Commitments and Contingencies (Details) ft² in Thousands	9 Months Ended
	Sep. 30, 2020 USD ($) ft²	Dec. 31, 2019 USD ($)
Property Subject to or Available for Operating Lease [Line Items]
Debt, net	$ 1,919,511,000	$ 2,144,418,000
4040 Wilson Venture
Property Subject to or Available for Operating Lease [Line Items]
Debt, net	150,000,000.0
Guarantees, maximum exposure amount	41,300,000
Unconsolidated Real Estate Ventures
Property Subject to or Available for Operating Lease [Line Items]
Real estate ventures aggregate indebtedness	$ 842,200,000
Radnor PA Property
Property Subject to or Available for Operating Lease [Line Items]
Rentable area (in square feet) | ft²	170
Payments to acquire buildings	$ 20,300,000
Payments for deposits on real estate acquisitions	1,000,000.0
One and Two Commerce Square | Preferred Equity Investment
Property Subject to or Available for Operating Lease [Line Items]
Other commitments	20,000,000.0
Drexel Square
Property Subject to or Available for Operating Lease [Line Items]
Other commitments	7,700,000
Estimated potential additional contribution obligation	2,600,000
Mortgage Lenders
Property Subject to or Available for Operating Lease [Line Items]
Associated letter of credit	$ 0