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 3 Months Ended 12 Months Ended
Dec. 31, 2010
Mar. 31, 2013
Mortagage_Note
Property
Dec. 31, 2012
Jan. 31, 2012
Dec. 31, 2010
Senior secured revolving credit facility [Member]
Mar. 31, 2013
Senior secured revolving credit facility [Member]
Mar. 31, 2013
Revolving Credit Facility [Member]
Mar. 31, 2013
Letter of Credit [Member]
Mar. 31, 2013
Mortgage Notes Payable [Member]
Mortagage_Note
Mortgage Notes Payable and Line of Credit (Additional Textual) [Abstract]                  
Number of fixed-rate mortgage notes payable                 31
Weighted average interest rate on all debt outstanding   5.44%             5.62%
Fair value of fixed-rate mortgage notes payable outstanding   $ 376,500,000              
Carrying value of fixed-rate mortgage notes payable outstanding   361,035,000 359,185,000            
Line of credit, outstanding 50,000,000       50,000,000 26,400,000      
Line of credit, maturity date         Dec. 28, 2013        
Line of credit at an interest rate   3.00%         3.00%    
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   65              
Number of long-term mortgages issued   1              
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   14              
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%         3.00%    
Line of credit facility, maximum additional amount drawn   $ 3,900,000