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Stockholders' Equity (Details) (USD $)
 3 Months Ended 12 Months Ended
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2010
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Class of Stock [Line Items]                
Common stock, shares outstanding 148,107,611       140,199,105 148,107,611 140,199,105  
Dividend paid on common stock   $ 0.50 $ 0.50 $ 0.50 $ 0.50      
Dividend shareholder of record date Dec. 31, 2011 Sep. 30, 2011 Jun. 30, 2011 Mar. 31, 2011 Dec. 31, 2010      
Dividends payable, date declared Dec. 14, 2011              
Dividends payable, amount per share $ 0.55         $ 0.55    
Dividend payable date Jan. 27, 2012 Oct. 31, 2011 Jul. 29, 2011 Apr. 29, 2011 Jan. 28, 2011      
At the market stock offering program, shares issued 148,186,511       140,278,005 148,186,511 140,278,005  
At the market stock offering program, aggregate value of common stock           $ 400,000,000    
At the market stock offering program, net proceeds           439,037,000    
Shares of its Common Stock in connection with the redemption of an equal number of OP Units           2,919,323 591,900  
Shares of Common Stock upon the exercise of options to purchase Common Stock by certain employees           316,159 638,957 242,507
Initial Atm Program [Member]
                
Class of Stock [Line Items]                
At the market stock offering program, shares issued 4,228,993         4,228,993    
At market stock offering program remained available for issuance           0    
At the market stock offering program maximum length of sale (years)           3    
At the market stock offering program, gross proceeds           400,000,000    
At the market stock offering program, net proceeds           394,700,000    
Second Atm Program [Member]
                
Class of Stock [Line Items]                
At the market stock offering program, shares issued 431,223         431,223    
At market stock offering program remained available for issuance           555,100,000    
At the market stock offering program, aggregate value of common stock           600,000,000    
At the market stock offering program maximum length of sale (years)           3    
At the market stock offering program, gross proceeds           44,900,000    
At the market stock offering program, net proceeds           $ 44,300,000