Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount		$ 0		$ 0
Average deviation of implied volatility, foreign currency pair		1.00%
Average deviation of implied volatility, at-the-money equity option		5.00%
Revaluation period		90 days
Unfunded commitments, equity securities measured at net asset value		$ 34		43
Derivative Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair value of level 3 assets		281	$ 343
Derivative Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair value of level 3 liabilities	[1]	$ 185	167
Residential Mortgage [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value Inputs, Loss Severity		69.00%
Impaired commercial loans [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value Inputs, Loss Severity		25.00%
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair value of level 3 assets		$ 2,152	3,351	2,207	$ 3,356
Fair value of level 3 liabilities		1,546	1,774	1,538	1,906
Fair Value Assets, Transfers Into Level 3		0	0
Fair Value Assets, Transfers Out of Level 3		0	0
Fair Value Liability, Transfers Into Level 3		3	5
Fair Value Liability, Transfers Out of Level 3		78	89
Level 3 [Member] | Domestic Deposits [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair value of level 3 liabilities	[1]	916	1,216	897	1,407
Fair Value Liability, Transfers Into Level 3	[1]	3	3
Fair Value Liability, Transfers Out of Level 3	[1]	34	46
Level 3 [Member] | Borrowings [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair value of level 3 liabilities	[1]	630	558	641	499
Fair Value Liability, Transfers Into Level 3	[1]	0	2
Fair Value Liability, Transfers Out of Level 3	[1]	44	43
Other asset-backed securities available-for-sale [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair value of level 3 assets	[2]	107	107	111	$ 105
Fair Value Assets, Transfers Into Level 3	[2]	0	0
Fair Value Assets, Transfers Out of Level 3	[2]	0	$ 0
Unfunded Loan Commitment [Member] | Standby Letters of Credit [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Deferred fees on commitments and standby letters of credit		$ 156		$ 158
Fair Value, Measurements, Recurring [Member] | Domestic Deposits [Member] | Option adjusted discounted cash flows [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value Inputs, Implied Volatility Currency Pairs	[3],[4]	6.00%		6.00%
Fair Value Inputs, Equity/Equity Index Volatility	[3],[4]	7.00%		7.00%
Fair Value Inputs, Equity/Equity and Equity/Index Correlation		42.00%		42.00%
Fair Value, Measurements, Recurring [Member] | Domestic Deposits [Member] | Option adjusted discounted cash flows [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value Inputs, Implied Volatility Currency Pairs	[3],[4]	9.00%		9.00%
Fair Value Inputs, Equity/Equity Index Volatility	[3],[4]	31.00%		42.00%
Fair Value Inputs, Equity/Equity and Equity/Index Correlation		52.00%		80.00%
Fair Value, Measurements, Recurring [Member] | Borrowings [Member] | Option adjusted discounted cash flows [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value Inputs, Implied Volatility Currency Pairs	[3],[4]	6.00%		6.00%
Fair Value Inputs, Equity/Equity Index Volatility	[3],[4]	7.00%		7.00%
Fair Value Inputs, Equity/Equity and Equity/Index Correlation	[3],[4]	42.00%		42.00%
Fair Value, Measurements, Recurring [Member] | Borrowings [Member] | Option adjusted discounted cash flows [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value Inputs, Implied Volatility Currency Pairs	[3],[4]	9.00%		9.00%
Fair Value Inputs, Equity/Equity Index Volatility	[3],[4]	31.00%		42.00%
Fair Value Inputs, Equity/Equity and Equity/Index Correlation	[3],[4]	78.00%		80.00%

[1] unrealized gains (losses) on fair value option liabilities attributable to our own credit spread, which are recorded in other comprehensive income (loss), gains (losses) on fair value option liabilities are included in gain on instruments designated at fair value and related derivatives in the consolidated statement of income (loss).

[2] Realized gains (losses) on securities available-for-sale are included in other securities gains, net in the consolidated statement of income (loss). Unrealized gains (losses) on securities available-for-sale are included in other comprehensive income (loss).

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

[4] 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.