Real Estate Assets Real Estate Assets (Details) ft² in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) a ft² property	Dec. 31, 2016 USD ($) a ft² property	Dec. 31, 2015 USD ($) ft² property
Acquisitions [Abstract]
Undeveloped land suitable for future development (in acres) | a	400
Asset acquisition, contingent consideration liability	$ 750	$ 0	$ 900
Acquisition-related costs			1,000
Dispositions [Abstract]
Rentable square feet of commercial real estate (in square feet) | ft²	30,700
Gains on disposition of discontinued operations	$ 0	414,496	0
Gains on disposition of property	54,157	14,807	11,444
Impairments [Abstract]
Impairments of real estate assets	1,445	$ 0	$ 0
Fee Simple Title Raleigh, NC (03/2017) [Member]
Acquisitions [Abstract]
Acquisition purchase price	$ 2,600
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
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
Raleigh NC Development Land (09/2016) [Member]
Acquisitions [Abstract]
Acquisition purchase price		$ 5,800
Nashville TN Land Acquisition (6/2016) [Member]
Acquisitions [Abstract]
Undeveloped land suitable for future development (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
2017 Dispositions [Member]
Dispositions [Abstract]
Number of buildings sold | property	15
Purchase price of real estate	$ 135,600
Closing credits excluded for unfunded building and/or tenant improvements	3,700
Gains on disposition of property	54,200
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
2017 Impairments [Member]
Impairments [Abstract]
Impairments of real estate assets	$ 1,400