Commitments and Contingencies (Textual) (Details)						3 Months Ended	9 Months Ended	12 Months Ended
		Oct. 18, 2017 USD ($) ft² property	Oct. 17, 2017 USD ($) property	Dec. 03, 2015 ft²	Apr. 02, 2015 USD ($)	Dec. 31, 2017 USD ($) ft² a property Parcel	Sep. 30, 2004 USD ($)	Dec. 31, 2017 USD ($) ft² a Parcel	Dec. 03, 2017 ft² Floor	Oct. 13, 2017 USD ($) ft²	Dec. 31, 2016 USD ($)	Jul. 02, 2016 a	Jul. 01, 2016 a	Dec. 31, 2015	Oct. 17, 2014 ft²	Jul. 31, 2014 ft²
Property Subject to or Available for Operating Lease [Line Items]
Letter of credit outstanding amount released		$ 10,000,000
Real estate ventures aggregate indebtedness to third parties						$ 768,912,000		$ 768,912,000			$ 997,466,000
Agreed holding period (not to sell) for properties acquired as part of the TRC acquisition								15 years
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						2,000,000		$ 2,000,000
Residual profit earned								1,500,000
NTE Amount						79,400,000		79,400,000
Project costs funded						67,100,000		67,100,000
Subaru Build-to-Service Center Project [Member]
Property Subject to or Available for Operating Lease [Line Items]
Lease terms				18 years
Project costs funded						15,500,000		15,500,000
Rentable Square Feet | ft²				83,000
Purchase option of lease from inception period				5 years
Project costs						46,900,000		46,900,000
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
4040 Wilson Venture [Member]
Property Subject to or Available for Operating Lease [Line Items]
Guarantees, maximum exposure amount						41,300,000		41,300,000
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
One Drexel Plaza [Member]
Property Subject to or Available for Operating Lease [Line Items]
Estimated of funding In the ground lease										$ 37,300,000
Rentable Square Feet | ft²										282,709
Drexel Square [Member]
Property Subject to or Available for Operating Lease [Line Items]
Required spending in capital improvements to property						3,600,000		3,600,000
Estimated potential additional contribution obligation						$ 900,000		$ 900,000
Garza Land Acquisition [Member]
Property Subject to or Available for Operating Lease [Line Items]
Acreage of land | a												34.6	34.6
Number of Parcels | Parcel						2		2
Infrastructure improvements to land, estimated cost						$ 13,500,000		$ 13,500,000
Garza Land Acquisition [Member] | Parcel One [Member]
Property Subject to or Available for Operating Lease [Line Items]
Area of land sold | a						8.4		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
Two Logan Square [Member]
Property Subject to or Available for Operating Lease [Line Items]
Leased Area Of Building | ft²								708,844
Amount to be paid if the Company takes fee title to Two Logan Square upon foreclosure of related mortgage						$ 2,900,000		$ 2,900,000
Liability related to acquisition of TRC							$ 700,000	$ 2,400,000
Commerce Square [Member] | Prentiss Properties Trust [Member]
Property Subject to or Available for Operating Lease [Line Items]
Non-controlling interest, ownership percentage by non-controlling Owners						1.00%		1.00%
One Commerce Square [Member]
Property Subject to or Available for Operating Lease [Line Items]
Tax guarantee obligation						$ 125,000,000		$ 125,000,000
Two Commerce Square [Member]
Property Subject to or Available for Operating Lease [Line Items]
Tax guarantee obligation						100,000,000		100,000,000
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								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		$ 8,000,000
Maximum [Member]
Property Subject to or Available for Operating Lease [Line Items]
Lease terms								71 years
Austin Joint Venture [Member]
Property Subject to or Available for Operating Lease [Line Items]
Number of Properties Sold | property		8	7			8
Equity method investment percentage		50.00%
Real estate ventures aggregate indebtedness to third parties		$ 150,968,000	$ 151,000,000
Rentable Square Feet | ft²		1,164,496				1,570,123		1,570,123							590,881	232,274
Unconsolidated Real Estate Ventures [Member]
Property Subject to or Available for Operating Lease [Line Items]
Equity method investment percentage						0.00%		0.00%			0.00%			0.00%
Real estate ventures aggregate indebtedness to third parties						$ 768,912,000		$ 768,912,000
Unconsolidated Real Estate Ventures [Member] | Minimum [Member]
Property Subject to or Available for Operating Lease [Line Items]
Equity method investment percentage						20.00%		20.00%			20.00%			20.00%
Unconsolidated Real Estate Ventures [Member] | Maximum [Member]
Property Subject to or Available for Operating Lease [Line Items]
Equity method investment percentage						70.00%		70.00%			70.00%			70.00%
PJP VII [Member]
Property Subject to or Available for Operating Lease [Line Items]
Equity method investment percentage	[1]					25.00%		25.00%
Real estate ventures aggregate indebtedness to third parties						$ 4,792,000		$ 4,792,000			$ 4,956,000
Guarantees, maximum exposure amount						400,000		400,000
Mortgage Lenders [Member]
Property Subject to or Available for Operating Lease [Line Items]
Associated letter of credit						$ 0		$ 0

[1] Ownership percentage represents the Company’s entitlement to residual distributions after payments of priority returns, where applicable.