Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Series B Preferred Shares [Member]	Dec. 31, 2010 Series B Preferred Shares [Member]	Dec. 31, 2011 Series C Preferred Shares [Member]	Dec. 31, 2010 Series C Preferred Shares [Member]	Dec. 31, 2011 Series D Preferred Shares [Member]	Dec. 31, 2010 Series D Preferred Shares [Member]	Dec. 31, 2011 Series E Preferred Shares [Member]	Dec. 31, 2010 Series E Preferred Shares [Member]
Rental properties, accumulated depreciation	$ 335,116,000	$ 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	48,062,593	47,769,422
Preferred Shares, par value	$ 0.01	$ 0.01
Preferred Shares, shares authorized	25,000,000	25,000,000
Preferred Shares, shares issued			0	3,200,000	5,400,000	5,400,000	4,600,000	4,600,000	3,450,000	3,450,000
Preferred Shares, liquidation preference			$ 80,000,000	$ 80,000,000	$ 135,000,000	$ 135,000,000	$ 115,000,000	$ 115,000,000	$ 86,250,000	$ 86,250,000
Treasury Shares, common shares	1,335,879	1,226,472