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Consolidated Statements of Equity (Parenthetical) - $ / shares
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Shares issued pursuant to dividend reinvestment plan (in shares) 572,928 266,011 186,797
Shares due to exercise of employee stock options and issuance of directors' deferred stock (in shares) 43,150 152,758 251,323
Issuance of partnership units pursuant to dividend reinvestment plan (in shares) 107,433 111,351 124,758
Issuance of partnership units (in shares) 284,113    
Series D Cumulative Redeemable Preferred Stock      
Issuance of Series D Cumulative preferred stock (in shares) 30,000    
Series C Cumulative Preferred Stock      
Redemption of Series C Cumulative preferred stock (in shares) 30,000    
Dividend declared      
Distributions payable common stock and partnership units (in usd per share) $ 0.53 $ 0.52 $ 0.51
Dividend declared | Series D Cumulative Redeemable Preferred Stock      
Distributions payable on preferred stock (in usd per share) 38.28    
Dividend declared | Series C Cumulative Preferred Stock      
Distributions payable on preferred stock (in usd per share) $ 42.97 $ 42.97 $ 42.97