Real Estate Assets Real Estate Assets (Details) ft² in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) ft² property	Dec. 31, 2014 USD ($) ft² property	Dec. 31, 2013 USD ($) ft² property	Dec. 31, 2012 USD ($)
Acquisitions [Abstract]
Asset acquisition, contingent consideration liability	$ 900	$ 3,300	$ 0
Gain on acquisition of controlling interest in unconsolidated affiliate	0	0	7,451
Acquisition-related costs	1,000	500	1,800
Dispositions [Abstract]
Gains/(losses) on disposition of discontinued operations	0	384	63,792
Gains/(losses) on disposition of property	11,444	44,352	(3)
Impairments [Abstract]
Impairments of real estate assets	$ 0	$ 588	$ 0
Tampa, FL Building Acquisition (09/2015) [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft) | ft²	528
Acquisition purchase price	$ 113,500
Tampa, FL Land Acquisition (09/2015) [Member]
Acquisitions [Abstract]
Acquisition purchase price	$ 2,200
Atlanta, GA Building Acquisition (09/2015) [Member]
Acquisitions [Abstract]
Number of office properties acquired | property	2
Net rentable square feet of acquisition (in sqft) | ft²	896
Acquisition purchase price	$ 290,300
Atlanta, GA Land Acquisition [Member] (6/2015) [Member]
Acquisitions [Abstract]
Acquisition purchase price	5,200
Asset acquisition, contingent consideration liability	$ 900
Orlando, FL Building Acquisition [Member] (6/2015) [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft) | ft²	168
Percentage of equity interest acquired in affiliate (in hundredths)	77.20%
Fair value of debt assumed from the acquisition	$ 19,300
Orlando, FL Office Property Acquisition (12/2014) [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft) | ft²		246
Acquisition purchase price		$ 67,400
Richmond, VA Office Property Acquisition (12/2014) [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft) | ft²		66
Percentage of equity interest acquired in affiliate (in hundredths)		50.00%
Acquisition purchase price		$ 4,200
Nashville, TN Land Acquisition (12/2014) [Member]
Acquisitions [Abstract]
Acquisition purchase price		15,800
Asset acquisition, contingent consideration liability		$ 3,300
Raleigh, NC Office Property Acquisition (9/2014) [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft) | ft²		374
Acquisition purchase price		$ 83,800
Atlanta, GA Office Properties Acquisition (9/2013) [Member]
Acquisitions [Abstract]
Number of office properties acquired | property			2
Net rentable square feet of acquisition (in sqft) | ft²			505
Percentage of equity interest acquired in affiliate (in hundredths)			57.00%
Acquisition purchase price			$ 44,500
Fair value of debt assumed from the acquisition			$ 37,600
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) | ft²			520
Acquisition purchase price			$ 150,100
Orlando, FL Office Properties Acquisition (9/2013) [Member]
Acquisitions [Abstract]
Number of office properties acquired | property			5
Net rentable square feet of acquisition (in sqft) | ft²			1,300
Percentage of equity interest acquired in affiliate (in hundredths)			60.00%
Purchase price of acquisition of unconsolidated affiliate			$ 112,800
Percentage of equity interest in joint venture, previously held (in hundredths)			40.00%
Fair value of assets recorded from the acquisition			$ 188,000
Fair value of debt assumed from the acquisition			$ 127,900
Effective interest rate of secured debt assumed from the acquisition (in hundredths)			3.11%
Previously held equity interest remeasured at fair value			$ 75,200
Gain on acquisition of controlling interest in unconsolidated affiliate			$ 7,500
Atlanta, GA Office Property Acquisition (6/2013) [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft) | ft²			553
Acquisition purchase price			$ 140,100
Greensboro, NC Office Properties Acquisition (3/2013) [Member]
Acquisitions [Abstract]
Number of office properties acquired | property			2
Net rentable square feet of acquisition (in sqft) | ft²			195
Acquisition purchase price			$ 30,800
Tampa, FL Office Properties Acquisition (3/2013) [Member]
Acquisitions [Abstract]
Number of office properties acquired | property			2
Net rentable square feet of acquisition (in sqft) | ft²			372
Acquisition purchase price			$ 52,500
Memphis, TN Development Land Acquisition (3/2013) [Member]
Acquisitions [Abstract]
Acquisition purchase price			$ 4,800
2015 Dispositions [Member]
Dispositions [Abstract]
Number of buildings sold | property	3
Purchase price of real estate	$ 27,800
Gains/(losses) on disposition of property	9,200
Income tax expense netted against gain	$ 500
2014 Dispositions [Member]
Dispositions [Abstract]
Number of buildings sold | property		33
Purchase price of real estate		$ 187,300
Closing credits excluded for unfunded building and/or tenant improvements		8,600
Closing credits excluded for free rent		2,900
Gains/(losses) on disposition of property		44,400
2013 Dispositions [Member]
Dispositions [Abstract]
Number of buildings sold | property			47
Purchase price of real estate			$ 260,500
Closing credits for unfunded tenant improvements			3,600
Closing credits included for free rent			2,000
Gains/(losses) on disposition of discontinued operations			62,300
Gains/(losses) on disposition of property			1,500
Additional purchase price consideration				$ 1,500
Greensboro, NC Office Property Impairment (6/2014) [Member]
Impairments [Abstract]
Impairments of real estate assets		$ 600
Winston-Salem, NC Office Properties (6/2013) [Member]
Impairments [Abstract]
Impairments of real estate assets			$ 1,100
Number of impaired properties | property			4
Atlanta, GA Industrial Properties (3/2013) [Member]
Impairments [Abstract]
Impairments of real estate assets			$ 1,100
Number of impaired properties | property			7