Securities Carried at Fair Value - Securities Carried at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Option, Quantitative Disclosures [Line Items]
Cost	$ 9,081	$ 16,037
Fair Value	10,586	19,705
Common Shares of Beneficial Interest [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Cost	8,920	15,876
Fair Value	10,360	19,694
Loan Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Cost	161	161
Fair Value	$ 226	$ 11