Real Estate (2020 Property Acquisitions and Dispositions) (Details)									3 Months Ended	12 Months Ended
		Sep. 02, 2021 USD ($) ft²	Jun. 14, 2021 USD ($) ft²	Jun. 01, 2021 USD ($) ft²	Apr. 30, 2021 USD ($) ft²	Dec. 31, 2020 USD ($)	Feb. 12, 2020 USD ($) ft²	Jan. 10, 2020 USD ($) ft² shares	Sep. 30, 2020 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 01, 2020 USD ($)
Business Acquisition and Disposition
DownREIT Partnership Units, issued in acquisition | shares								163,322
Mortgage loans assumed with acquisition	[1]									$ 0	$ 8,903,000	$ 98,041,000
Impairment charge										0	57,218,000	0
Fair value of property						$ 57,000,000					57,000,000
Gain on sale of real estate										89,950,000	98,117,000	$ 116,393,000
Chesterbrook
Business Acquisition and Disposition
Square footage of real estate property | ft²	[2]				90,000
Grossmont Center
Business Acquisition and Disposition
Square footage of real estate property | ft²	[2]			933,000
Camelback Colonnade
Business Acquisition and Disposition
Square footage of real estate property | ft²	[2]		642,000
Hilton Village
Business Acquisition and Disposition
Square footage of real estate property | ft²	[2]		93,000
Twinbrooke Shopping Centre
Business Acquisition and Disposition
Square footage of real estate property | ft²		106,000
Two properties 2021
Business Acquisition and Disposition
Sales price of real estate										141,600,000
Gain on sale of real estate										$ 88,300,000
Fairfax Junction
Business Acquisition and Disposition
Square footage of real estate property | ft²								49,000
Purchase price of real estate acquisition	[3]							$ 22,300,000
Hoboken
Business Acquisition and Disposition
Square footage of real estate property | ft²							12,000
Purchase price of real estate acquisition	[4]						$ 14,300,000
The Shops at Sunset Place
Business Acquisition and Disposition
Impairment charge									$ 57,200,000
Sales price of real estate						65,500,000
Gain on sale of real estate						$ 9,200,000
The Shops at Sunset Place | Nonrecourse | Mortgages
Business Acquisition and Disposition
Debt Instrument, Face Amount													$ 60,600,000
Hoboken2Buildings | Mortgages
Business Acquisition and Disposition
Mortgage loans assumed with acquisition							8,900,000
Three properties 2020
Business Acquisition and Disposition
Sales price of real estate											186,100,000
Gain on sale of real estate											98,100,000
Other Assets | Above Market Leases | Chesterbrook
Business Acquisition and Disposition
Above market leases					$ 600,000
Other Assets | Above Market Leases | Grossmont Center
Business Acquisition and Disposition
Above market leases				$ 2,600,000
Other Assets | Above Market Leases | Hilton Village
Business Acquisition and Disposition
Above market leases			$ 1,100,000
Other Assets | Above Market Leases | Twinbrooke Shopping Centre
Business Acquisition and Disposition
Above market leases		$ 300,000
Other Assets | Above Market Leases | Fairfax Junction
Business Acquisition and Disposition
Above market leases								500,000
Other Assets | Above Market Leases | Hoboken
Business Acquisition and Disposition
Above market leases							100,000
Other Liabilities | Chesterbrook
Business Acquisition and Disposition
Below market leases					$ 8,000,000
Other Liabilities | Grossmont Center
Business Acquisition and Disposition
Below market leases				$ 14,700,000
Other Liabilities | Camelback Colonnade
Business Acquisition and Disposition
Below market leases			28,300,000
Other Liabilities | Hilton Village
Business Acquisition and Disposition
Below market leases			$ 3,600,000
Other Liabilities | Twinbrooke Shopping Centre
Business Acquisition and Disposition
Below market leases		$ 2,700,000
Other Liabilities | Fairfax Junction
Business Acquisition and Disposition
Below market leases								$ 400,000
Other Liabilities | Hoboken
Business Acquisition and Disposition
Below market leases							$ 3,300,000
Condominiums | Pike & Rose
Business Acquisition and Disposition
Sales price of real estate											$ 2,100,000
Number of condominium units sold											2

[1] See our Annual Report on Form 10-K for the year ended December 31, 2020 for additional disclosures relating to the mortgages entered into and assumed as a result of the Hoboken acquisition

[2] These acquisitions were completed through newly formed joint ventures, for which we own the controlling interest listed above, and therefore, these properties are consolidated in our financial statements.

[3] ) This property is adjacent to, and is operated as part of the property acquired in 2019. The purchase price was paid with a combination of cash and the issuance of 163,322 downREIT operating partnership units. Approximately $0.5 million and $0.4 million of net assets acquired were allocated to other assets for "above market leases," and other liabilities for "below market leases," respectively.

[4] The purchase price includes the assumption of $8.9 million of mortgage debt, and is in addition to the 37 buildings previously acquired in 2019, and was completed through the same joint venture. Less than $0.1 million and approximately $3.3 million of net assets acquired were allocated to other assets for "above market leases," and other liabilities for "below market leases," respectively.