Organization of The Parent Company and The Operating Partnership - Narrative (Details)	9 Months Ended
	Sep. 30, 2021 ft² a property parcel
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of real estate properties | property	81
Rentable Square Feet	13,651,650
Unconsolidated Properties | Unconsolidated Real Estate Ventures
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of real estate properties | property	10
Parent Company
Organization of The Parent Company and The Operating Partnership [Line Items]
Areas of land held for development (in acres) | a	176.1
Area of land held for sale | a	10.0
Lease agreement term	99 years
Area of additional undeveloped parcels of land with option to purchase (in acres) | a	55.5
Total potential development capacity (in square feet)	13,400,000
Parent Company | Land Parcel One
Organization of The Parent Company and The Operating Partnership [Line Items]
Lease agreement term	99 years
Parent Company | Held-for-sale
Organization of The Parent Company and The Operating Partnership [Line Items]
Total potential development capacity (in square feet)	100,000
Parent Company | Leashold Interest Land
Organization of The Parent Company and The Operating Partnership [Line Items]
Areas of land held for development (in acres) | a	0.8
Number of parcels of land | parcel	1
Wholly-owned Management Company Subsidiaries
Organization of The Parent Company and The Operating Partnership [Line Items]
Rentable Square Feet	24,400,000
Wholly-owned Management Company Subsidiaries | Wholly Owned Properties
Organization of The Parent Company and The Operating Partnership [Line Items]
Rentable Square Feet	13,700,000
Wholly-owned Management Company Subsidiaries | Partially Owned Properties
Organization of The Parent Company and The Operating Partnership [Line Items]
Rentable Square Feet	10,700,000
Brandywine Operating Partnership LP
Organization of The Parent Company and The Operating Partnership [Line Items]
Ownership in the operating partnership	99.50%