Consolidated Balance Sheets (Parenthetical) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Real Estate Investment Property, Accumulated Depreciation	$ 883,174,000	$ 741,334,000
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	77,226,443	76,858,632
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	2,878,587	2,733,552
Series C Preferred Shares [Member]
Preferred Stock, Value, Issued	$ 54,000	$ 54,000
Preferred Stock, Shares Issued	5,394,050	5,399,050
Auction Market Preferred Securities, Stock Series, Liquidation Value	$ 134,851,250	$ 134,976,250
Series E Preferred Shares [Member]
Preferred Stock, Value, Issued	$ 34,000	$ 34,000
Preferred Stock, Shares Issued	3,447,381	3,449,115
Auction Market Preferred Securities, Stock Series, Liquidation Value	$ 86,184,525	$ 86,227,875
Series G Preferred Stock [Member]
Preferred Stock, Value, Issued	$ 60,000	$ 60,000
Preferred Stock, Shares Issued	6,000,000	6,000,000
Auction Market Preferred Securities, Stock Series, Liquidation Value	$ 150,000,000	$ 150,000,000