Investments in Loans (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended		12 Months Ended
	Jun. 30, 2013 Loan		Dec. 31, 2012 Loan
Mortgage Loans on Real Estate [Line Items]
Unpaid Principal Balance	$ 1,189,325		$ 1,107,583
Deferred fees	(115)		(1,231)
Total investments in loans, Unpaid Principal Balance	1,189,210		1,106,352
Total investment in loans, Unamortized (Discounts) Premiums	(27,444)		(31,223)
Carrying Amount	1,161,881		1,076,360
Commercial mortgages, mezzanine loans and other loans, Carrying Amount	1,161,766		1,075,129
Number of Loans	157		150
Weighted-Average Coupon	7.10%	[1]	7.30%	[2]
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	813,289		728,774
Total investment in loans, Unamortized (Discounts) Premiums	(22,428)		(25,807)
Carrying Amount	790,861		702,967
Number of Loans	60		50
Weighted-Average Coupon	6.30%	[1]	6.40%	[2]
Range of Maturity Dates, Minimum	Aug. 01, 2013		Mar. 01, 2013
Range of Maturity Dates, Maximum	Jul. 01, 2023		Jan. 01, 2023
Mezzanine Loans
Mortgage Loans on Real Estate [Line Items]
Unpaid Principal Balance	276,264		275,457
Total investment in loans, Unamortized (Discounts) Premiums	(3,933)		(4,355)
Carrying Amount	272,331		271,102
Number of Loans	80		83
Weighted-Average Coupon	9.40%	[1]	9.30%	[2]
Range of Maturity Dates, Minimum	Aug. 01, 2013		Mar. 01, 2013
Range of Maturity Dates, Maximum	Apr. 01, 2024		Nov. 01, 2038
Preferred Equity Interests
Mortgage Loans on Real Estate [Line Items]
Unpaid Principal Balance	64,753		64,752
Total investment in loans, Unamortized (Discounts) Premiums	(986)		(1,031)
Carrying Amount	63,767		63,721
Number of Loans	15		15
Weighted-Average Coupon	9.80%	[1]	9.70%	[2]
Range of Maturity Dates, Minimum	Mar. 01, 2014		Mar. 01, 2014
Range of Maturity Dates, Maximum	Aug. 01, 2025		Aug. 01, 2025
CRE Loans
Mortgage Loans on Real Estate [Line Items]
Unpaid Principal Balance	1,154,306		1,068,983
Total investment in loans, Unamortized (Discounts) Premiums	(27,347)		(31,193)
Carrying Amount	1,126,959		1,037,790
Number of Loans	155		148
Weighted-Average Coupon	7.20%	[1]	7.40%	[2]
Other loans
Mortgage Loans on Real Estate [Line Items]
Unpaid Principal Balance	35,019		38,600
Total investment in loans, Unamortized (Discounts) Premiums	(97)		(30)
Carrying Amount, Other loans	$ 34,922		$ 38,570
Number of Loans	2		2
Weighted-Average Coupon	4.60%	[1]	4.90%	[2]
Range of Maturity Dates, Minimum	Aug. 01, 2013		Mar. 01, 2013
Range of Maturity Dates, Maximum	Oct. 01, 2016		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] Weighted-average coupon is calculated on the unpaid principal amount of the underlying instruments which does not necessarily correspond to the carrying amount.