Real Estate Investments - Summary of Land Parcels Sold (Parenthetical) (Details)	3 Months Ended			12 Months Ended
	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2018 USD ($) Parcel	Dec. 31, 2017 USD ($) Parcel	Dec. 31, 2016 USD ($)	Jun. 30, 2018 USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Gain/(Loss) on Sale				$ 2,932,000	$ 31,657,000	$ 116,983,000
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				3	2
Aggregate deferred gain recognized				$ 200,000			$ 2,800,000
Bishops Gate [Member] | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Provision for impairment on assets held for sale	$ 300,000	$ 3,000,000.0
Carrying value of real estate prior to impairment	5,900,000
Aggregate carrying value	$ 5,600,000
Greenhills Land [Member] | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Provision for impairment on assets held for sale			$ 300,000
Gain/(Loss) on Sale						$ 0