Commitments and Contingencies (Textual) (Details)			3 Months Ended	9 Months Ended
	Dec. 03, 2015 ft² Floor	Apr. 02, 2015 USD ($)	Sep. 30, 2017 USD ($) a	Sep. 30, 2017 USD ($) a	Jul. 01, 2016 a Parcel
Subaru Corporate Headquarters Project [Member]
Property Subject to or Available for Operating Lease [Line Items]
Number of floors | Floor	5
Area of office property | ft²	250,000
Residual profit recognized			$ 2,100,000	$ 2,100,000
Residual profit earned			800,000	1,600,000
NTE Amount			79,400,000	79,400,000
Project costs funded			57,200,000	57,200,000
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			44,300,000	44,300,000
Subaru Build-to-Service Center Project [Member] | Other Assets [Member]
Property Subject to or Available for Operating Lease [Line Items]
Project costs funded			12,400,000	12,400,000
1919 Venture [Member]
Property Subject to or Available for Operating Lease [Line Items]
Guarantees, maximum exposure amount			88,900,000	88,900,000
Unconsolidated Real Estate Ventures [Member]
Property Subject to or Available for Operating Lease [Line Items]
Real estate ventures aggregate indebtedness to third parties			915,000,000	915,000,000
evo at Cira Centre South Venture [Member]
Property Subject to or Available for Operating Lease [Line Items]
Guarantees, maximum exposure amount			55,400,000	55,400,000
PJP VII [Member]
Property Subject to or Available for Operating Lease [Line Items]
Guarantees, maximum exposure amount			400,000	400,000
618 Market Street [Member]
Property Subject to or Available for Operating Lease [Line Items]
Contingent consideration, liability		$ 2,000,000	$ 1,800,000	$ 1,800,000
Fair value of contingent consideration		1,600,000
Interest expense		$ 2,000,000
Garza Land Acquisition [Member]
Property Subject to or Available for Operating Lease [Line Items]
Acreage of land | a					34.6
Number of Parcels | Parcel					2
Garza Land Acquisition [Member] | Parcel One [Member]
Property Subject to or Available for Operating Lease [Line Items]
Area of land sold | a			8.4	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	1.7
Garza Land Acquisition [Member] | A1000 Atrium Way And Libertyview
Property Subject to or Available for Operating Lease [Line Items]
Infrastructure improvements to land, estimated cost			$ 13,200,000	$ 13,200,000
Put Agreement [Member]
Property Subject to or Available for Operating Lease [Line Items]
Purchase price			35,000,000	$ 35,000,000
Minimum [Member]
Property Subject to or Available for Operating Lease [Line Items]
Lease terms				12 years
Maximum [Member]
Property Subject to or Available for Operating Lease [Line Items]
Lease terms				72 years
Mortgage Lenders [Member]
Property Subject to or Available for Operating Lease [Line Items]
Associated letter of credit			$ 10,000,000	$ 10,000,000