Mortgage-Backed Debt and Related Collateral - Additional Information (Detail) (USD $)			9 Months Ended			9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Trust 2006-1 [Member]	Sep. 30, 2013 Trust 2005-1 [Member]	Sep. 30, 2013 VIE Primary Beneficiary [Member]	Sep. 30, 2013 VIE Primary Beneficiary [Member] Residual Trusts [Member]	Sep. 30, 2013 VIE Primary Beneficiary [Member] Non-Residual Trusts [Member] Trust
Beneficial Interests And Related Collateral [Line Items]
Mandatory clean-up calls must be performed on the Non-Residual Trusts when each loan pool falls to this percent of the original principal balance							10.00%
Outstanding balance of collateral for mandatory clean-up call obligation	$ 419,000,000						$ 419,000,000
Range of final maturities for Residual Trusts						2029 to 2050
Delinquency rate			9.30%
Delinquency rate trigger			8.00%	8.00%
Cumulative loss rate			7.80%
Cumulative loss rate trigger			6.00%
Expected period to exercise mandatory clean-up call obligation							2017 and continuing through 2019
Residual and Non-Residual Trusts outstanding debt	2,000,000,000
Total mortgage-backed debt collateral					2,400,000,000
Number of Non-Residual Trusts with reimbursement obligations consolidated on balance sheet							7
Number of securitization trusts related to Non-Residual Trusts							10
Notional amount of accounts receivable recorded at fair value							46,300,000
Receivables related to Non-Residual Trusts at fair value	$ 45,849,000	$ 53,975,000					$ 45,800,000