Quarterly report pursuant to Section 13 or 15(d)

Shareholders' Equity (Details)

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Shareholders' Equity (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
 3 Months Ended 6 Months Ended 0 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Dec. 31, 2011
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
Net proceeds from issuance of Series L preferred shares     $ 165,902 $ 0   $ 165,700  
Series L cumulative preferred shares, redemption price per share           $ 25.00  
Preferred stock, liquidation preference value in aggregate $ 388,833   $ 388,833   $ 216,333 $ 172,500  
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)     54,550        
Common Stock, Conversion Basis for Common Unit     1        
Dividends declared per common share (in dollars per share) $ 0.275 $ 0.4125 $ 0.55 $ 0.825