Mortgage-Backed Debt and Related Collateral - Additional Information (Detail) (USD $)			6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Trust 2006-1 [Member]	Jun. 30, 2013 Trust 2005-1 [Member]	Jun. 30, 2013 VIE Primary Beneficiary [Member] Residual Trusts [Member]	Jun. 30, 2013 VIE Primary Beneficiary [Member] Non-Residual Trusts [Member] Trust	Jun. 30, 2013 VIE [Member]
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 residential loans	$ 418,300,000					$ 418,300,000
Range of final maturities for Residual Trusts					2029 to 2050
Delinquency rate			9.64%
Delinquency rate trigger			8.00%	8.00%
Cumulative loss rate			7.51%
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	2,500,000,000
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							51,500,000
Receivables related to Non-Residual Trusts at fair value	$ 50,890,000	$ 53,975,000