Basis of Presentation (Textual) (Details)		3 Months Ended
	Mar. 15, 2017 USD ($) ft²	Mar. 31, 2019 USD ($) Ground		Mar. 31, 2018 USD ($)	Jan. 15, 2024 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)
Basis Of Presentation [Line Items]
Direct financing leases		$ 0
Operating leases expiration dates extending		The operating leases have various expiration dates.
Cumulative effect of accounting change		$ (5,336,000)
Amount of general reserve related to leases		$ 11,300,000
Number of long-term ground leases | Ground		6
Right of use asset - operating leases		$ 22,175,000	[1]			$ 22,400,000	$ 0
Lease liability - operating leases		22,402,000				$ 22,400,000	0
Proceeds from the sale of properties		5,273,000		$ 14,921,000
Assets held for sale, net		7,345,000					$ 11,599,000
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	12,000,000.0
Proceeds from the sale of properties	11,200,000
Balance payment on purchases	$ 9,400,000				$ 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
Uncollectible Leases [Member]
Basis Of Presentation [Line Items]
Cumulative effect of accounting change		(700,000)
Depreciation Out of Period Adjustment [Member]
Basis Of Presentation [Line Items]
Depreciation expense		$ 800,000

[1] On January 1, 2019, as a result of the adoption of Topic 842, Leases, the Company recognized operating ground leases for which it is a lessee on its consolidated balance sheets for the period ended March 31, 2019. See Note 2, “Basis of Presentation,” for further information.