Consolidated Balance Sheets (Parenthetical) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Real Estate Investment Property, Accumulated Depreciation	$ 741,334,000	$ 635,535,000
Common Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01
Common Stock, Shares Authorized	100,000,000	100,000,000
Common Stock, Shares, Issued	76,858,632	66,263,487
Preferred Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01
Preferred Stock, Shares Authorized	25,000,000	25,000,000
Treasury Stock, Shares	2,733,552	2,616,406
Series C Preferred Shares [Member]
Preferred Stock, Value, Issued	$ 54,000	$ 54,000
Preferred Stock, Shares Issued	5,399,050	5,399,050
Auction Market Preferred Securities, Stock Series, Liquidation Value	$ 134,976,250	$ 134,976,250
Series E Preferred Shares [Member]
Preferred Stock, Value, Issued	$ 34,000	$ 35,000
Preferred Stock, Shares Issued	3,449,115	3,450,000
Auction Market Preferred Securities, Stock Series, Liquidation Value	$ 86,227,875	$ 86,250,000
Series F Preferred Stock [Member]
Preferred Stock, Value, Issued	$ 0	$ 50,000
Preferred Stock, Shares Issued	0	5,000,000
Auction Market Preferred Securities, Stock Series, Liquidation Value	$ 0	$ 125,000,000
Series G Preferred Stock [Member]
Preferred Stock, Value, Issued	$ 60,000	$ 0
Preferred Stock, Shares Issued	6,000,000	0
Auction Market Preferred Securities, Stock Series, Liquidation Value	$ 150,000,000	$ 0