Property Transactions (Gain (Loss) on Sales of Discontinued Operations) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Document Fiscal Year Focus	2013
Gain on sale of investment properties included in discontinued operations
Gain on sale of discontinued investment properties	$ 11,489	$ 18,407	$ 8,519
Third party management and leasing business [Member]
Gain on sale of investment properties included in discontinued operations
Gain on sale of discontinued investment properties	4,577	7,459	0
Tiffany Springs MarketCenter Kansas City [Member]
Gain on sale of investment properties included in discontinued operations
Gain on sale of discontinued investment properties	3,697	0	0
Inhibitex Suburban Atlanta [Member]
Gain on sale of investment properties included in discontinued operations
Gain on sale of discontinued investment properties	2,989	0	0
King Mill Distribution Park - Building 3 [Member]
Gain on sale of investment properties included in discontinued operations
Gain on sale of discontinued investment properties	275	307	4,977
Lakeside Ranch Business Park - Building 20 [Member]
Gain on sale of investment properties included in discontinued operations
Gain on sale of discontinued investment properties	25	(59)	1,121
Cosmopolitan Center [Member]
Gain on sale of investment properties included in discontinued operations
Gain on sale of discontinued investment properties	0	2,064	0
Galleria 75 [Member]
Gain on sale of investment properties included in discontinued operations
Gain on sale of discontinued investment properties	0	569	0
One Georgia Center [Member]
Gain on sale of investment properties included in discontinued operations
Gain on sale of discontinued investment properties	0	(104)	2,805
Jefferson Mill Business Park - Building A [Member]
Gain on sale of investment properties included in discontinued operations
Gain on sale of discontinued investment properties	5	0	(394)
Avenue Webb Gin Suburban Atlanta [Member]
Gain on sale of investment properties included in discontinued operations
Gain on sale of discontinued investment properties	(2)	3,590	0
The Avenue Forsyth Suburban Atlanta, GA [Member]
Gain on sale of investment properties included in discontinued operations
Gain on sale of discontinued investment properties	(77)	4,508	0
Other Disposal Groups [Member]
Gain on sale of investment properties included in discontinued operations
Gain on sale of discontinued investment properties	$ 0	$ 73	$ 10