Commitments and Contingencies (Textual) (Details)			3 Months Ended
	Dec. 03, 2015 ft²	Apr. 02, 2015 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($) a Parcel	Mar. 16, 2018 a	Oct. 13, 2017 USD ($)	Apr. 28, 2017 a	Jan. 30, 2017 a	Jul. 01, 2016 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
Subaru Build-to-Service Center Project [Member] | Other Assets [Member]
Property Subject to or Available for Operating Lease [Line Items]
Project costs funded				16,800,000
Unconsolidated Real Estate Ventures [Member]
Property Subject to or Available for Operating Lease [Line Items]
Real estate ventures aggregate indebtedness to third parties				639,300,000
PJP VII [Member]
Property Subject to or Available for Operating Lease [Line Items]
Guarantees, maximum exposure amount				400,000
Real estate property debt				4,300,000
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		$ 1,800,000
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]
Estimated of funding In the ground lease						$ 37,300,000
One Drexel Plaza [Member] | Scenario Forecast [Member]
Property Subject to or Available for Operating Lease [Line Items]
Estimated aggregate cost			$ 83,100,000
Acquisition cost, inclusive in estimated aggregate cost			$ 37,800,000
3025 JFK Boulevard [Member]
Property Subject to or Available for Operating Lease [Line Items]
Lease terms				99 years
Acreage of land | a				1.0
Estimated gross purchase price				$ 20,500,000
Drexel Square [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				$ 900,000
Garza Land Acquisition [Member]
Property Subject to or Available for Operating Lease [Line Items]
Acreage of land | a									34.6
Number of Parcels | Parcel				3
Infrastructure improvements to land, estimated cost				$ 13,200,000
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				71 years
Mortgage Lenders [Member]
Property Subject to or Available for Operating Lease [Line Items]
Associated letter of credit				$ 0