Real Estate Assets (Details) ft² in Thousands, $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2016 USD ($) a ft² property	Jun. 30, 2016 USD ($) a	Mar. 31, 2016 USD ($) ft²	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) a ft² property	Sep. 30, 2015 USD ($)
Acquisitions [Abstract]
Acquisition of development land (in acres) | a	450				450
Dispositions [Abstract]
Gains on disposition of property	$ 3,902			$ 7,012	$ 14,160	$ 10,581
Gains on disposition of discontinued operations	$ 0			$ 0	$ 414,496	$ 0
Rentable square feet of building (in sq feet) | ft²	31,100				31,100
Raleigh NC Building Acquisition (09/2016) [Member]
Acquisitions [Abstract]
Rentable square feet of acquisition (in sqft) | ft²	243				243
Acquisition purchase price	$ 76,900
Acquisition-related costs	300
Fee Simple Title Pittsburgh, PA (09/2016) [Member]
Acquisitions [Abstract]
Acquisition purchase price	18,500
Acquisition-related costs	$ 500
Number of buildings | property	1				1
Raleigh NC Development Land (09/2016) [Member]
Acquisitions [Abstract]
Acquisition purchase price	$ 5,800
Nashville TN Land Acquisition (6/2016) [Member]
Acquisitions [Abstract]
Acquisition purchase price		$ 9,100
Acquisition of development land (in acres) | a		14
2016 Dispositions [Member]
Dispositions [Abstract]
Purchase price of real estate	6,800	$ 14,200	$ 6,400
Gains on disposition of property	3,900	$ 5,900	2,000
Deferred gain on sale of property	$ 400				$ 400
Disposition closing credits excluded			500
Division Exit [Member]
Dispositions [Abstract]
Purchase price of real estate			660,000
Gains on disposition of property			1,300
Disposition closing credits excluded			4,800
Gains on disposition of discontinued operations			414,500
Related Party Office Building Disposition [Member]
Dispositions [Abstract]
Purchase price of real estate			4,700
Gains on disposition of property			1,100
Disposition closing credits excluded			$ 100
Rentable square feet of building (in sq feet) | ft²			32
Buyer occupancy percentage (in hundredths)			79.00%