Mortgage-Backed Debt and Related Collateral - Summary of Collateral for Mortgage-Backed Debt (Detail) (USD $)	Jun. 30, 2013	Dec. 31, 2012
Financial Instruments Owned and Pledged as Collateral [Line Items]
Residential loans of securitization trusts, principal balance	$ 1,602,614,000	$ 1,662,183,000
Receivables related to Non-Residual Trusts	50,890,000	53,975,000
Real estate owned, net	68,380,000	64,959,000
Restricted cash and cash equivalents	1,057,503,000	653,338,000
Total mortgage-backed debt collateral	2,500,000,000
VIE Primary Beneficiary [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Restricted cash and cash equivalents	57,278,000	58,253,000
VIE Primary Beneficiary [Member] | Mortgage-backed Debt [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Residential loans of securitization trusts, principal balance	2,353,972,000	2,458,678,000
Receivables related to Non-Residual Trusts	50,890,000	53,975,000
Real estate owned, net	42,944,000	43,115,000
Restricted cash and cash equivalents	57,278,000	58,253,000
Total mortgage-backed debt collateral	$ 2,505,084,000	$ 2,614,021,000