Consolidated Balance Sheets (Parenthetical) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Rental properties, accumulated depreciation	$ 776,404,000	$ 741,334,000
Common Shares, par value	$ 0.01	$ 0.01
Common Shares, shares authorized	100,000,000	100,000,000
Common Shares, shares issued	77,161,235	76,858,632
Preferred Shares, par value	$ 0.01	$ 0.01
Preferred Shares, shares authorized	25,000,000	25,000,000
Treasury Shares, common shares	2,842,294	2,733,552
Series C Preferred Shares [Member]
Preferred Shares, shares issued	5,399,050	5,399,050
Preferred Shares, liquidation preference	$ 134,976,250	$ 134,976,250
Series E Preferred Shares [Member]
Preferred Shares, shares issued	3,447,381	3,449,115
Preferred Shares, liquidation preference	$ 86,184,525	$ 86,227,875
Series G Preferred Stock [Member]
Preferred Shares, shares issued	6,000,000	6,000,000
Preferred Shares, liquidation preference	$ 150,000,000	$ 150,000,000