Quarterly report pursuant to Section 13 or 15(d)

Mortgage Notes Payable and Line of Credit (Details Textual)

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Mortgage Notes Payable and Line of Credit (Details Textual) (USD $)
 1 Months Ended 9 Months Ended 12 Months Ended 1 Months Ended
Dec. 31, 2010
Sep. 30, 2012
Mortagage_Note
Property
Sep. 30, 2011
Jan. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Senior secured revolving credit facility [Member]
Sep. 30, 2012
Senior secured revolving credit facility [Member]
Sep. 30, 2012
Letter of Credit [Member]
Sep. 30, 2012
University of Phoenix Mortgage [Member]
Sep. 30, 2012
Mortgage Notes Payable One [Member]
Sep. 30, 2012
Mortgage Notes Payable [Member]
Mortagage_Note
Feb. 29, 2012
Mortgage Notes Payables Twenty Three [Member]
Mortgage Notes Payable and Line of Credit (Additional Textual) [Abstract]                        
Number of fixed-rate mortgage notes payable                     28  
Weighted average interest rate on all debt outstanding                   5.65%    
Mortgage repayment   $ 5,544,000 $ 5,585,000                 $ 2,300,000
Original maturity date of mortgage                       2030-04
Fair value of fixed-rate mortgage notes payable outstanding                 348,500,000      
Carrying value of fixed-rate mortgage notes payable outstanding   337,931,000     285,350,000       337,900,000      
Total mortgage note issued                     45,200,000  
Line of Credit, outstanding 50,000,000         50,000,000 5,500,000          
Line of Credit, maturity date           Dec. 28, 2013            
Letters of credit, outstanding               6,100,000        
Mortgage Notes Payable and Line of Credit (Textual) [Abstract]                        
Number of properties collateralized in fixed-rate mortgage notes payable   69                    
Number of long-term mortgages issued   8                    
Standby letters of credit sublimit 20,000,000                      
Line of Credit, maximum borrowing capacity amended       75,000,000                
Number of properties pledged as collateral under Line of Credit   6                    
Line of Credit, variable spread on interest rate 3.00%                      
Line of Credit, annual maintenance fees, percentage   0.25%                    
Line of Credit covenant limit of distributions to shareholders percentage       95.00%                
Line of Credit, weighted average interest rate   3.00%                    
Line of credit facility , maximum amount drawn   $ 8,300,000