Real Estate Assets Real Estate Assets (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Acquisitions [Abstract]
Asset acquisition, contingent consideration liability	$ 3,300,000	$ 0	$ 0
Gain on acquisition of controlling interest in unconsolidated affiliate	0	7,451,000	0
Acquisition-related costs	500,000	1,800,000	1,500,000
Dispositions [Abstract]
Gains/(losses) on disposition of discontinued operations	384,000	63,792,000	29,455,000
Gains/(losses) on disposition of property	44,352,000	(3,000)	0
Impairments [Abstract]
Impairments of real estate assets	588,000	0	0
Orlando, FL Office Property Acquisition (12/2014) [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft)	246,000
Acquisition purchase price	67,400,000
Richmond, VA Office Property Acquisition (12/2014) [Member]
Acquisitions [Abstract]
Percentage of equity interest acquired in affiliate (in hundredths)	50.00%
Net rentable square feet of acquisition (in sqft)	66,000
Acquisition purchase price	4,200,000
Nashville, TN Land Acquisition (12/2014) [Member]
Acquisitions [Abstract]
Acquisition purchase price	15,800,000
Asset acquisition, contingent consideration liability	3,300,000
Raleigh, NC Office Property Acquisition (9/2014) [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft)	374,000
Acquisition purchase price	83,800,000
Atlanta, GA Office Properties Acquisition (9/2013) [Member]
Acquisitions [Abstract]
Percentage of equity interest acquired in affiliate (in hundredths)		57.00%
Number of office properties acquired		2
Net rentable square feet of acquisition (in sqft)		505,000
Acquisition purchase price		44,500,000
Fair value of debt assumed from the acquisition		37,600,000
Effective interest rate of secured debt assumed from the acquisition (in hundredths)		3.34%
Nashville, TN Office Property Acquisition (9/2013) [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft)		520,000
Acquisition purchase price		150,100,000
Orlando, FL Office Properties Acquisition (9/2013) [Member]
Acquisitions [Abstract]
Percentage of equity interest acquired in affiliate (in hundredths)		60.00%
Number of office properties acquired		5
Net rentable square feet of acquisition (in sqft)		1,300,000
Purchase price of acquisition of unconsolidated affiliate		112,800,000
Percentage of equity interest in joint venture, previously held (in hundredths)		40.00%
Fair value of assets recorded from the acquisition		188,000,000
Fair value of debt assumed from the acquisition		127,900,000
Effective interest rate of secured debt assumed from the acquisition (in hundredths)		3.11%
Previously held equity interest remeasured at fair value		75,200,000
Gain on acquisition of controlling interest in unconsolidated affiliate		7,500,000
Atlanta, GA Office Property Acquisition (6/2013) [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft)		553,000
Acquisition purchase price		140,100,000
Greensboro, NC Office Properties Acquisition (3/2013) [Member]
Acquisitions [Abstract]
Number of office properties acquired		2
Net rentable square feet of acquisition (in sqft)		195,000
Acquisition purchase price		30,800,000
Tampa, FL Office Properties Acquisition (3/2013) [Member]
Acquisitions [Abstract]
Number of office properties acquired		2
Net rentable square feet of acquisition (in sqft)		372,000
Acquisition purchase price		52,500,000
Memphis, TN Development Land Acquisition (3/2013) [Member]
Acquisitions [Abstract]
Acquisition purchase price		4,800,000
Atlanta, GA Office Property Acquisition (2012) [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft)			492,000
Acquisition purchase price			144,900,000
Pittsburgh PA Office Property Acquisition (2012) [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft)			616,000
Acquisition purchase price			91,200,000
Greensboro, NC Medical Office Properties Acquisition (2012) [Member]
Acquisitions [Abstract]
Number of office properties acquired			3
Acquisition purchase price			29,600,000
Common Units issued to acquire real estate assets (in shares)			66,864
Business combination, contingent consideration at fair value			700,000
Fair value of debt assumed from the acquisition			7,900,000
Effective interest rate of secured debt assumed from the acquisition (in hundredths)			4.06%
Nashville TN Development Land Acquisition (2012) [Member]
Acquisitions [Abstract]
Acquisition purchase price			15,000,000
Square footage zoned for future development			1,300,000
Cary, NC Office Property Acquisition (2012) [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft)			178,300
Acquisition purchase price			26,000,000
Atlanta, GA Industrial Property/Land Parcel Disposition (10/2014) [Member]
Dispositions [Abstract]
Purchase price of real estate	11,400,000
Gains/(losses) on disposition of property	1,700,000
Raleigh, NC Land Disposition (12/2014) [Member]
Dispositions [Abstract]
Number of land parcels sold	2
Purchase price of real estate	1,700,000
Gains/(losses) on disposition of property	500,000
Winston-Salem, NC Office Properties Disposition (12/2014) [Member]
Dispositions [Abstract]
Purchase price of real estate	9,900,000
Closing credits excluded for unfunded building and/or tenant improvements	1,600,000
Closing credits excluded for free rent	700,000
Gains/(losses) on disposition of property	(100,000)
Raleigh, NC Office Properties Disposition (9/2014) [Member]
Dispositions [Abstract]
Number of office properties sold	5
Purchase price of real estate	58,700,000
Gains/(losses) on disposition of property	11,700,000
Richmond, VA Office Properties Disposition (9/2014) [Member]
Dispositions [Abstract]
Number of office properties sold	11
Purchase price of real estate	40,700,000
Gains/(losses) on disposition of property	17,600,000
Greensboro, NC Office Properties Disposition (9/2014) [Member]
