Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Tenant and other receivables, allowance for doubtful accounts	$ 1,099	$ 1,142
Accrued rental income, allowance	1,126	1,499
Unsecured senior notes, discount	$ 11,899	$ 12,296
Excess stock, par value	$ 0.01	$ 0.01
Excess stock, shares authorized	150,000,000	150,000,000
Excess stock, shares issued	0	0
Excess stock, shares outstanding	0	0
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	50,000,000	50,000,000
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	250,000,000	250,000,000
Common stock, shares issued	153,481,007	153,192,845
Common stock, shares outstanding	153,402,107	153,113,945
Treasury common stock at cost, shares	78,900	78,900
Series B Cumulative Redeemable Preferred Stock [Member]
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	92,000	92,000
Series B Dividend Rate Percentage	5.25%	5.25%
Series B, Liquidation Preference Per Share (dollars per share)	$ 2,500.00	$ 2,500.00
Series B, shares issued	80,000	80,000
Series B, Shares Outstanding (in shares)	80,000	80,000