Fair Value Measurements (Details) (USD $)
3 Months Ended 6 Months Ended
Jun. 30, 2012
Property
Jun. 30, 2011
Property
Jun. 30, 2012
Property
Jun. 30, 2011
Property
Jun. 30, 2012
Fair value, measurements, recurring
Dec. 31, 2011
Fair value, measurements, recurring
Jun. 30, 2012
Fair value, measurements, recurring
Quoted prices in active markets for identical assets (Level 1)
Dec. 31, 2011
Fair value, measurements, recurring
Quoted prices in active markets for identical assets (Level 1)
Jun. 30, 2012
Fair value, measurements, recurring
Significant other observable inputs (Level 2)
Dec. 31, 2011
Fair value, measurements, recurring
Significant other observable inputs (Level 2)
Jun. 30, 2012
Fair value, measurements, recurring
Significant unobservable inputs (Level 3)
Dec. 31, 2011
Fair value, measurements, recurring
Significant unobservable inputs (Level 3)
Jun. 30, 2012
Estimate of fair value
Significant other observable inputs (Level 2)
Dec. 31, 2011
Estimate of fair value
Significant other observable inputs (Level 2)
Jun. 30, 2012
Estimate of fair value
Significant unobservable inputs (Level 3)
Jun. 30, 2012
Carrying value
Dec. 31, 2011
Carrying value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                                  
Investment in real estate assets, net, carrying amount before impairment                               $ 3,800,000  
Number of properties impaired 1 0 1 0                          
Investment in real estate assets         1,751,000   0   0   1,751,000       1,800,000    
Impairment of real estate assets 1,979,000 0 1,979,000 0                          
Mortgage notes receivable                         86,600,000 85,300,000   75,100,000 76,700,000
Notes payable and line of credit                         1,800,000,000 1,800,000,000   1,700,000,000 1,800,000,000
Liabilities:                                  
Interest rate swaps         $ 2,770,000 $ 3,558,000 $ 0 $ 0 $ 2,770,000 $ 3,558,000 $ 0 $ 0