Investments in Loans (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2014 Loan		Dec. 31, 2013 Loan
Mortgage Loans on Real Estate [Line Items]
Unpaid Principal Balance	$ 1,389,276		$ 1,146,574
Total investment in loans, Unamortized (Discounts) Premiums	(18,310)		(23,397)
Carrying Amount	1,370,966		1,123,177
Number of Loans	174		153
Weighted-Average Coupon	6.60%	[1]	7.20%	[1]
Deferred fees	(1,184)		(800)
Total investments in loans, Unpaid Principal Balance	1,388,092		1,145,774
Commercial mortgages, mezzanine loans and other loans, Carrying Amount	1,369,782		1,122,377
CRE Loans
Mortgage Loans on Real Estate [Line Items]
Unpaid Principal Balance	1,369,138		1,115,949
Total investment in loans, Unamortized (Discounts) Premiums	(18,363)		(23,469)
Carrying Amount	1,350,775		1,092,480
Number of Loans	173		151
Weighted-Average Coupon	6.60%	[1]	7.30%	[1]
Deferred Fee
Mortgage Loans on Real Estate [Line Items]
Total investment in loans, Unamortized (Discounts) Premiums	0		0
Commercial mortgages
Mortgage Loans on Real Estate [Line Items]
Unpaid Principal Balance	1,068,977	[2]	792,526	[3]
Total investment in loans, Unamortized (Discounts) Premiums	(15,787)	[2]	(19,257)	[3]
Carrying Amount	1,053,190	[2]	773,269	[3]
Number of Loans	86	[2]	59	[3]
Weighted-Average Coupon	5.90%	[1],[2]	6.40%	[1],[3]
Range of Maturity Dates, Minimum	Dec. 01, 2014	[2]	Mar. 01, 2014	[3]
Range of Maturity Dates, Maximum	Jul. 01, 2044	[2]	Jan. 01, 2029	[3]
Mezzanine Loans
Mortgage Loans on Real Estate [Line Items]
Unpaid Principal Balance	257,553		269,034
Total investment in loans, Unamortized (Discounts) Premiums	(2,576)		(3,273)
Carrying Amount	254,977		265,761
Number of Loans	77		81
Weighted-Average Coupon	9.40%	[1]	9.60%	[1]
Range of Maturity Dates, Minimum	Dec. 01, 2014		Mar. 01, 2014
Range of Maturity Dates, Maximum	Jan. 01, 2029		Jan. 01, 2029
Preferred Equity Interests
Mortgage Loans on Real Estate [Line Items]
Unpaid Principal Balance	42,608		54,389
Total investment in loans, Unamortized (Discounts) Premiums	0		(939)
Carrying Amount	42,608		53,450
Number of Loans	10		11
Weighted-Average Coupon	7.70%	[1]	8.60%	[1]
Range of Maturity Dates, Minimum	May 01, 2015		May 01, 2015
Range of Maturity Dates, Maximum	Aug. 01, 2025		Aug. 01, 2025
Other loans
Mortgage Loans on Real Estate [Line Items]
Unpaid Principal Balance	20,138		30,625
Total investment in loans, Unamortized (Discounts) Premiums	53		72
Carrying Amount, Other loans	$ 20,191		$ 30,697
Number of Loans	1		2
Weighted-Average Coupon	2.80%	[1]	4.30%	[1]
Range of Maturity Dates, Minimum	Oct. 01, 2016		Mar. 01, 2014
Range of Maturity Dates, Maximum			Oct. 01, 2016

[1] Weighted-average coupon is calculated on the unpaid principal amount of the underlying instruments, which does not necessarily correspond to the carrying amount.

[2] Commercial mortgages includes 11 conduit loans with an unpaid principal balance and carrying amount of $107,164, a weighted-average coupon of 4.6% and maturity dates ranging from September 2021 through July 2044. These commercial mortgages are classified as loans held for sale.

[3] Commercial mortgages includes six conduit loans with an unpaid principal balance and carrying amount of $61,825, a weighted-average coupon of 5.3% and maturity dates ranging from January 2024 through January 2029. These commercial mortgages are classified as loans held for sale.