Securitized Debt - Narrative (Detail) (Securitized Loans [Member], USD $) In Millions, unless otherwise specified							9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure [Member] Loans Held for Investment [Member]	Dec. 31, 2011 Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure [Member] Loans Held for Investment [Member]	Sep. 30, 2012 Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure [Member] Non-Agency RMBS [Member]	Dec. 31, 2011 Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure [Member] Non-Agency RMBS [Member]	Sep. 30, 2012 Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure [Member] Minimum [Member] Loans Held for Investment [Member]	Sep. 30, 2012 Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure [Member] Minimum [Member] Non-Agency RMBS [Member]	Sep. 30, 2012 Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure [Member] Maximum [Member] Loans Held for Investment [Member]	Sep. 30, 2012 Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure [Member] Maximum [Member] Non-Agency RMBS [Member]
Debt Instrument [Line Items]
Principal balance of debt	$ 2,300.0	$ 1,900.0	$ 1,000.0	$ 222.9	$ 1,300.0	$ 1,700.0
Year the debt matures							2023	2035	2042	2047
Weighted average cost of financing			3.63%	5.48%	4.96%	5.09%