Fair Value Measurements - Trust's Financial Assets for Which the Fair Value Option Was Elected (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Financial instruments at fair value	$ 10,586	$ 19,705
Securities Carried at Fair Value [Member] | REIT Common Shares [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Financial instruments at fair value	10,360	19,694
Loan Securities Carried at Fair Value [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Financial instruments at fair value	$ 226	$ 11