Commitments and Contingencies (Textual) (Details) ft² in Thousands	3 Months Ended
Mar. 31, 2020 USD ($) ft²	Dec. 31, 2019 USD ($)
Property Subject to or Available for Operating Lease [Line Items]
Loan outstanding principal balance	$ 2,192,600,000	$ 2,144,418,000
4040 Wilson Venture
Property Subject to or Available for Operating Lease [Line Items]
Loan outstanding principal balance	150,000,000.0
Unconsolidated Real Estate Ventures
Property Subject to or Available for Operating Lease [Line Items]
Real estate ventures aggregate indebtedness	$ 611,600,000
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
Drexel Square
Property Subject to or Available for Operating Lease [Line Items]
Required spending in capital improvements to property	8,400,000
Estimated potential additional contribution obligation	2,700,000
Maximum \| 4040 Wilson Venture
Property Subject to or Available for Operating Lease [Line Items]
Guarantees, maximum exposure amount	41,300,000
Mortgage Lenders
Property Subject to or Available for Operating Lease [Line Items]
Associated letter of credit	$ 0

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