Investments in Real Estate - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended			1 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Aug. 06, 2013 Subsequent Event	Jun. 30, 2013 Series of Individually Immaterial Business Acquisitions Loan	Jun. 30, 2013 Series of Individually Immaterial Business Acquisitions Multi-family real estate properties Property
Real Estate Properties [Line Items]
Investments in real estate	$ 949,649,000		$ 949,649,000		$ 918,189,000
Mortgages held by third parties	143,957,000		143,957,000		144,645,000
Mortgages held by consolidated securitizations	870,472,000		870,472,000		830,077,000
Number of multi family property acquired								1
Investments in acquisition of real estate							36,950,000	35,400,000
Number of original loans converted							2
Gains (losses) on assets	224,000	2,518,000	221,000	2,529,000			1,550
Investment contributed revenue	27,858,000	25,540,000	55,027,000	50,371,000			0
Net income allocable to common shares	(65,877,000)	(6,951,000)	(156,409,000)	(113,970,000)			0
Acquisition related cost							0
Commercial real estate loans carrying value		24,871,000		24,871,000		15,000,000
Proceed from sale of real estate property						3,300,000
Losses on sale of real estate property						$ (1,326,000)