Basis of Presentation (Textual) (Details)					3 Months Ended			6 Months Ended
		Mar. 22, 2018 USD ($) a	Mar. 15, 2017 USD ($) ft²	Jul. 01, 2016 USD ($) a	Jun. 30, 2018 USD ($) a Parcel	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2018 USD ($) a Parcel	Jun. 30, 2017 USD ($)	Mar. 16, 2018 a	Dec. 31, 2017 USD ($)	Apr. 28, 2017 a	Jan. 30, 2017 a
Basis Of Presentation [Line Items]
Prepaid leasehold interests in land held for development	[1]				$ 40,177,000			$ 40,177,000			$ 0
Income tax provision					20,000	$ (339,000)		158,000	$ (239,000)
Increase in operating cash flows								3,656,000	2,262,000
Lease agreement variable income								0
Third party management contracts assets outstanding					0			0
Third party management contracts liabilities outstanding					0			0
Net gain on sale of undepreciated real estate					2,837,000	0		2,859,000	0
Proceeds from the sale of properties								16,771,000	102,083,000
Assets held for sale, net					4,254,000			4,254,000			$ 392,000
Gain (Loss) on Sale					(35,000)	1,088,000		(35,000)	8,411,000
Philadelphia Marine Center (Marine Piers) [Member]
Basis Of Presentation [Line Items]
Rentable Square Feet | ft²			181,900
Sales Price			$ 21,400,000
Payment by cash on purchases			12,000,000
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
Deferred gain on sale			$ 9,400,000
ASU 2016-18 [Member]
Basis Of Presentation [Line Items]
Restricted cash					$ 900,000	33,200,000		$ 900,000	33,200,000
Increase in operating cash flows									500,000
Increase in investing cash flows									32,000,000
General and Administrative Expenses [Member]
Basis Of Presentation [Line Items]
Income tax provision						300,000			200,000
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						$ 300,000			$ 200,000
3001-3003 JFK Boulevard [Member]
Basis Of Presentation [Line Items]
Lease agreement term		99 years
Area of developed parcels of land (in acres) | a		1
Prepaid leasehold interests in land held for development		$ 24,600,000
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
Garza Land Acquisition [Member] | Parcel One [Member]
Basis Of Presentation [Line Items]
Area of land sold | a												8.4
Garza Land Acquisition [Member] | Parcel Two [Member]
Basis Of Presentation [Line Items]
Area of land sold | a													1.7
Garza Land Acquisition [Member] | Parcel Three [Member]
Basis Of Presentation [Line Items]
Area of land sold | a										6.6
Garza Land Acquisition [Member] | Garza Land Sale [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
Net gain on sale of undepreciated real estate					$ 2,800,000
Garza Land Acquisition [Member] | Garza Land Sale [Member] | Parcel One [Member]
Basis Of Presentation [Line Items]
Area of land sold | a					8.4			8.4
Garza Land Acquisition [Member] | Garza Land Sale [Member] | Parcel One [Member] | Third Party Developers [Member]
Basis Of Presentation [Line Items]
Date of parcels sold								Jan. 30, 2017
Garza Land Acquisition [Member] | Garza Land Sale [Member] | Parcel Two [Member]
Basis Of Presentation [Line Items]
Area of land sold | a					1.7			1.7
Garza Land Acquisition [Member] | Garza Land Sale [Member] | Parcel Two [Member] | Third Party Developers [Member]
Basis Of Presentation [Line Items]
Date of parcels sold								Apr. 28, 2017
Garza Land Acquisition [Member] | Garza Land Sale [Member] | Parcel Three [Member]
Basis Of Presentation [Line Items]
Area of land sold | a					6.6			6.6
Garza Land Acquisition [Member] | Garza Land Sale [Member] | Parcel Three [Member] | Third Party [Member]
Basis Of Presentation [Line Items]
Date of parcels sold								Mar. 16, 2018
Other Assets [Member] | 3001-3003 JFK Boulevard [Member]
Basis Of Presentation [Line Items]
Prepaid leasehold interests in land held for development		$ 24,900,000
Impairment Charges [Member]
Basis Of Presentation [Line Items]
Out-of-period error immaterial in current year							$ 1,200,000

[1] As of June 30, 2018, this caption comprised leasehold interests in prepaid 99-year ground leases at 3025 and 3001-3003 JFK Boulevard, in Philadelphia, Pennsylvania. See Note 3, “Real Estate Investments,” for further information