Real Estate - Properties Sold and Held for Sale Narrative (Details) ft² in Thousands, $ in Thousands							1 Months Ended		3 Months Ended			9 Months Ended
	Apr. 21, 2020 USD ($) ft²	Dec. 19, 2019 USD ($) ft²	Aug. 27, 2019 USD ($)	Aug. 21, 2019 USD ($)	Jul. 23, 2019 USD ($)	Jun. 26, 2019 USD ($) ft² building	Dec. 31, 2019 USD ($) ft²	Jun. 30, 2019 buyer agreement	Sep. 30, 2020 USD ($) ft²	Sep. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2020 USD ($) ft²	Sep. 30, 2019 USD ($)
Properties Sold and Held for Sale [Line Items]
(Loss) gain on sale									$ 0	$ 0		$ (7,539)	$ (1,046)
Real estate impairment									0	0		0	8,374
(Loss) gain on sale									0	339,024		0	339,024
Loss on extinguishment of debt									$ 0	$ 764		$ 0	$ 764
2020 Properties
Properties Sold and Held for Sale [Line Items]
Rentable square feet (sq ft) | ft²									223			223
Contract sales price									$ 57,000			$ 57,000
2019 Properties
Properties Sold and Held for Sale [Line Items]
Rentable square feet (sq ft) | ft²							2,184
Contract sales price							$ 728,750
Quantico Corporate Center | Office
Properties Sold and Held for Sale [Line Items]
Number of buildings | building						2
Rentable square feet (sq ft) | ft²						272
Contract sales price						$ 33,000
(Loss) gain on sale						$ 1,000
Real estate impairment											$ 8,400
Quantico Corporate Center | Disposal group, not discontinued operations | Office | 2019 Properties
Properties Sold and Held for Sale [Line Items]
Rentable square feet (sq ft) | ft²						272
Contract sales price						$ 33,000
(Loss) gain on sale						$ (1,046)
Shopping Center Portfolio and Power Center Portfolio
Properties Sold and Held for Sale [Line Items]
Number of purchase and sale agreements | agreement								2
Number of buyers | buyer								2
Shopping Center Portfolio
Properties Sold and Held for Sale [Line Items]
(Loss) gain on sale					$ 333,000
Frederick Crossing and Frederick County Square
Properties Sold and Held for Sale [Line Items]
(Loss) gain on sale				$ 9,500
Centre at Hagerstown
Properties Sold and Held for Sale [Line Items]
(Loss) gain on sale			$ (3,500)
John Marshall II | Held for sale | Office
Properties Sold and Held for Sale [Line Items]
Contract sales price per agreement	$ 57,000						63,400
John Marshall II | Disposal group, not discontinued operations | Office | 2020 Properties
Properties Sold and Held for Sale [Line Items]
Rentable square feet (sq ft) | ft²	223
Contract sales price	$ 57,000
(Loss) gain on sale	$ (6,855)
1776 G Street | Disposal group, not discontinued operations | Office | 2019 Properties
Properties Sold and Held for Sale [Line Items]
Rentable square feet (sq ft) | ft²		262
Contract sales price		$ 129,500					129,500
(Loss) gain on sale		$ 61,007					$ 61,000