Fair Value of Financial Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Dealer marks of securities	56.00%	56.00%
Percentage difference of internal valuation than dealer marks	(1.00%)	(1.00%)
Residential | Senior Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair value option elected aggregate carrying amount, asset	$ 5	$ 36
Residential Loans
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair value option elected aggregate carrying amount, asset	$ 524
Asset-backed Securities
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Dealer marks of securities	55.00%	55.00%
Percentage difference of internal valuation than dealer marks	8.00%	8.00%