Investments in Unconsolidated Companies (Summarized Financial Information) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Equity Method Investments and Joint Ventures [Abstract]
Rental revenue	$ 272,937	$ 228,378	$ 254,787
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	10,709	19,202	9,760
Land, buildings and tenant improvements, net	2,051,412	1,687,228
Construction in progress	12,208	120,834
Undeveloped land	177,742	177,473
Other assets	309,409	242,461
Total assets	2,550,771	2,227,996
Indebtedness	1,317,554	1,082,823
Other liabilities	71,241	66,471
Total liabilities	1,388,795	1,149,294
Owners' equity	1,161,976	1,078,702
Total liabilities and equity	$ 2,550,771	$ 2,227,996