Property Transactions (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Gain on sale of investment properties included in discontinued operations
Gain on sale of investment properties	$ 8,519	$ 7,226	$ 147
Gain on extinguishment of debt			12,498
King Mill Distribution Park Building 3 [Member]
Gain on sale of investment properties included in discontinued operations
Gain on sale of investment properties	4,977
One Georgia Center [Member]
Gain on sale of investment properties included in discontinued operations
Gain on sale of investment properties	2,805
Lakeside Ranch Business Park Building Twenty [Member]
Gain on sale of investment properties included in discontinued operations
Gain on sale of investment properties	1,121
Jefferson Mill Business Park Building A [Member]
Gain on sale of investment properties included in discontinued operations
Gain on sale of investment properties	(394)
San Jose Market Center [Member]
Gain on sale of investment properties included in discontinued operations
Gain on sale of investment properties	10	6,572
8995 Westside Parkway [Member]
Gain on sale of investment properties included in discontinued operations
Gain on sale of investment properties		654
Other [Member]
Gain on sale of investment properties included in discontinued operations
Gain on sale of investment properties			$ 147