Related-Party Transactions - Fees and Reimbursements Paid by the Trust (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	$ 2,725	$ 2,542	$ 8,142	$ 7,244
Base Asset Management [Member]
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	2,309	2,316	6,866	6,641
Property Management [Member]
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	304	193	895	506
Construction Management [Member]
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	$ 112	$ 33	$ 381	$ 97