Basis of Presentation (Textual) (Details)					3 Months Ended			9 Months Ended		12 Months Ended
	Aug. 23, 2018	Mar. 15, 2017 USD ($) ft²	Jul. 01, 2016 USD ($) a	Dec. 03, 2015 ft²	Sep. 30, 2018 USD ($) a Parcel	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2018 USD ($) a Parcel	Sep. 30, 2017 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2018 USD ($)	Mar. 16, 2018 a	Dec. 31, 2017 USD ($)	Apr. 28, 2017 a	Jan. 30, 2017 a
Basis Of Presentation [Line Items]
Income tax provision					$ 0	$ (793,000)		$ 158,000	$ (1,032,000)
Increase in operating cash flows								9,831,000	8,361,000
Lease agreement variable income								0
Third party management contracts assets outstanding					0			0
Third party management contracts liabilities outstanding					0			0
Proceeds from the sale of properties								16,771,000	114,821,000
Assets held for sale, net					297,194,000			297,194,000					$ 392,000
Gain (Loss) on Sale					0	0		$ (35,000)	8,411,000
Scenario Plan [Member]
Basis Of Presentation [Line Items]
Increase in operating expenses										$ 3,600,000
Scenario Plan [Member] | Land Lease Arrangements
Basis Of Presentation [Line Items]
Increase in operating expenses										$ 800,000
Subaru Build-to-Service Center Project [Member]
Basis Of Presentation [Line Items]
Rentable Square Feet | ft²				83,000
Lease terms				18 years
Purchase option of lease from inception period	5 years			5 years
Sales-type lease, interest income					$ 400,000
Garza Land Acquisition [Member]
Basis Of Presentation [Line Items]
Area of developed parcels of land (in acres) | a			34.6
Gross purchase value of assets			$ 20,600,000
Number of Parcels | Parcel					3			3
Parcel One [Member] | Garza Land Acquisition [Member]
Basis Of Presentation [Line Items]
Area of land sold | a														8.4
Parcel Two [Member] | Garza Land Acquisition [Member]
Basis Of Presentation [Line Items]
Area of land sold | a															1.7
Parcel Three [Member] | Garza Land Acquisition [Member]
Basis Of Presentation [Line Items]
Area of land sold | a												6.6
Garza Land Sale [Member] | Garza Land Acquisition [Member]
Basis Of Presentation [Line Items]
Area of developed parcels of land (in acres) | a			34.6
Gross purchase value of assets			$ 20,600,000
Number of Parcels | Parcel					3			3
Aggregate deferred gain recognized					$ 0			$ 0			$ 2,800,000
Garza Land Sale [Member] | Parcel One [Member] | Garza Land Acquisition [Member]
Basis Of Presentation [Line Items]
Area of land sold | a					8.4			8.4
Garza Land Sale [Member] | Parcel One [Member] | Garza Land Acquisition [Member] | Third Party Developers [Member]
Basis Of Presentation [Line Items]
Date of parcels sold								Jan. 30, 2017
Garza Land Sale [Member] | Parcel Two [Member] | Garza Land Acquisition [Member]
Basis Of Presentation [Line Items]
Area of land sold | a					1.7			1.7
Garza Land Sale [Member] | Parcel Two [Member] | Garza Land Acquisition [Member] | Third Party Developers [Member]
Basis Of Presentation [Line Items]
Date of parcels sold								Apr. 28, 2017
Garza Land Sale [Member] | Parcel Three [Member] | Garza Land Acquisition [Member]
Basis Of Presentation [Line Items]
Area of land sold | a					6.6			6.6
Garza Land Sale [Member] | Parcel Three [Member] | Garza Land Acquisition [Member] | Third Party [Member]
Basis Of Presentation [Line Items]
Date of parcels sold								Mar. 16, 2018
Philadelphia Marine Center (Marine Piers) [Member]
Basis Of Presentation [Line Items]
Aggregate deferred gain recognized		$ 9,400,000
Rentable Square Feet | ft²		181,900
Sales Price		$ 21,400,000
Payment by cash on purchases		12,000,000.0
Proceeds from the sale of properties		11,200,000
Balance payment on purchases		$ 9,400,000
Date of second installment payment		Jan. 31, 2020
Assets held for sale, net		$ 4,700,000
Gain (Loss) on Sale		$ 6,500,000
ASU 2016-18 [Member]
Basis Of Presentation [Line Items]
Restricted cash					$ 800,000	1,300,000		$ 800,000	1,300,000
Increase in operating cash flows									600,000
General and Administrative Expenses [Member]
Basis Of Presentation [Line Items]
Income tax provision						800,000			1,000,000.0
Net Income Tax Provision [Member]
Basis Of Presentation [Line Items]
Reclassification of net income tax provision out of general and administrative expenses into net income tax provision						$ 800,000			$ 1,000,000.0
Impairment Charges [Member]
Basis Of Presentation [Line Items]
Out-of-period error immaterial in current year							$ 1,200,000