Acquisitions and Developments in Progress - Summary of Developments in Progress (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) a	Dec. 31, 2017 USD ($)
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Developments in progress		$ 113,353	$ 48,369
Aggregate proceeds, net		44,656	197,887	$ 896,456
Circle East
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Developments in progress		33,628	22,383
Plaza del Lago
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Developments in progress		0	536
One Loudoun Downtown
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Developments in progress		27,868	0
Carillon
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Developments in progress		26,407	0
Accelerated depreciation	$ 26,330
Redevelopment properties
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Developments in progress		87,903	22,919
One Loudoun Uptown
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Land held for future development		$ 25,450	$ 25,450
Acres of land | a			58
Acres of land that are developable | a			32
Circle East, air rights
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Aggregate proceeds, net			$ 11,820