Consolidated Balance Sheets (Parenthetical) (USD $)	Jun. 30, 2011	Dec. 31, 2010
Rental properties, accumulated depreciation	$ 316,580,000	$ 296,784,000
Common Shares, par value	$ 0.01	$ 0.01
Common Shares, shares authorized	75,000,000	75,000,000
Common Shares, shares issued	47,992,569	47,769,422
Preferred Shares, par value	$ 0.01	$ 0.01
Preferred Shares, shares authorized	25,000,000	25,000,000
Treasury Shares, common shares	1,335,879	1,226,472
Series B Preferred Shares [Member]
Preferred Shares, shares issued	3,200,000	3,200,000
Preferred Shares, liquidation preference	80,000,000	80,000,000
Series C Preferred Shares [Member]
Preferred Shares, shares issued	5,400,000	5,400,000
Preferred Shares, liquidation preference	135,000,000	135,000,000
Series D Preferred Shares[Member]
Preferred Shares, shares issued	4,600,000	4,600,000
Preferred Shares, liquidation preference	115,000,000	115,000,000
Series E Preferred Shares [Member]
Preferred Shares, shares issued	3,450,000	3,450,000
Preferred Shares, liquidation preference	$ 86,250,000	$ 86,250,000