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EQUITY (Details) (USD $)
 9 Months Ended
Jan. 31, 2013
Jan. 31, 2012
Jan. 20, 2012
Class of Stock [Line Items]      
Proceeds from issuance of Series B preferred units $ 111,357,000 $ 0  
Aggregate public offering price, maximum 150,000,000    
Aggregate gross sales price of common shares of beneficial interest, maximum     100,000,000
Proceeds from Issuance of Common Stock 2,448,000 2,970,000  
Common shares issued under 2008 Incentive Plan Award (in shares) 53,000 53,000  
Value of common shares issued under 2008 Incentive Plan Award 398,000 443,000  
Units converted to common shares (in shares) 203,000 759,000  
Units converted to common shares 846,000 3,500,000  
Shares issued 2,846,000 3,413,000  
Continuous equity offering program [Member]
      
Class of Stock [Line Items]      
Shares issued, shares (in shares) 300,000 366,000  
Proceeds from Issuance of Common Stock 2,100,000 2,700,000  
401(k) plan [Member]
      
Class of Stock [Line Items]      
Shares issued, shares (in shares) 43,000 52,000  
Shares issued 337,000 372,000  
Distribution Reinvestment and Share Purchase Plan [Member]
      
Class of Stock [Line Items]      
Shares issued, shares (in shares) 4,300,000 4,000,000  
Shares issued 34,100,000 28,800,000  
Public Offering [Member]
      
Class of Stock [Line Items]      
Proceeds from issuance of Series B preferred shares 111,400,000    
Per share amount of redeemable nonvoting stocks (in dollars per share) $ 25.00    
Per share amount of annual distributions accrued annually (in dollars per share) $ 1.9875    
Liquidation preference rate of preferred shares (in hundredths) 7.95%    
Liquidation preference (in dollars per share) $ 25.00    
Amount of liquidation preference 115,000,000    
Proceeds from issuance of Series B preferred units $ 4,600,000