Discontinued Operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Discontinued Operations [Abstract]
Rental and other revenues									$ 20,812	$ 41,176	$ 52,490
Operating expenses:
Rental property and other expenses									8,283	14,456	19,412
Depreciation and amortization									5,753	11,970	15,647
Total operating expenses									14,036	26,426	35,059
Interest expense									0	283	489
Income from discontinued operations									6,776	14,467	16,942
Impairment of real estate assets									(2,194)	0	(2,429)
Net gains on disposition of discontinued operations									63,792	29,455	2,573
Total discontinued operations	12,362	39,460	14,092	2,460	2,384	26,179	5,742	9,617	68,374	43,922	17,086
Assets, held for sale [Abstract]
Land	0				41,875				0	41,875
Buildings and tenant improvements	0				204,525				0	204,525
Land held for development	0				2,368				0	2,368
Less - accumulated depreciation	0				(71,121)				0	(71,121)
Net real estate assets	0				177,647				0	177,647
Accrued straight-line rents receivable, net	0				7,591				0	7,591
Deferred leasing costs, net	0				6,059				0	6,059
Prepaid expenses and other assets	0				30				0	30
Real estate and other assets, net, held for sale	$ 0				$ 191,327				$ 0	$ 191,327
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					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					2
Tampa, FL Office Properties Disposition (12/2013) [Member]
Assets, held for sale [Abstract]
Number of office properties included in real estate and other assets, net, held for sale					2					2
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					5					5
Greenville, SC Office Properties Disposition (12/2013) [Member]
Assets, held for sale [Abstract]
Number of office properties included in real estate and other assets, net, held for sale					8					8