Real Estate Assets Real Estate Assets (Details) ft² in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) a ft² property parcel	Dec. 31, 2017 USD ($) property	Dec. 31, 2016 USD ($) a ft² property
Acquisitions [Abstract]
Undeveloped land suitable for future development (in acres) | a	350
Acquisition-related costs
Dispositions [Abstract]
Rentable square feet of commercial real estate (in square feet) | ft²	30,500
Gains on disposition of discontinued operations	$ 0	0	414,496
Gains on disposition of property	37,638	54,157	14,807
Impairments [Abstract]
Impairments of real estate assets	$ 423	1,445	$ 0
Nashville TN Land Acquisition (03/2018) [Member]
Acquisitions [Abstract]
Number of development parcels acquired (in parcels) | parcel	2
Undeveloped land suitable for future development (in acres) | a	9
Acquisition purchase price	$ 50,600
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
2018 Dispositions [Member]
Dispositions [Abstract]
Number of buildings sold | property	3
Purchase price of real estate	$ 90,600
Gains on disposition of property	37,600
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%
2018 Impairments [Member]
Impairments [Abstract]
Impairments of real estate assets	$ 400
2017 Impairments [Member]
Impairments [Abstract]
Impairments of real estate assets		$ 1,400