Real Estate and Accumulated Depreciation (Details 1) (USD $) In Thousands, unless otherwise specified					12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2011 Corporate Property [Member]	Dec. 31, 2010 Corporate Property [Member]	Dec. 31, 2009 Corporate Property [Member]	Dec. 31, 2011 Construction in Progress [Member]	Dec. 31, 2010 Construction in Progress [Member]	Dec. 31, 2009 Construction in Progress [Member]	Dec. 31, 2011 Real Estate [Member]	Dec. 31, 2010 Real Estate [Member]	Dec. 31, 2009 Real Estate [Member]
Additions during the period:
Total Property					$ 885	$ 316	$ 284	$ 105,685	$ 63,301	$ 85,120	$ 187,158	$ 337,223	$ 141,579
Accumulated Depreciation					1,429	740	784				85,440	72,825	67,680
Retirement/dispositions:
Total Property					(1,150)	(7)	(3)				(65,682)	(37,155)	(96,954)
Accumulated Depreciation					(1,146)	(7)	(3)				(48,507)	(11,801)	(23,395)
Real Estate, Gross, Beginning Balance	2,887,265	2,614,557	2,250,879	2,120,853
Real Estate Accumulated Depreciation, Beginning Balance	541,304	504,088	442,331	397,265
Real Estate, Gross, Ending Balance	2,887,265	2,614,557	2,250,879	2,120,853	14,675						2,801,602
Real Estate Accumulated Depreciation, Ending Balance	$ 541,304	$ 504,088	$ 442,331	$ 397,265