Fair Value Measurements - Additional Information Relating To Asset-Backed Securities and Collateralized Debt Obligations (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	$ 470		$ 557
Level 2 [Member] | Trading asset-backed securities and related collateral [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading collateralized debt obligations and related collateral	74	[1]
Level 2 [Member] | Alt [Member] | Trading asset-backed securities and related collateral [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	90	[1]
Available-for-sale securities backed by collateral	335	[1]
Level 2 [Member] | Subprime [Member] | Trading asset-backed securities and related collateral [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	73	[1]
Level 2 [Member] | Commercial Mortgages [Member] | Trading asset-backed securities and related collateral [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	135	[1]
Level 3 [Member] | Trading asset-backed securities and related collateral [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading collateralized debt obligations and related collateral	245	[1]
Estimate of Fair Value Measurement [Member] | Trading asset-backed securities and related collateral [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	163	[1]
Available-for-sale securities backed by collateral	470	[1]
AAA - A Rating [Member] | Level 2 [Member] | Trading asset-backed securities and related collateral [Member] | Student Loans [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading collateralized debt obligations and related collateral	74	[1]
AAA - A Rating [Member] | Level 2 [Member] | Alt [Member] | Trading asset-backed securities and related collateral [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	100	[1]
AAA - A Rating [Member] | Level 2 [Member] | Alt [Member] | Trading asset-backed securities and related collateral [Member] | Residential Mortgage [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	90	[1]
AAA - A Rating [Member] | Level 2 [Member] | Alt [Member] | Trading asset-backed securities and related collateral [Member] | Home Equity Line of Credit [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	100	[1]
AAA - A Rating [Member] | Level 2 [Member] | Subprime [Member] | Trading asset-backed securities and related collateral [Member] | Residential Mortgage [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	69	[1]
AAA - A Rating [Member] | Level 2 [Member] | Commercial Mortgages [Member] | Trading asset-backed securities and related collateral [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	135	[1]
AAA - A Rating [Member] | Level 2 [Member] | Commercial Mortgages [Member] | Trading asset-backed securities and related collateral [Member] | Residential Mortgage [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	135	[1]
AAA - A Rating [Member] | Level 3 [Member] | Trading asset-backed securities and related collateral [Member] | Student Loans [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading collateralized debt obligations and related collateral	0	[1]
AAA - A Rating [Member] | Estimate of Fair Value Measurement [Member] | Trading asset-backed securities and related collateral [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	235	[1]
AAA - A Rating [Member] | Estimate of Fair Value Measurement [Member] | Trading asset-backed securities and related collateral [Member] | Residential Mortgage [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	159	[1]
Available-for-sale securities backed by collateral	135	[1]
AAA - A Rating [Member] | Estimate of Fair Value Measurement [Member] | Trading asset-backed securities and related collateral [Member] | Home Equity Line of Credit [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	100	[1]
CCC - Unrated [Member] | Level 2 [Member] | Alt [Member] | Trading asset-backed securities and related collateral [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	132	[1]
CCC - Unrated [Member] | Level 2 [Member] | Alt [Member] | Trading asset-backed securities and related collateral [Member] | Residential Mortgage [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	2	[1]
CCC - Unrated [Member] | Level 2 [Member] | Alt [Member] | Trading asset-backed securities and related collateral [Member] | Home Equity Line of Credit [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	130	[1]
CCC - Unrated [Member] | Level 2 [Member] | Subprime [Member] | Trading asset-backed securities and related collateral [Member] | Residential Mortgage [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	4	[1]
CCC - Unrated [Member] | Estimate of Fair Value Measurement [Member] | Trading asset-backed securities and related collateral [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	132	[1]
CCC - Unrated [Member] | Estimate of Fair Value Measurement [Member] | Trading asset-backed securities and related collateral [Member] | Residential Mortgage [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	4	[1]
Available-for-sale securities backed by collateral	2	[1]
CCC - Unrated [Member] | Estimate of Fair Value Measurement [Member] | Trading asset-backed securities and related collateral [Member] | Home Equity Line of Credit [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	130	[1]
BBB -B Rating [Member] | Level 2 [Member] | Trading asset-backed securities and related collateral [Member] | Other Debt Obligations [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading collateralized debt obligations and related collateral	0	[1]
BBB -B Rating [Member] | Level 2 [Member] | Alt [Member] | Trading asset-backed securities and related collateral [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	103	[1]
BBB -B Rating [Member] | Level 2 [Member] | Alt [Member] | Trading asset-backed securities and related collateral [Member] | Other Line Of Credit [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	103	[1]
BBB -B Rating [Member] | Level 3 [Member] | Trading asset-backed securities and related collateral [Member] | Other Debt Obligations [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading collateralized debt obligations and related collateral	245	[1]
BBB -B Rating [Member] | Estimate of Fair Value Measurement [Member] | Trading asset-backed securities and related collateral [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	103	[1]
BBB -B Rating [Member] | Estimate of Fair Value Measurement [Member] | Trading asset-backed securities and related collateral [Member] | Other Line Of Credit [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	$ 103	[1]

[1] We utilize Standard & 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.