Composition of the Company's Loan Portfolio (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans carried at fair value	$ 390	[1]	$ 433	[1]
Residential Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans carried at fair value	390		431
Residential Portfolio Segment [Member] | Residential Nonguaranteed [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans carried at fair value	$ 390		$ 431

[1] 2 Includes loans of consolidated VIEs374 3,322