Real Estate Assets (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2019 USD ($) a ft² propertyUnit parcel	Dec. 31, 2019 USD ($) a ft² propertyUnit parcel	Dec. 31, 2018 USD ($) a propertyUnit parcel	Dec. 31, 2017 USD ($) propertyUnit
Acquisitions [Abstract]
Rentable square feet of commercial real estate (in square feet) | ft²	31,700,000	31,700,000
Undeveloped land suitable for future development (in acres) | a	275	275
Dispositions [Abstract]
Gains on disposition of property		$ 39,517	$ 37,638	$ 54,157
Impairments [Abstract]
Impairments of real estate assets		$ 5,849	$ 423	1,445
Submarket Acquisition [Member]
Acquisitions [Abstract]
Rentable square feet of commercial real estate (in square feet) | ft²	841,000	841,000
Purchase price of acquisition	$ 436,000	$ 399,100
2019 Land Acquisitions [Member]
Acquisitions [Abstract]
Number of development parcels acquired (in parcels) | parcel	4	4
Undeveloped land suitable for future development (in acres) | a	10	10
Purchase price of acquisition		$ 12,400
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
Purchase price of acquisition			$ 50,600
Fee Simple Title Raleigh, NC (03/2017) [Member]
Acquisitions [Abstract]
Purchase price of acquisition				$ 2,600
2019 Dispositions [Member]
Dispositions [Abstract]
Number of buildings sold | propertyUnit	6	6
Purchase price of real estate		$ 136,400
Gains on disposition of property		39,500
2018 Dispositions [Member]
Dispositions [Abstract]
Number of buildings sold | propertyUnit			3
Purchase price of real estate			$ 90,600
Gains on disposition of property			37,600
2017 Dispositions [Member]
Dispositions [Abstract]
Number of buildings sold | propertyUnit				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
2019 Impairments [Member]
Impairments [Abstract]
Impairments of real estate assets		$ 5,800
2018 Impairments [Member]
Impairments [Abstract]
Impairments of real estate assets			$ 400
2017 Impairments [Member]
Impairments [Abstract]
Impairments of real estate assets				$ 1,400