Commitments and Contingencies (Textual) (Details)					6 Months Ended
	May 18, 2018	Jul. 01, 2016 USD ($) a	Dec. 03, 2015 ft²	Apr. 02, 2015 USD ($)	Jun. 30, 2018 USD ($) Parcel	May 04, 2018 USD ($)	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
Estimated project costs					$ 46,900,000
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					22,900,000
Unconsolidated Real Estate Ventures [Member]
Property Subject to or Available for Operating Lease [Line Items]
Real estate ventures aggregate indebtedness					645,500,000
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
618 Market Street [Member]
Property Subject to or Available for Operating Lease [Line Items]
Contingent consideration, liability				$ 2,000,000	1,900,000
Fair value of contingent consideration				1,600,000
Interest expense				$ 2,000,000
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					3,400,000
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
Put Agreement [Member]
Property Subject to or Available for Operating Lease [Line Items]
Purchase price						$ 35,000,000
Minimum [Member]
Property Subject to or Available for Operating Lease [Line Items]
Lease terms					11 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
Maximum [Member]
Property Subject to or Available for Operating Lease [Line Items]
Lease terms					67 years
Mortgage Lenders [Member]
Property Subject to or Available for Operating Lease [Line Items]
Associated letter of credit					$ 0