Related-Party Transactions - Fees and Reimbursements Paid by the Trust (Detail) - Winthrop Management [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	$ 1,798	$ 2,715	$ 5,759	$ 8,335
Base Asset Management Fee [Member]
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	1,540	2,296	4,859	7,070
Property Management Fee [Member]
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	248	381	793	1,038
Construction Management Fee [Member]
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	$ 10	$ 38	$ 107	$ 227