Commitments and Contingencies (Textual) (Details)				6 Months Ended
	Feb. 02, 2017 ft² property	Dec. 03, 2015 ft²	Apr. 02, 2015 USD ($)	Jun. 30, 2017 USD ($) a property	Aug. 01, 2017 USD ($)	Jul. 01, 2016 USD ($) a Parcel
Property Subject to or Available for Operating Lease [Line Items]
Expected acquisition date for replacement property				Aug. 01, 2017
Latest acquisition date to avoid payment of tax liability				Aug. 01, 2017
Loss contingency on acquisition of replacement property				$ 0
Subaru Corporate Headquarters Project [Member]
Property Subject to or Available for Operating Lease [Line Items]
NTE Amount				79,400,000
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
Subaru Build-to-Service Center Project [Member] | Other Assets [Member]
Property Subject to or Available for Operating Lease [Line Items]
Project costs funded				$ 11,300,000
Concord Airport Plaza [Member] | Subsequent Event [Member]
Property Subject to or Available for Operating Lease [Line Items]
Purchase commitment amount to satisfy tax liabilities					$ 13,500,000
Office Properties [Member]
Property Subject to or Available for Operating Lease [Line Items]
Number of Properties Sold | property				12
Office Properties [Member] | Concord Airport Plaza [Member]
Property Subject to or Available for Operating Lease [Line Items]
Number of Properties Sold | property	2
Rentable Square Feet | ft²	350,256
1919 Venture [Member]
Property Subject to or Available for Operating Lease [Line Items]
Guarantees, maximum exposure amount				$ 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				952,700,000
evo at Cira Centre South Venture [Member]
Property Subject to or Available for Operating Lease [Line Items]
Guarantees, maximum exposure amount				55,400,000
PJP VII [Member]
Property Subject to or Available for Operating Lease [Line Items]
Guarantees, maximum exposure amount				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
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
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] | A1000 Atrium Way And Libertyview
Property Subject to or Available for Operating Lease [Line Items]
Infrastructure improvements to land, estimated cost						$ 14,300,000
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				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