Real Estate - Properties Sold and Held for Sale (Details) ft² in Thousands, $ in Thousands						9 Months Ended	12 Months Ended
	Sep. 22, 2021 USD ($) ft² property	Jul. 26, 2021 USD ($) ft² property	Dec. 17, 2020 USD ($) ft²	Dec. 02, 2020 USD ($) ft²	Apr. 21, 2020 USD ($) ft²	Sep. 30, 2021 USD ($) ft² property	Dec. 31, 2020 USD ($) ft²
Properties Sold and Held for Sale [Line Items]
Gain (loss) on sale of real estate		$ (11,200)
Office
Properties Sold and Held for Sale [Line Items]
Number of properties held for sale | property		12				12
Retail
Properties Sold and Held for Sale [Line Items]
Number of properties held for sale | property	8					8
2021 Properties | Disposal group
Properties Sold and Held for Sale [Line Items]
Rentable Square Feet (sq ft) | ft²						3,063
Contract Sales Price						$ 934,314
Gain (loss) on sale of real estate						$ 46,441
2021 Properties | Disposal group | Office Portfolio | Office
Properties Sold and Held for Sale [Line Items]
Rentable Square Feet (sq ft) | ft²		2,370
Contract Sales Price		$ 766,000
Gain (loss) on sale of real estate		$ (11,220)
2021 Properties | Disposal group | Retail Profile | Retail
Properties Sold and Held for Sale [Line Items]
Rentable Square Feet (sq ft) | ft²	693
Contract Sales Price	$ 168,314
Gain (loss) on sale of real estate	$ 57,661
2020 Properties | Disposal group
Properties Sold and Held for Sale [Line Items]
Rentable Square Feet (sq ft) | ft²							565
Contract Sales Price							$ 163,500
Gain (loss) on sale of real estate							$ (14,325)
2020 Properties | Disposal group | John Marshall II | Office
Properties Sold and Held for Sale [Line Items]
Rentable Square Feet (sq ft) | ft²					223
Contract Sales Price					$ 57,000
Gain (loss) on sale of real estate					$ (6,855)
2020 Properties | Disposal group | Monument II | Office
Properties Sold and Held for Sale [Line Items]
Rentable Square Feet (sq ft) | ft²				207
Contract Sales Price				$ 53,000
Gain (loss) on sale of real estate				$ (8,595)
2020 Properties | Disposal group | 1227 25th Street | Office
Properties Sold and Held for Sale [Line Items]
Rentable Square Feet (sq ft) | ft²			135
Contract Sales Price			$ 53,500
Gain (loss) on sale of real estate			$ 1,125