Fair Value Measurements - Additional Information Relating To Asset-Backed Securities and Collateralized Debt Obligations (Detail) $ in Millions	Dec. 31, 2015 USD ($)	[1]
Level 2 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	$ 203
Level 3 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	221
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	197
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	173
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	89
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	221
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	9
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	89
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	424
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	197
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	173
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	89
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	221
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	9
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	89
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	61
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	75
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	61
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	75
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	47
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	47
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.