Fair Value Measurements - Additional Information Relating To Asset-Backed Securities and Collateralized Debt Obligations (Detail) $ in Millions	Sep. 30, 2017 USD ($)	[1]
Level 2 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	$ 163
Level 3 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	125
AAA - A Rating [Member] | Level 2 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	147
AAA - A Rating [Member] | Level 2 [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	454
AAA - A Rating [Member] | Level 3 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	0
AAA - A Rating [Member] | Level 3 [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	110
Student loans [Member] | AAA - A Rating [Member] | Level 2 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	91
Student loans [Member] | AAA - A Rating [Member] | Level 3 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	0
Collateralized debt obligations [Member] | BBB -B Rating [Member] | Level 2 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	0
Collateralized debt obligations [Member] | BBB -B Rating [Member] | Level 3 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	125
Other [Member] | AAA - A Rating [Member] | Level 2 [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	400
Other [Member] | AAA - A Rating [Member] | Level 3 [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	110
Estimate of fair value [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	288
Estimate of fair value [Member] | AAA - A Rating [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	147
Estimate of fair value [Member] | AAA - A Rating [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	564
Estimate of fair value [Member] | Student loans [Member] | AAA - A Rating [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	91
Estimate of fair value [Member] | Collateralized debt obligations [Member] | BBB -B Rating [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	125
Estimate of fair value [Member] | Other [Member] | AAA - A Rating [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	510
Alt [Member] | Residential Mortgages [Member] | AAA - A Rating [Member] | Level 2 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	31
Alt [Member] | Residential Mortgages [Member] | AAA - A Rating [Member] | Level 3 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	0
Alt [Member] | Home Equity [Member] | AAA - A Rating [Member] | Level 2 [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	54
Alt [Member] | Home Equity [Member] | AAA - A Rating [Member] | Level 3 [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	0
Alt [Member] | Estimate of fair value [Member] | Residential Mortgages [Member] | AAA - A Rating [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	31
Alt [Member] | Estimate of fair value [Member] | Home Equity [Member] | AAA - A Rating [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	54
Subprime [Member] | Residential Mortgages [Member] | AAA - A Rating [Member] | Level 2 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	25
Subprime [Member] | Residential Mortgages [Member] | AAA - A Rating [Member] | Level 3 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	0
Subprime [Member] | Residential Mortgages [Member] | CCC - Unrated [Member] | Level 2 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	16
Subprime [Member] | Residential Mortgages [Member] | CCC - Unrated [Member] | Level 3 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	0
Subprime [Member] | Estimate of fair value [Member] | Residential Mortgages [Member] | AAA - A Rating [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	25
Subprime [Member] | Estimate of fair value [Member] | Residential Mortgages [Member] | CCC - Unrated [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	$ 16

[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.