Related-Party Transactions - Fees and Reimbursements Paid by the Trust (Parenthetical) (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Related Party Transaction [Line Items]
Management fee paid	$ 10,912,000	$ 9,805,000	$ 8,228,000
FUR Advisors [Member]
Related Party Transaction [Line Items]
Management fee paid	$ 100,000	$ 139,000	$ 49,000