Condensed Consolidated Balance Sheets (Parenthetical) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Series D Depositary Share [Member]	Dec. 31, 2012 Series D Depositary Share [Member]	Sep. 30, 2013 Series E Depositary Share [Member]	Dec. 31, 2012 Series E Depositary Share [Member]
Receivables, allowance	$ 2,936,000	$ 855,000
Accrued rental income allowance	3,086,000	3,270,000
Accumulated amortization - debt costs	19,556,000	17,965,000
Unamortized premium - mortgages payable	144,000	187,000
Unamortized discount - notes payable - convertible	0	2,072,000
Unamortized discount - notes payable	$ 11,116,000	$ 9,338,000
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	15,000,000	15,000,000
Series D, depositary shares issued (in shares)			11,500,000	0	11,500,000	0
Series D, depositary shares outstanding (in shares)			11,500,000	0	11,500,000	0
Series D, stated liquidation value per share (in dollars per share)			$ 25	$ 0	$ 25.0	$ 0
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	375,000,000	375,000,000
Common stock, shares issued (in shares)	121,856,503	111,554,997
Common stock, shares outstanding (in shares)	121,856,503	111,554,997
Excess stock, par value (in dollars per share)	$ 0.01	$ 0.01
Excess stock, shares authorized (in shares)	390,000,000	390,000,000
Excess stock, shares issued (in shares)	0	0
Excess stock, shares outstanding (in shares)	0	0