Related-Party Transactions - Fees and Reimbursements Paid by the Trust (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	$ 2,738	$ 2,463	$ 5,417	$ 4,606
Base Asset Management [Member]
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	2,291	2,298	4,557	4,325
Property Management [Member]
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	309	133	591	249
Construction Management [Member]
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	$ 138	$ 32	$ 269	$ 32