Consolidated Balance Sheets (Parenthetical) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Real Estate Investment Property, Accumulated Depreciation	$ 635,535,000	$ 534,303,000
Common Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01
Common Stock, Shares Authorized	100,000,000
Common Stock, Shares, Issued	66,263,487	63,195,182
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	2,616,406	2,371,198
Series C Preferred Shares [Member]
Preferred Stock, Shares Issued	5,399,050	5,400,000
Auction Market Preferred Securities, Stock Series, Liquidation Value	$ 134,976,250	$ 135,000,000
Series E Preferred Shares [Member]
Preferred Stock, Shares Issued	3,450,000	3,450,000
Auction Market Preferred Securities, Stock Series, Liquidation Value	$ 86,250,000	$ 86,250,000
Series F Preferred Stock [Member]
Preferred Stock, Shares Issued	5,000,000	5,000,000
Auction Market Preferred Securities, Stock Series, Liquidation Value	$ 125,000,000	$ 125,000,000