Quarterly report pursuant to Section 13 or 15(d)

Shareholders' Equity (Details)

v2.4.0.6
Shareholders' Equity (Details) (USD $)
 9 Months Ended 0 Months Ended 3 Months Ended 0 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Dec. 31, 2011
Aug. 07, 2012
Series G Preferred Stock [Member]
Sep. 30, 2012
Series G Preferred Stock [Member]
Aug. 06, 2012
Series G Preferred Stock [Member]
Jun. 27, 2012
Public Offering
Series L Cumulative Preferred Stock
Jun. 27, 2012
Public Offering
Series L Preferred Units
Class of Stock [Line Items]                
Preferred shares issued to the public (in shares) 6,900,000 4,600,000         6,900,000  
Stock Issued During Period Per Share Value New Issues             $ 25.00  
Preferred Stock                
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           $ 25.00    
Stock Redeemed or Called During Period, Value $ (55,000,000)     $ 55,000,000        
Net proceeds from issuance of Series L preferred shares 165,662,000 0         165,700,000  
Series L and Series G cumulative preferred shares, redemption price per share             $ 25.00  
Preferred stock, liquidation preference value in aggregate 333,833,000   216,333,000       172,500,000  
Issuance costs associated with redeemed preferred shares         $ 1,800,000      
Annual dividend per share (in dollars per share)             $ 1.84375  
Series L cumulative preferred shares, annual dividend rate (paid quarterly)             7.375%  
Common Shares                
Number of common shares for each converted common unit (in shares) 94,550              
Common Stock, Conversion Basis for Common Unit 1