Real Estate Investments - Summary of Land Parcels Sold (Details) $ in Thousands				3 Months Ended		9 Months Ended
	Mar. 16, 2018 USD ($) a Parcel		Jan. 10, 2018 USD ($) a Parcel	Sep. 30, 2018 USD ($) a Parcel	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) a Parcel	Sep. 30, 2017 USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Proceeds from the sale of properties						$ 16,771	$ 114,821
Gain on Sale				$ 0	$ 953	$ 2,859	$ 953
Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel				2		2
Acreage of land | a				19.7		19.7
Sales Price				$ 15,056		$ 15,056
Proceeds from the sale of properties						14,921
Gain on Sale						1,446
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	$ 1,424	[1]				$ 1,424
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] As of March 31, 2018, the Company had not transferred control to the buyer of this land parcel, or two other parcels at this site which were sold during 2017, because of a completion guarantee which required the Company, as developer, to complete certain infrastructure improvements on behalf of the buyers of the land parcels. The cash received at settlement was recorded as “Deferred income, gains and rent” on the Company’s consolidated balance sheets. During the three months ended June 30, 2018, the infrastructure improvements were substantially completed, at which time the Company transferred control of the land parcels. As a result, the Company then recognized the sale. See Note 2, “Basis of Presentation,” for further discussion of the infrastructure improvements and related revenue recognition.