Condensed Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Receivables, allowance	$ 1,353	$ 855
Accrued rental income allowance	3,154	3,270
Accumulated amortization - debt costs	18,002	17,965
Unamortized premium - mortgages payable	173	187
Unamortized discount - notes payable - convertible	951	2,072
Unamortized discount - notes payable	$ 9,103	$ 9,338
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	15,000,000	15,000,000
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)	116,891,268	111,554,997
Common stock, shares outstanding (in shares)	116,891,268	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
Series D Depositary Share [Member]
Series D, depositary shares issued (in shares)	11,500,000	0
Series D, depositary shares outstanding (in shares)	11,500,000	0
Series D, stated liquidation value per share (in dollars per share)	$ 25	$ 0