Fair Value Measurements - Additional Information Relating To Asset-Backed Securities and Collateralized Debt Obligations (Detail) $ in Millions	Sep. 30, 2019 USD ($)
AAA - A Rating [Member] | Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	$ 139	[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	53	[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	31	[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	86	[1]
Other [Member] | AAA - A Rating [Member] | Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	54	[1]
Other [Member] | AAA - A Rating [Member] | Level 2 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	0	[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	61	[1]
Other [Member] | BBB -B Rating [Member] | Level 2 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	0	[1]
Trading assets, excluding derivatives [Member] | Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	187	[1]
Available-for-sale securities [Member] | Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	115
Available-for-sale securities [Member] | Level 2 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	33
Available-for-sale securities [Member] | AAA - A Rating [Member] | Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	54	[1]
Available-for-sale securities [Member] | AAA - A Rating [Member] | Level 2 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	33	[1]
Estimate of fair value [Member] | AAA - A Rating [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	139	[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	53	[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	31	[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	86	[1]
Estimate of fair value [Member] | Other [Member] | AAA - A Rating [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	54	[1]
Estimate of fair value [Member] | Other [Member] | BBB -B Rating [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	61	[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	187	[1]
Estimate of fair value [Member] | Available-for-sale securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	148
Estimate of fair value [Member] | Available-for-sale securities [Member] | AAA - A Rating [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	87	[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	0	[1]
Alt [Member] | Home equity [Member] | AAA - A Rating [Member] | Level 2 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	33	[1]
Alt [Member] | Estimate of fair value [Member] | Home equity [Member] | AAA - A Rating [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	33	[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	17	[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	$ 17	[1]

[1] We utilize Standard and Poor's ("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.