Dispositions [Abstract]
Number of office properties sold	6
Number of industrial properties sold	8
Purchase price of real estate	28,200,000
Closing credits excluded for unfunded building and/or tenant improvements	1,200,000
Closing credits excluded for free rent	400,000
Gains/(losses) on disposition of property	4,700,000
Greenville, SC Office Property Disposition (9/2014) [Member]
Dispositions [Abstract]
Purchase price of real estate	27,200,000
Closing credits excluded for unfunded building and/or tenant improvements	5,800,000
Closing credits excluded for free rent	1,800,000
Gains/(losses) on disposition of property	2,200,000
Atlanta, GA Land Disposition (6/2014) [Member]
Dispositions [Abstract]
Number of land parcels sold	2
Purchase price of real estate	9,500,000
Gains/(losses) on disposition of property	5,900,000
Greenville, SC Office Properties Disposition (12/2013) [Member]
Dispositions [Abstract]
Number of office properties sold		8
Purchase price of real estate		57,900,000
Closing credits for unfunded tenant improvements		100,000
Closing credits included for free rent		300,000
Gains/(losses) on disposition of discontinued operations		3,100,000
Tampa, FL Office Properties Disposition (12/2013) [Member]
Dispositions [Abstract]
Purchase price of real estate		11,500,000
Closing credits for unfunded tenant improvements		600,000
Gains/(losses) on disposition of discontinued operations		2,800,000
Atlanta, GA Office Property Dispositon (10/2013) [Member]
Dispositions [Abstract]
Purchase price of real estate		13,800,000
Gains/(losses) on disposition of discontinued operations		3,000,000
Winston-Salem, NC Office Property Disposition 2 (10/2013) [Member]
Dispositions [Abstract]
Purchase price of real estate		5,300,000
Gains/(losses) on disposition of discontinued operations		2,500,000
Winston-Salem, NC Office Properties Disposition (9/2013) [Member]
Dispositions [Abstract]
Number of office properties sold		4
Purchase price of real estate		6,200,000
Gains/(losses) on disposition of discontinued operations		100,000
Atlanta, GA Industrial Properties/Land Disposition (9/2013) [Member]
Dispositions [Abstract]
Number of industrial properties sold		16
Purchase price of real estate		91,600,000
Closing credits for unfunded tenant improvements		300,000
Closing credits included for free rent		300,000
Gains/(losses) on disposition of discontinued operations		36,700,000
Gains/(losses) on disposition of property		100,000
Tampa, FL Office Property Disposition (9/2013) [Member]
Dispositions [Abstract]
Purchase price of real estate		11,600,000
Gains/(losses) on disposition of discontinued operations		1,200,000
Atlanta, GA Industrial Property Disposition (6/2013) [Member]
Dispositions [Abstract]
Number of industrial properties sold		5
Purchase price of real estate		4,500,000
Closing credits included for free rent		100,000
Gains/(losses) on disposition of discontinued operations		100,000
Atlanta, GA Industrial Property/Land Disposition 2 (6/2013) [Member]
Dispositions [Abstract]
Number of industrial properties sold		6
Purchase price of real estate		38,700,000
Closing credits for unfunded tenant improvements		1,800,000
Closing credits included for free rent		1,300,000
Gains/(losses) on disposition of discontinued operations		13,200,000
Atlanta, GA Industrial Property Disposition 3 (6/2013) [Member]
Dispositions [Abstract]
Number of industrial properties sold		2
Purchase price of real estate		4,800,000
Gains/(losses) on disposition of discontinued operations		(100,000)
Orlando, FL Office Properties Disposition (3/2013) [Member]
Dispositions [Abstract]
Number of office properties sold		2
Purchase price of real estate		14,600,000
Closing credits for unfunded tenant improvements		800,000
Gains/(losses) on disposition of discontinued operations		(300,000)
Atlanta GA and Jackson MS Office Properties Disposition (2012) [Member]
Dispositions [Abstract]
Number of office properties sold			3
Purchase price of real estate			86,500,000
Gains/(losses) on disposition of discontinued operations			14,000,000
Gains/(losses) on disposition of property		1,500,000
Additional purchase price consideration			1,500,000
Nashville, TN Office Properties Disposition (2012) [Member]
Dispositions [Abstract]
Number of office properties sold			5
Purchase price of real estate			41,000,000
Gains/(losses) on disposition of discontinued operations			7,000,000
Pinellas County, FL Office Property Disposition (2012) [Member]
Dispositions [Abstract]
Purchase price of real estate			9,500,000
Gains/(losses) on disposition of discontinued operations			1,400,000
Kansas City, MO Office Property Disposition (2012) [Member]
Dispositions [Abstract]
Purchase price of real estate			6,500,000
Gains/(losses) on disposition of discontinued operations			1,900,000
Kansas City, MO Rental Residential Disposition (2012) [Member]
Dispositions [Abstract]
Residential units (in units)			96
Purchase price of real estate			11,000,000
Gains/(losses) on disposition of discontinued operations			5,100,000
Raleigh NC Residential Condominiums Disposition (2012) [Member]
Dispositions [Abstract]
Residential units (in units)			17
Purchase price of real estate			5,500,000
Gains/(losses) on disposition of discontinued operations			400,000
Greensboro, NC Office Property Impairment (6/2014) [Member]
Impairments [Abstract]
Impairments of real estate assets	600,000
Winston-Salem, NC Office Properties (6/2013) [Member]
Impairments [Abstract]
Number of impaired properties		4
Impairments of real estate assets		1,100,000
Atlanta, GA Industrial Properties (3/2013) [Member]
Impairments [Abstract]
Number of impaired properties		7
Impairments of real estate assets		$ 1,100,000