Fair Value Measurements - Additional Information Relating To Asset-Backed Securities and Collateralized Debt Obligations (Detail) $ in Millions	Sep. 30, 2015 USD ($)	[1]
Level 2 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	$ 207
Level 3 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	233
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	201
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	194
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	0
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	88
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	233
Commercial mortgages [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	22
Commercial mortgages [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
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	93
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	0
Estimate of fair value [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	440
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	201
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	194
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	88
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	233
Estimate of fair value [Member] | Commercial mortgages [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	22
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	93
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	65
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	79
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	65
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	79
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	48
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	6
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	48
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	$ 6

[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 collateralized debt obligations represent the ratings associated with the underlying collateral.