Annual report pursuant to Section 13 and 15(d)

Shareholders' Equity (Details)

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Shareholders' Equity (Details) (USD $)
 1 Months Ended 12 Months Ended 0 Months Ended 3 Months Ended 12 Months Ended 0 Months Ended
Oct. 31, 2012
May 31, 2011
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Aug. 06, 2012
Series G Preferred Shares
Sep. 30, 2012
Series G Preferred Shares
Dec. 31, 2012
Preferred Shares
Dec. 31, 2012
Series H
Dec. 31, 2012
Series J
Dec. 31, 2012
Series K
Dec. 31, 2012
Series L Cumulative Preferred Stock
Jun. 27, 2012
Public Offering
Series L Preferred Units
Jun. 27, 2012
Public Offering
Series L Cumulative Preferred Stock
Preferred Stock                            
Number of preferred shares of beneficial interest authorized (in dollars per share)               $ 0.01            
Number of shares issued     12,821,667         25,000,000 2,000,000 3,390,000 531,667 6,900,000    
Aggregate Liquidation Preference     $ 333,833,000           $ 50,000,000 $ 84,750,000 $ 26,583,000 $ 172,500,000    
Annual Dividend Yield           8.00%     7.50% 7.625% 5.60% 7.375%    
Annual Dividend Per Share (in dollars per share)                 $ 1.87500 $ 1.90625 $ 2.80000 $ 1.84375    
Number of common shares to be issued on conversion of each preferred share                     0.8163      
Net proceeds from issuance of Series L preferred shares     165,652,000 0 0                 165,700,000
Number of Series L preferred units of operating partnerships received in exchange for offering proceeds (in units)                         6,900,000  
Preferred Stock, Redemption Price Per Share           $ 25                
Stock Redeemed or Called During Period, Value     (55,000,000)     55,000,000                
Issuance costs associated with redeemed preferred shares             1,800,000              
Common Shares                            
Common and preferred shares issued to the public (in shares) 8,625,000 4,600,000           6,900,000           6,900,000
Per share value of commonand preferred shares issued to the public (in dollars per share) $ 24.75 $ 33                       $ 25
Net proceeds from issue of shares after underwriter discounts but before offering expenses 204,900,000 145,700,000 165,652,000 145,367,000 245,621,000                  
At-market-stock, offering program established, aggregate value $ 150,000,000                          
Number of common shares for each converted common unit (in shares)     1                      
Number of operating parterships units converted into common shares (in units)     234,246 100,939 663,498                  
Dividends declared per common share     $ 1.1000 $ 1.65 $ 1.61