Securities Carried at Fair Value - Securities Carried at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Value, Option, Quantitative Disclosures [Line Items]
Cost	$ 6,479	$ 16,037
Fair Value	7,300	19,705
Common Shares of Beneficial Interest [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Cost	6,318	15,876
Fair Value	7,074	19,694
Loan Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Cost	161	161
Fair Value	$ 226	$ 11