Consolidated Balance Sheets (Parenthetical) (USD $)	Jun. 30, 2014	Dec. 31, 2013
Rental properties, accumulated depreciation	$ 439,242,000	$ 409,643,000
Common Shares, par value	$ 0.01	$ 0.01
Common Shares, shares authorized	75,000,000	75,000,000
Common Shares, shares issued	55,254,911	53,361,261
Preferred Shares, par value	$ 0.01	$ 0.01
Preferred Shares, shares authorized	25,000,000	25,000,000
Treasury Shares, common shares	1,784,058	1,706,109
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