Commitments and Contingencies (Textual) (Details)		9 Months Ended	12 Months Ended
	Apr. 02, 2015 USD ($)	Sep. 30, 2004 USD ($)	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)	Oct. 13, 2017 USD ($)
Property Subject to or Available for Operating Lease [Line Items]
Real estate ventures aggregate indebtedness			$ 370,319,000	$ 768,912,000
Loan outstanding principal balance			$ 2,028,046,000	1,930,828,000
Agreed holding period (not to sell) for properties acquired as part of the TRC acquisition			15 years
4040 Wilson Venture [Member]
Property Subject to or Available for Operating Lease [Line Items]
Guarantees, maximum exposure amount			$ 41,300,000
Loan outstanding principal balance			$ 150,000,000.0
Prentiss Properties Trust [Member] | Commerce Square [Member]
Property Subject to or Available for Operating Lease [Line Items]
Non-controlling interest, ownership percentage by non-controlling Owners			1.00%
Unconsolidated Real Estate Ventures [Member]
Property Subject to or Available for Operating Lease [Line Items]
Real estate ventures aggregate indebtedness			$ 370,319,000
PJP VII [Member]
Property Subject to or Available for Operating Lease [Line Items]
Real estate ventures aggregate indebtedness			3,777,000	$ 4,792,000
Guarantees, maximum exposure amount			300,000
Loan outstanding principal balance			3,800,000
618 Market Street [Member]
Property Subject to or Available for Operating Lease [Line Items]
Contingent consideration, liability	$ 2,000,000.0		1,900,000
Fair value of contingent consideration	1,600,000
Interest expense	$ 2,000,000.0
The Bulletin Building [Member]
Property Subject to or Available for Operating Lease [Line Items]
Funding requirement met for capital improvements			1,200,000
Estimated of funding In the ground lease			37,300,000
Estimated aggregate cost			83,100,000
Acquisition cost, inclusive in estimated aggregate cost					$ 37,800,000
Drexel Square [Member]
Property Subject to or Available for Operating Lease [Line Items]
Required spending in capital improvements to property			2,700,000
Estimated potential additional contribution obligation			$ 600,000
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
Liability related to acquisition of TRC		$ 700,000	2,700,000
One Commerce Square [Member]
Property Subject to or Available for Operating Lease [Line Items]
Tax guarantee obligation			125,000,000.0
Two Commerce Square [Member]
Property Subject to or Available for Operating Lease [Line Items]
Tax guarantee obligation			$ 100,000,000.0
Minimum [Member]
Property Subject to or Available for Operating Lease [Line Items]
Lease terms			10 years
Minimum [Member] | The Bulletin Building [Member]
Property Subject to or Available for Operating Lease [Line Items]
Required spending in capital improvements to property			$ 8,000,000.0
Maximum [Member]
Property Subject to or Available for Operating Lease [Line Items]
Lease terms			66 years
Mortgage Lenders [Member]
Property Subject to or Available for Operating Lease [Line Items]
Associated letter of credit			$ 0