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Consolidated Statements of Stockholders' Equity (Parenthetical) - $ / shares
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Pursuant to dividend reinvestment plan, shares 201,212 197,638 475,162
Exercise of employee stock options and issuance of directors' deferred stock, shares 117,886 172,887 56,002
Issuance of partnership units pursuant to dividend reinvestment plan, shares 107,037 196,183 88,309
Series A Preferred Stock      
Redemption of preferred stock, shares   16,000 24,000
Series B Preferred Stock      
Redemption of preferred stock, shares     31,731
Series C Preferred Stock      
Issuance of common stock, shares   16,000 56,000
Dividend Declared      
Distributions payable common stock (in usd per share) $ 0.43 $ 0.40 $ 0.36
Distributions partnership units (in usd per share) 0.43 0.40 0.36
Dividend Declared | Series A Preferred Stock      
Preferred stock (in usd per share)     50.00
Dividend Declared | Series C Preferred Stock      
Preferred stock (in usd per share) $ 42.97 $ 42.97 $ 42.97