Related-Party Transactions - Fees and Reimbursements Paid by the Trust (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	$ 10,912,000	$ 9,805,000	$ 8,228,000
Base Asset Management Fee [Member]
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	9,289,000	8,953,000	7,690,000
Property Management Fee [Member]
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	1,226,000	724,000	537,000
Construction Management Fee [Member]
Related Party Transaction [Line Items]
Fees and reimbursements paid by the Trust	$ 397,000	$ 128,000	$ 1,000