Real Estate Assets Real Estate Assets (Details) ft² in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) a ft² property	Dec. 31, 2015 USD ($) ft² property	Dec. 31, 2014 USD ($) ft² property
Acquisitions [Abstract]
Acquisition of development land (in acres) | a	400
Asset acquisition, contingent consideration liability	$ 0	$ 900	$ 3,300
Acquisition-related costs		1,000	500
Dispositions [Abstract]
Rentable square feet of commercial real estate (in square feet) | ft²	31,100
Gains on disposition of discontinued operations	$ 414,496	0	384
Gains on disposition of property	14,807	11,444	44,352
Impairments [Abstract]
Impairments of real estate assets	$ 0	$ 0	$ 588
Raleigh NC Building Acquisition (09/2016) [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft) | ft²	243
Acquisition purchase price	$ 76,900
Acquisition-related costs	300
Raleigh NC Development Land (09/2016) [Member]
Acquisitions [Abstract]
Acquisition purchase price	$ 5,800
Fee Simple Title Pittsburgh, PA (09/2016) [Member]
Acquisitions [Abstract]
Number of office properties | property	1
Acquisition purchase price	$ 18,500
Acquisition-related costs	$ 500
Nashville TN Land Acquisition (6/2016) [Member]
Acquisitions [Abstract]
Acquisition of development land (in acres) | a	14
Acquisition purchase price	$ 9,100
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 | 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 partner's interest in asset acquired (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 partner's interest in asset acquired (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
2016 Dispositions [Member]
Dispositions [Abstract]
Number of buildings sold | property	2
Purchase price of real estate	$ 31,100
Closing credits excluded for unfunded building and/or tenant improvements	500
Gains on disposition of property	12,400
Additional purchase price consideration	400
Division Exit [Member]
Dispositions [Abstract]
Purchase price of real estate	660,000
Closing credits excluded for unfunded building and/or tenant improvements	4,800
Gains on disposition of discontinued operations	414,500
Gains on disposition of property	$ 1,300
Related Party Office Building Disposition [Member]
Dispositions [Abstract]
Rentable square feet of commercial real estate (in square feet) | ft²	32
Purchase price of real estate	$ 4,700
Closing credits excluded for unfunded building and/or tenant improvements	100
Gains on disposition of property	$ 1,100
Buyer occupancy percentage (in hundredths)	79.00%
2015 Dispositions [Member]
Dispositions [Abstract]
Number of buildings sold | property		3
Purchase price of real estate		$ 27,800
Gains 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 on disposition of property			44,400
Greensboro, NC Office Property Impairment (6/2014) [Member]
Impairments [Abstract]
Impairments of real estate assets			$ 600