Consolidated Statements of Stockholders' Equity (Parenthetical) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Issuance of common stock, shares	531,164	753,607	734,786
Restricted shares			186,968
Pursuant to dividend reinvestment plan, shares	475,162	595,388	498,248
Exercise of employee stock options and issuance of directors' deferred stock, shares	56,002	158,219	49,570
Partnership units, shares			1,497,814
Issuance of partnership units pursuant to dividend reinvestment plan, shares	88,309
Series A Preferred Stock
Redemption of preferred stock, shares	24,000
Series B Preferred Stock
Redemption of preferred stock, shares	31,731
Series C Preferred Stock
Issuance of common stock, shares	56,000
Dividend Declared
Distributions payable common stock (in usd per share)	$ 0.36	$ 0.36	$ 0.36
Distributions partnership units (in usd per share)	$ 0.36	$ 0.36	$ 0.36
Dividend Declared | Series A Preferred Stock
Preferred stock (in usd per share)	$ 50.00	$ 50.00	$ 50.00
Dividend Declared | Series B Preferred Stock
Preferred stock (in usd per share)		$ 56.25	$ 56.25
Dividend Declared | Series C Preferred Stock
Preferred stock (in usd per share)	$ 42.97