Discontinued Operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Discontinued operations [Abstract]
Rental and other revenues	$ 2,810	$ 5,671	$ 10,398	$ 24,211
Operating expenses:
Rental property and other expenses	1,049	1,622	3,389	7,796
Depreciation and amortization	665	1,610	3,166	7,108
Total operating expenses	1,714	3,232	6,555	14,904
Interest expense	0	35	0	283
Income from discontinued operations	1,096	2,404	3,843	9,024
Impairment of real estate assets	0	0	(2,194)	0
Net gains on disposition of discontinued operations	37,946	22,936	52,353	29,455
Total discontinued operations	39,042	25,340	54,002	38,479
Assets, held for sale [Abstract]
Land	1,948		1,948		28,598
Buildings and tenant improvements	18,498		18,498		132,361
Land held for development	0		0		2,368
Less accumulated depreciation	(5,931)		(5,931)		(43,730)
Net real estate assets	14,515		14,515		119,597
Accrued straight line rents receivable	1,187		1,187		5,759
Deferred leasing costs, net	614		614		4,014
Prepaid expenses and other assets	0		0		30
Real estate and other assets, net, held for sale	$ 16,316		$ 16,316		$ 129,400
Winston-Salem, NC Office Properties Disposition (9/2013) [Member]
Assets, held for sale [Abstract]
Number of office properties included in real estate and other assets, net, held for sale	4		4		4
Atlanta, GA Industrial Properties/Land Disposition (9/2013) [Member]
Assets, held for sale [Abstract]
Number of industrial properties included in real estate and other assets, net, held for sale					29
Orlando, FL Office Properties Disposition (3/2013) [Member]
Assets, held for sale [Abstract]
Number of office properties included in real estate and other assets, net, held for sale					2