Investment and Disposition Activities - 2015 Transactions - Additional Information (Detail)								1 Months Ended	6 Months Ended	12 Months Ended
	Sep. 16, 2015 USD ($)	Jul. 01, 2015 USD ($)	Jun. 25, 2015 USD ($) Assets SecurityLoan	Jun. 01, 2015 USD ($)	Apr. 14, 2015 USD ($)	Feb. 05, 2015 USD ($)	Jan. 02, 2015 USD ($)	May 31, 2015 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2016 USD ($) Property	Dec. 31, 2015 USD ($)	Oct. 09, 2015 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Investment And Disposition Activities [Line Items]
Proceeds from sale of equity investments				$ 82,471,000
Distributions received									$ 4,959,000
Outstanding principal balance on loan receivable										$ 8,400,000	$ 5,280,000			$ 24,005,000	$ 101,100,000
Edens Center and Norridge Commons [Member]
Investment And Disposition Activities [Line Items]
Principal payment of loans						$ 15,275,000
Outstanding principal balance on loan receivable													$ 97,000
Loan participation interest percentage													30.00%
Minimum value of properties for entitlement to loan participation interest													$ 115,000,000
Description of Trust's participation interest in loan										Upon satisfaction of the loan, Winthrop was entitled to a participation interest equal to 30% of the value of both of the properties which collateralized the loan in excess of $115,000,000.
Loan receivable												$ 3,100,000
Aggregate price in sale of general partnership interest										$ 493,000
Number of properties sold | Property										2
Vintage Housing Holdings [Member]
Investment And Disposition Activities [Line Items]
Additional contribution in equity investments							$ 5,645,000
Liquidation value of investment														82,928,000
44 Monroe [Member] | Phoenix AZ [Member]
Investment And Disposition Activities [Line Items]
Ownership percentage in real estate investment					83.70%
Proceeds from sale of real estate, gross					$ 50,650,000
Proceeds from sale of real estate, net					$ 49,143,000
Liquidation value of property														50,650,000
Concord Debt Holdings LLC [Member]
Investment And Disposition Activities [Line Items]
Distributions received								$ 20,173,000
CDH CDO LLC [Member]
Investment And Disposition Activities [Line Items]
Proceeds from sale of equity investments			$ 54,122,000
Number of bond assets sold | Assets			4
Number of loan asset sold | SecurityLoan			1
Amount received from the venture		$ 6,200,000
Cerritos [Member] | California [Member]
Investment And Disposition Activities [Line Items]
Proceeds from sale of real estate, gross	$ 30,500,000
Proceeds from sale of real estate, net	$ 6,174,000
Liquidation value of property														$ 29,916,000
701 Seventh WRT Investor LLC [Member]
Investment And Disposition Activities [Line Items]
Additional capital contributions											8,865,000
Capital contributions											$ 115,489,000