Real Estate Assets (Details) (USD $)	3 Months Ended		6 Months Ended						3 Months Ended
	Jun. 30, 2013 acre	Jun. 30, 2012	Jun. 30, 2013 acre	Jun. 30, 2012	Jun. 30, 2013 Atlanta, GA Office Property Acquisition (6/2013) [Member] sqft	Mar. 31, 2013 Tampa, FL Office Properties Acquisition (3/2013) [Member] property sqft	Mar. 31, 2013 Greensboro, NC Office Properties Acquisition (3/2013) [Member] sqft property	Mar. 31, 2013 Memphis, TN Development Land Acquisition (3/2013) [Member] acre	Jun. 30, 2013 Atlanta, GA Industrial Property Disposition (6/2013) [Member] property	Jun. 30, 2013 Atlanta, GA Industrial Property/Land Disposition 2 (6/2013) [Member] property	Jun. 30, 2013 Atlanta, GA Industrial Property Disposition 3 (6/2013) [Member] property	Mar. 31, 2013 Orlando, FL Office Properties Disposition (3/2013) [Member] property	Mar. 31, 2013 Jackson, MS Office Property Disposition (9/2012) [Member]	Jun. 30, 2013 Winston-Salem, NC Office Properties (6/2013) [Member] property	Mar. 31, 2013 Atlanta, GA Industrial Properties (3/2013) [Member] property	Mar. 31, 2013 Atlanta, GA Industrial Properties Held For Sale (3/2013) [Member] property
Acquisitions [Abstract]
Number of office properties acquired						2	2
Rentable square feet of acquisition (in sq. ft)					553,000	372,000	195,000
Acquisition development land (in acres)	634		634					5
Acquisition purchase price					$ 140,100,000	$ 52,500,000	$ 30,800,000	$ 4,800,000
Acquisition-related costs	400,000		900,000
Dispositions [Abstract]
Number of office properties sold												2
Number of industrial real estate properties sold									5	6	2
Proceeds received from sale of real estate									4,500,000	38,700,000	4,800,000	14,600,000
Closing credits for unfunded tenant improvements										1,800,000		800,000
Closing credits for free rent									100,000	1,300,000
Gain/(loss) on disposition of discontinued operations	13,163,000	1,385,000	13,694,000	6,519,000					100,000	13,200,000	(100,000)	(300,000)	1,500,000
Impairments [Abstract]
Number of impaired properties														4	2	5
Impairments of real estate assets	1,066,000	0	1,066,000	0										1,100,000	400,000
Impairments of real estate assets held for sale																$ 700,000