Real Estate Investments - Summary of Land Parcels Sold (Details) $ in Thousands		3 Months Ended
		Mar. 31, 2018 USD ($) a Parcel	Mar. 31, 2017 USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Proceeds from the sale of properties		$ 14,921	$ 74,159
Gain on Sale		$ 22	$ 0
Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel		2
Acreage of land | a		19.7
Sales Price		$ 15,056
Proceeds from the sale of properties		14,921
Gain on Sale		$ 22
Garza Ranch - Office [Member] | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel		1
Acreage of land | a		6.6
Sales Price		$ 14,571
Proceeds from the sale of properties		14,509
Gain on Sale	[1]	$ 0
Westpark Land [Member] | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel		1
Acreage of land | a		13.1
Sales Price		$ 485
Proceeds from the sale of properties		412
Gain on Sale		$ 22

[1] The Company has a continuing involvement, through a completion guarantee, which requires the Company as developer to complete certain infrastructure improvements on behalf of the buyers of the land parcels. The Company does not transfer control of the land parcel until the infrastructure improvements are complete and the guarantee is released. Accordingly, the cash received at settlement was recorded as “Deferred income, gains and rent” on the Company’s consolidated balance sheets and the Company will recognize the sale once the infrastructure improvements are complete. See Note 14, “Commitments and Contingencies,” for further discussion of the infrastructure improvements.