Fair Value Measurements - Additional Information Relating To Asset-Backed Securities and Collateralized Debt Obligations (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	$ 593	$ 818
Trading asset-backed securities and related collateral [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	250
Available-for-sale securities backed by collateral	594
Trading asset-backed securities and related collateral [Member] | AAA -A [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	246
Available-for-sale securities backed by collateral	453
Trading asset-backed securities and related collateral [Member] | BBB -B [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	77
Trading asset-backed securities and related collateral [Member] | CCC -Unrated [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	4
Available-for-sale securities backed by collateral	64
Trading asset-backed securities and related collateral [Member] | Residential Mortgage [Member] | AAA -A [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	246
Trading asset-backed securities and related collateral [Member] | Residential Mortgage [Member] | CCC -Unrated [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	4
Available-for-sale securities backed by collateral	2
Trading asset-backed securities and related collateral [Member] | Commercial Mortgages [Member] | AAA -A [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	264
Trading asset-backed securities and related collateral [Member] | Home Equity Mortgages [Member] | AAA -A [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	104
Trading asset-backed securities and related collateral [Member] | Home Equity Mortgages [Member] | BBB -B [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	77
Trading asset-backed securities and related collateral [Member] | Home Equity Mortgages [Member] | CCC -Unrated [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	62
Trading asset-backed securities and related collateral [Member] | Other securities [Member] | AAA -A [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	85
Trading asset-backed securities and related collateral [Member] | Level 2 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading collateralized debt obligations and related collateral	58
Available-for-sale securities backed by collateral	264
Trading asset-backed securities and related collateral [Member] | Level 2 [Member] | AAA -A [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading collateralized debt obligations and related collateral	58
Available-for-sale securities backed by collateral	264
Trading asset-backed securities and related collateral [Member] | Level 2 [Member] | Commercial Mortgages [Member] | AAA -A [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	264
Trading asset-backed securities and related collateral [Member] | Level 2 [Member] | Student loans [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading collateralized debt obligations and related collateral	58
Trading asset-backed securities and related collateral [Member] | Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading collateralized debt obligations and related collateral	679
Trading asset-backed securities and related collateral [Member] | Level 3 [Member] | BBB -B [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading collateralized debt obligations and related collateral	614
Trading asset-backed securities and related collateral [Member] | Level 3 [Member] | CCC -Unrated [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading collateralized debt obligations and related collateral	65
Trading asset-backed securities and related collateral [Member] | Level 3 [Member] | Commercial Mortgages [Member] | BBB -B [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading collateralized debt obligations and related collateral	122
Trading asset-backed securities and related collateral [Member] | Level 3 [Member] | Commercial Mortgages [Member] | CCC -Unrated [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading collateralized debt obligations and related collateral	65
Trading asset-backed securities and related collateral [Member] | Level 3 [Member] | Other securities [Member] | BBB -B [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading collateralized debt obligations and related collateral	162
Trading asset-backed securities and related collateral [Member] | Level 3 [Member] | Corporate Loans [Member] | BBB -B [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading collateralized debt obligations and related collateral	330
Alt-A [Member] | Trading asset-backed securities and related collateral [Member] | Level 2 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	68
Available-for-sale securities backed by collateral	330
Alt-A [Member] | Trading asset-backed securities and related collateral [Member] | Level 2 [Member] | AAA -A [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	68
Available-for-sale securities backed by collateral	189
Alt-A [Member] | Trading asset-backed securities and related collateral [Member] | Level 2 [Member] | BBB -B [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	77
Alt-A [Member] | Trading asset-backed securities and related collateral [Member] | Level 2 [Member] | CCC -Unrated [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	64
Alt-A [Member] | Trading asset-backed securities and related collateral [Member] | Level 2 [Member] | Residential Mortgage [Member] | AAA -A [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	68
Alt-A [Member] | Trading asset-backed securities and related collateral [Member] | Level 2 [Member] | Residential Mortgage [Member] | CCC -Unrated [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	2
Alt-A [Member] | Trading asset-backed securities and related collateral [Member] | Level 2 [Member] | Home Equity Mortgages [Member] | AAA -A [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	104
Alt-A [Member] | Trading asset-backed securities and related collateral [Member] | Level 2 [Member] | Home Equity Mortgages [Member] | BBB -B [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	77
Alt-A [Member] | Trading asset-backed securities and related collateral [Member] | Level 2 [Member] | Home Equity Mortgages [Member] | CCC -Unrated [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	62
Alt-A [Member] | Trading asset-backed securities and related collateral [Member] | Level 2 [Member] | Other securities [Member] | AAA -A [Member]
Fair Value Of Other Financial Instrument [Line Items]
Available-for-sale securities backed by collateral	85
Sub-prime [Member] | Trading asset-backed securities and related collateral [Member] | Level 2 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	182
Sub-prime [Member] | Trading asset-backed securities and related collateral [Member] | Level 2 [Member] | AAA -A [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	178
Sub-prime [Member] | Trading asset-backed securities and related collateral [Member] | Level 2 [Member] | CCC -Unrated [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	4
Sub-prime [Member] | Trading asset-backed securities and related collateral [Member] | Level 2 [Member] | Residential Mortgage [Member] | AAA -A [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	178
Sub-prime [Member] | Trading asset-backed securities and related collateral [Member] | Level 2 [Member] | Residential Mortgage [Member] | CCC -Unrated [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	$ 4