Fair Value Measurements - Additional Information Relating To Asset-Backed Securities and Collateralized Debt Obligations (Detail) $ in Millions	Jun. 30, 2018 USD ($)
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	$ 92	[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]
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	0	[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	131	[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] | AAA - A Rating [Member] | Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	49	[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]
Other [Member] | BBB -B Rating [Member] | Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	58	[1]
Available-for-sale securities [Member] | Level 2 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	47
Available-for-sale securities [Member] | Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	107
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	47	[1]
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	49	[1]
Trading assets, excluding derivatives [Member] | Level 2 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	109	[1]
Trading assets, excluding derivatives [Member] | Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	131	[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	92	[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	131	[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	49	[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	58	[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	154
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	96	[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	240	[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	47	[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] | 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	47	[1]
Subprime [Member] | Residential Mortgages [Member] | CCC - Unrated [Member] | Level 2 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	17	[1]
Subprime [Member] | Residential Mortgages [Member] | CCC - Unrated [Member] | Level 3 [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 Mortgages [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 collateralized debt obligations represent the ratings associated with the underlying collateral.