Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012 Property	Dec. 31, 2011	Dec. 31, 2010
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Receivable measured at fair value	$ 2,972,000
Impairment charges on real estate assets	4,967,000	19,214,000
Number of real estate properties held held for sale	111
Real estate held for sale, net	25,340,000
Impairment charges	13,942,000	20,226,000
Assumed holding periods for unobservable inputs	15 years
Assumed annual average rent for unobservable inputs	2.00%
Rate of income capitalization for unobservable inputs	8.00%
Cash flows discounted rate for unobservable inputs	7.00%
Maturity period of liquid investments	3 months
Stock based compensation expenses	757,000	643,000	480,000
Fair Value, Measurements, Recurring [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Receivable measured at fair value	$ 2,972,000
USTs [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Property plant and equipment useful life	10 years
Minimum [Member] | Building and Building Improvements [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Property plant and equipment useful life	16 years
Maximum [Member] | Building and Building Improvements [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Property plant and equipment useful life	25 years