Fair Value Measurements - Additional Information Relating To Asset-Backed Securities and Collateralized Debt Obligations (Detail) $ in Millions	Sep. 30, 2020 USD ($)
AAA - A Rating [Member] | Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	$ 0	[1]
AAA - A Rating [Member] | Level 2 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	107	[1]
Collateralized debt obligations [Member] | AAA - A Rating [Member] | Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	0	[1]
Collateralized debt obligations [Member] | AAA - A Rating [Member] | Level 2 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	39	[1]
Collateralized debt obligations [Member] | BBB -B Rating [Member] | Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	0	[1]
Collateralized debt obligations [Member] | BBB -B Rating [Member] | Level 2 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	25	[1]
Student loans [Member] | AAA - A Rating [Member] | Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	0	[1]
Student loans [Member] | AAA - A Rating [Member] | Level 2 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	68	[1]
Other [Member] | BBB -B Rating [Member] | Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	100	[1]
Trading assets, excluding derivatives [Member] | Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	15	[1]
Trading assets, excluding derivatives [Member] | Level 2 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	132	[1]
Available-for-sale securities [Member] | Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	127
Estimate of fair value [Member] | AAA - A Rating [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	107	[1]
Estimate of fair value [Member] | Collateralized debt obligations [Member] | AAA - A Rating [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	39	[1]
Estimate of fair value [Member] | Collateralized debt obligations [Member] | BBB -B Rating [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	25	[1]
Estimate of fair value [Member] | Student loans [Member] | AAA - A Rating [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	68	[1]
Estimate of fair value [Member] | Trading assets, excluding derivatives [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	147	[1]
Alt [Member] | Home equity [Member] | AAA - A Rating [Member] | Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	27	[1]
Subprime [Member] | Residential mortgage - Subprime [Member] | CCC - Unrated [Member] | Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	15	[1]
Subprime [Member] | Residential mortgage - Subprime [Member] | CCC - Unrated [Member] | Level 2 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	0	[1]
Subprime [Member] | Estimate of fair value [Member] | Residential mortgage - Subprime [Member] | CCC - Unrated [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	$ 15	[1]

[1] We utilize S&P as the primary source of credit ratings in the tables above. If S&P ratings are not available, ratings by Moody's and Fitch are used in that order. Ratings for CDOs represent the ratings associated with the underlying collateral.