0.00
Measurement Input, Equity Dividend Yields [Member] | Equity contracts [Member] | Maximum [Member] | Valuation Technique, Option Pricing Model [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	[2]	0.14	0.08
Measurement Input, Credit Default Swap Spreads [Member] | Credit contracts [Member] | Minimum [Member] | Valuation Technique, Option Pricing Model [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input		0.0095	0.0154
Measurement Input, Credit Default Swap Spreads [Member] | Credit contracts [Member] | Maximum [Member] | Valuation Technique, Option Pricing Model [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input		0.0115	0.0174
Measurement Input, Conversion Rate [Member] | Other Contracts [Member] | Valuation Technique, Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input		1.6	1.6
Measurement Input, Expected Duration [Member] | Other Contracts [Member] | Minimum [Member] | Valuation Technique, Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Derivative Contract, Duration, Measurement Input		2 years	2 years
Measurement Input, Expected Duration [Member] | Other Contracts [Member] | Maximum [Member] | Valuation Technique, Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Derivative Contract, Duration, Measurement Input		4 years	4 years
Measurement Input, Market Assumptions Related to Yields for Comparable Instruments [Member] | Other asset-backed securities available-for-sale [Member] | Minimum [Member] | Valuation Technique, Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Measurement Input		0.01	0.01
Measurement Input, Market Assumptions Related to Yields for Comparable Instruments [Member] | Other asset-backed securities available-for-sale [Member] | Maximum [Member] | Valuation Technique, Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Measurement Input		0.03	0.03
Measurement Input, Client Transfer Rates Based on Rating [Member] | Minimum [Member] | Valuation Technique, Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Other Assets, Measurement Input		0.50	0.50
Measurement Input, Client Transfer Rates Based on Rating [Member] | Maximum [Member] | Valuation Technique, Discounted Cash Flows [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Other Assets, Measurement Input		0.95	0.95

[1] Structured deposits and structured notes contain embedded derivative features whose fair value measurements contain significant Level 3 inputs.

[2] We are the client-facing entity and we enter into identical but opposite derivatives to transfer the resultant risks to our affiliates. With the exception of counterparty credit risks, we are market neutral. The corresponding intra-group derivatives are presented as equity derivatives and foreign exchange derivatives in the table.

[3] Securities available-for-sale are recorded at fair value through other comprehensive income (loss).

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