Investments in Joint Ventures (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Condensed combined balance sheets
Gross Real Estate Investment			$ 41,258
Less: Accumulated Depreciation	(658,729)	(509,634)
Net Investment in Real Estate	2,333,367	2,109,133
Total Assets	2,666,657	2,750,054
Debt	1,483,803
Total Liabilities and Equity	2,666,657	2,750,054
Carrying Value of the Company's investments in Joint Ventures	1,674	2,451
Corporate Joint Venture [Member]
Condensed combined balance sheets
Gross Real Estate Investment	155,555	210,567
Less: Accumulated Depreciation	(41,342)	(47,286)
Net Investment in Real Estate	114,213	163,281
Other Assets	23,364	33,351
Total Assets	137,577	196,632
Debt	112,261	157,431
Other Liabilities	5,779	10,849
Equity	19,537	28,352
Total Liabilities and Equity	137,577	196,632
Company's share of Equity	3,029	4,344
Basis Differentials	(1,564)	(2,089)
Carrying Value of the Company's investments in Joint Ventures	$ 1,465	$ 2,255