Related-Party Transactions - Fees and Reimbursements Paid by the Trust (Detail) - Winthrop Management [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	$ 7,471	$ 10,812	$ 10,912
Base Asset Management Fee [Member]
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	6,367	9,040	9,289
Property Management Fee [Member]
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	974	1,394	1,226
Construction Management Fee [Member]
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	$ 130	$ 378	$ 397