Schedule III - Real Estate and Accumulated Depreciation (Details Textual) (USD $)						12 Months Ended
	Dec. 31, 2012 property	Dec. 31, 2011 property	Dec. 31, 2010 property	Dec. 31, 2012 Mortgage notes payable [Member] 5.000%-7.625% Mortgage Notes Payable [Member]	Dec. 31, 2011 Mortgage notes payable [Member] 5.000%-7.625% Mortgage Notes Payable [Member]	Dec. 31, 2012 Building and improvements [Member] Minimum [Member]	Dec. 31, 2012 Building and improvements [Member] Maximum [Member]	Dec. 31, 2012 Lease intangibles [Member] Minimum [Member]	Dec. 31, 2012 Lease intangibles [Member] Maximum [Member]	Dec. 31, 2012 Personal property [Member] Minimum [Member]	Dec. 31, 2012 Personal property [Member] Maximum [Member]	Dec. 31, 2012 Land improvements [Member] Minimum [Member]	Dec. 31, 2012 Land improvements [Member] Maximum [Member]
Real Estate and Accumulated Depreciation (Textual) [Abstract]
Number of real estate assets held for sale	1	15	11
Gross value of assets held for sale	$ 9,600,000	$ 52,800,000	$ 43,000,000
Accumulated depreciation on real estate assets held for sale	6,300,000	24,600,000	19,500,000
Estimated aggregate total cost of total assets for federal income tax purposes	2,800,000,000	2,700,000,000	2,500,000,000
Asset estimated useful life						3 years 3 months 18 days	39 years	2 years	93 years 1 month 6 days	3 years	15 years 9 months 18 days	15 years	38 years 1 month 6 days
Unaccreted discount				$ 2,755,000	$ 3,728,000