Acquisitions, Dispositions, Leasing and Financing Activities - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended				3 Months Ended	6 Months Ended		1 Months Ended		3 Months Ended	6 Months Ended			1 Months Ended		1 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Recourse Secured Financings [Member]	Dec. 31, 2012 Recourse Secured Financings [Member]	Jun. 30, 2013 1515 Market Street [Member]	Jun. 30, 2013 1515 Market Street [Member]	Jan. 31, 2013 1515 Market Street [Member]	Jun. 11, 2013 Deer Valley [Member]	Jun. 14, 2013 Fenway Shea [Member]	Jun. 30, 2013 Queensridge [Member]	Jun. 30, 2013 Queensridge [Member]	Jun. 30, 2013 Churchill [Member]	Jun. 28, 2013 Churchill [Member] First Mortgage [Member]	Apr. 30, 2013 1515 Market Street Loan Modification [Member] Original Mortgage [Member]	Apr. 25, 2013 1515 Market Street Loan Modification [Member] Original Mortgage [Member]	Apr. 30, 2013 1515 Market Street Loan Modification [Member] First Mortgage [Member]	Apr. 25, 2013 1515 Market Street Loan Modification [Member] First Mortgage [Member]	Jun. 30, 2013 Atrium Property [Member] Times sqft
Loans At Acquisition Date [Line Items]
Increased outstanding principal amount									$ 71,629,000
Maturity date of loan									Feb. 01, 2016
Mortgage loans on real estate adjusted interest rate minimum									7.50%
LIBOR plus									6.50%
Increased loan balance through future funding advances									6,000,000
Future advances									500,000
First mortgage loan of Trust									70,000,000						5,100,000				43,000,000
Interest rate increase for each future advances									0.10%
Percentage for participation									40.00%								40.00%
Acquired interest rate through indirect									49.00%								49.00%
Payment to acquire indirect interest								10,000
Revenue							2,632,000	4,493,000
Net income							576,000	1,072,000
Gross proceeds from sales										20,500,000
Net proceeds										19,585,000
Gain on sale of property	6,752,000		9,527,000							6,752,000
Principal amount of the loan											2,250,000
Amount of nor-performing mortage loan acquired																	33,157,000			10,650,000
Non-performing mortgage loan acquisition cost			21,437,000	44,096,000																6,625,000
Joint venture paid																				1,137,000
Investment by trust																				3,935,000
Area of collateralized building																				71,000
Lease expiration date																				2014-09
Number of Automatic Extensions for building lease																				6
Extension period for lease																				5 years
Loan receivable												14,960,000	25,077,000
Pay downs of recourse debt					13,653,000	23,770,000
Loan bears interest, description of variable rate basis																		LIBOR plus 2.0%
Maturity date of loan														Aug. 01, 2024				May 01, 2016
Mortgage loan, LIBOR rate																		0.50%
Loan proceeds received																		38,472,000
Interest rate on mortgage loan														3.50%		12.90%
Mortgage loans on real estate reduced investment amount																	21,098,000
Mortgage loans on real estate effective interest rate on investment																19.60%
Net proceeds														$ 4,922,000