Consolidated Balance Sheets (Parenthetical) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Rental properties, accumulated depreciation	$ 511,949,000	$ 465,660,000
Common Shares, par value	$ 0.01	$ 0.01
Common Shares, shares authorized	75,000,000	75,000,000
Common Shares, shares issued	61,498,390	58,952,404
Preferred Shares, par value	$ 0.01	$ 0.01
Preferred Shares, shares authorized	25,000,000	25,000,000
Treasury Shares, common shares	2,340,474	1,826,463
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 E Preferred Shares [Member]
Preferred Shares, shares issued	3,450,000	3,450,000
Preferred Shares, liquidation preference	$ 86,250,000	$ 86,250,000
Series F Preferred Stock [Member]
Preferred Shares, shares issued	5,000,000	5,000,000
Preferred Shares, liquidation preference	$ 125,000,000	$ 125,000,000