Real Estate Securities - Additional Information (Detail) (USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2013 Investment	Mar. 31, 2013	Jun. 30, 2012	Jun. 30, 2013 Investment	Jun. 30, 2012	Dec. 31, 2012 Investment
Investment Holdings [Line Items]
Trading securities	$ 141,615,000			$ 141,615,000		$ 33,172,000
Realized gain on sale of available for sale securities	193,000	12,000,000	6,989,000	12,231,000	14,774,000
Available-for-sale securities	1,172,690,000			1,172,690,000		1,075,581,000
Number of AFS securities	294			294		284
Number of securities in unrealized loss position	47			47		22
Number of securities in a continuous unrealized loss position for twelve consecutive months or longer	9			9		14
Senior Securities
Investment Holdings [Line Items]
Trading securities	141,116,000			141,116,000		32,543,000
Available-for-sale securities	634,846,000			634,846,000		711,789,000
Trading Securities
Investment Holdings [Line Items]
Unpaid principal balance	12,000,000			12,000,000		12,000,000
Interest Only Securities
Investment Holdings [Line Items]
Trading securities	141,000,000			141,000,000
Residential Subordinate Securities
Investment Holdings [Line Items]
Trading securities	242,000			242,000
Re-REMIC
Investment Holdings [Line Items]
Available-for-sale securities	154,167,000			154,167,000		163,035,000
Subordinate Securities
Investment Holdings [Line Items]
Trading securities	499,000			499,000		629,000
Available-for-sale securities	383,677,000			383,677,000		200,757,000
Residential Prime | Senior Securities
Investment Holdings [Line Items]
Trading securities	123,591,000			123,591,000		10,409,000
Securities financed through non-recourse resecuritization entity	154,000,000			154,000,000		172,000,000
Loans underlying	8,000,000,000			8,000,000,000
Available-for-sale securities	410,159,000			410,159,000		466,523,000
Residential Prime | Re-REMIC
Investment Holdings [Line Items]
Loans underlying	5,000,000,000			5,000,000,000
Residential Prime | Subordinate Securities
Investment Holdings [Line Items]
Trading securities	354,000			354,000		468,000
Loans underlying	16,000,000,000			16,000,000,000
Available-for-sale securities	381,631,000			381,631,000		184,528,000
Residential non-prime | Senior Securities
Investment Holdings [Line Items]
Trading securities	17,525,000			17,525,000		22,134,000
Securities financed through non-recourse resecuritization entity	141,000,000			141,000,000		152,000,000
Loans underlying	4,000,000,000			4,000,000,000
Available-for-sale securities	224,687,000			224,687,000		245,266,000
Residential non-prime | Re-REMIC
Investment Holdings [Line Items]
Loans underlying	106,000,000			106,000,000
Residential non-prime | Subordinate Securities
Investment Holdings [Line Items]
Trading securities	145,000			145,000		161,000
Loans underlying	431,000,000			431,000,000
Available-for-sale securities	2,046,000			2,046,000		2,160,000
Residential
Investment Holdings [Line Items]
Marketable securities, due from five to ten years	3,000,000			3,000,000
Marketable securities, less than five years	3,000			3,000
Credit reserve	190,410,000	190,690,000		190,410,000		187,032,000
Available-for-sale securities	1,172,690,000			1,172,690,000		1,061,512,000
Residential | Senior Securities
Investment Holdings [Line Items]
Credit reserve	50,000,000			50,000,000
Percentage of outstanding loan serious delinquencies	10.08%			10.08%
Residential | Re-REMIC
Investment Holdings [Line Items]
Credit reserve	44,000,000			44,000,000
Loans underlying	5,000,000,000			5,000,000,000
Percentage of outstanding loan serious delinquencies	9.70%			9.70%
Residential | Subordinate Securities
Investment Holdings [Line Items]
Credit reserve	96,000,000			96,000,000
Loans underlying	$ 16,000,000,000			$ 16,000,000,000
Percentage of outstanding loan serious delinquencies	4.18%			4.18%