Commitments and Contingencies (Textual) (Details)						9 Months Ended
	Aug. 23, 2018	May 18, 2018	Jul. 01, 2016 USD ($) a	Dec. 03, 2015 ft²	Apr. 02, 2015 USD ($)	Sep. 30, 2018 USD ($) Parcel	Mar. 16, 2018 a	Oct. 13, 2017 USD ($)	Apr. 28, 2017 a	Jan. 30, 2017 a
Subaru Build-to-Service Center Project [Member]
Property Subject to or Available for Operating Lease [Line Items]
Lease terms				18 years
Rentable Square Feet | ft²				83,000
Purchase option of lease from inception period	5 years			5 years
Estimated project costs						$ 47,600,000
Developer profit percentage		8.00%
Period following substantial completion of project to determine closing date		30 days
Period following completion of punch list items to determine closing date		3 days
Subaru Build-to-Service Center Project [Member] | Other Assets [Member]
Property Subject to or Available for Operating Lease [Line Items]
Project costs funded						34,500,000
Unconsolidated Real Estate Ventures [Member]
Property Subject to or Available for Operating Lease [Line Items]
Real estate ventures aggregate indebtedness						664,200,000
PJP VII [Member]
Property Subject to or Available for Operating Lease [Line Items]
Guarantees, maximum exposure amount						400,000
Real estate property debt						4,200,000
4040 Wilson Venture [Member]
Property Subject to or Available for Operating Lease [Line Items]
Guarantees, maximum exposure amount						41,300,000
Loan total borrowing capacity						150,000,000.0
618 Market Street [Member]
Property Subject to or Available for Operating Lease [Line Items]
Contingent consideration, liability					$ 2,000,000.0	1,900,000
Fair value of contingent consideration					1,600,000
Interest expense					$ 2,000,000.0
One Drexel Plaza [Member]
Property Subject to or Available for Operating Lease [Line Items]
Expected excess redevelopment funding						37,300,000
Estimated aggregate cost						83,100,000
Acquisition cost, inclusive in estimated aggregate cost								$ 37,800,000
Schuylkill Yards [Member]
Property Subject to or Available for Operating Lease [Line Items]
Required spending in capital improvements to property						2,700,000
Estimated potential additional contribution obligation						$ 700,000
Garza Land Acquisition [Member]
Property Subject to or Available for Operating Lease [Line Items]
Acreage of land | a			34.6
Gross purchase value of assets			$ 20,600,000
Number of Parcels | Parcel						3
Garza Land Acquisition [Member] | Parcel One [Member]
Property Subject to or Available for Operating Lease [Line Items]
Area of land sold | a									8.4
Garza Land Acquisition [Member] | Parcel Two [Member]
Property Subject to or Available for Operating Lease [Line Items]
Area of land sold | a										1.7
Garza Land Acquisition [Member] | Parcel Three [Member]
Property Subject to or Available for Operating Lease [Line Items]
Area of land sold | a							6.6
Minimum [Member]
Property Subject to or Available for Operating Lease [Line Items]
Lease terms						10 years
Minimum [Member] | One Drexel Plaza [Member]
Property Subject to or Available for Operating Lease [Line Items]
Required spending in capital improvements to property						$ 8,000,000.0
Maximum [Member]
Property Subject to or Available for Operating Lease [Line Items]
Lease terms						66 years
Mortgage Lenders [Member]
Property Subject to or Available for Operating Lease [Line Items]
Associated letter of credit						$ 0