Related-Party Transactions - Fees and Reimbursements Paid by the Trust (Detail) - Winthrop Management [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	$ 1,475	$ 1,962	$ 3,079	$ 3,961
Base Asset Management Fee [Member]
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	1,298	1,635	2,718	3,319
Property Management Fee [Member]
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	174	261	358	545
Construction Management Fee [Member]
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	$ 3	$ 66	$ 3	$ 97