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Basis of Presentation and Principles of Consolidation and Significant Accounting Policies - Additional Information (Details) $ in Thousands	1 Months Ended	9 Months Ended
	Oct. 31, 2019 USD ($) ApartmentUnit	Sep. 30, 2019 USD ($) ft² Property Community State ApartmentUnit shares	Sep. 30, 2018 shares	Dec. 31, 2021 Property	Sep. 30, 2021 Property	Dec. 31, 2020 Property	Sep. 30, 2020 Property	Dec. 31, 2019 Property	Oct. 28, 2019 ApartmentUnit	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($) shares
Real Estate Properties [Line Items]
General Partners' Capital Account, Units Outstanding | shares		114,065,859	113,838,139
Number of owned or owned interests of apartment communities | Property		303
Development and capital improvements in progress		$ 101,469									$ 59,506
Number of States in which Entity Operates | State		17
Buildings, Improvements, and Other		$ 11,825,934									11,670,216
Land		1,898,509									1,868,828
Accrued expenses and other liabilities		521,374									$ 413,850
Operating Lease, Right-of-Use Asset		53,000
Operating Lease, Liability		$ 32,957
Operating Lease, Weighted Average Remaining Lease Term		32 years
Operating Lease, Weighted Average Discount Rate, Percent		4.40%
Percentage Of Rental Revenue From Leasing Of Apartment Homes		93.00%
Percentage Of Reimbursable Revenue From Leasing Of Apartment Homes		6.00%
Percentage of Rental Revenue and Other Income From Non-leasing Activities		1.00%
Subsequent Event [Member]
Real Estate Properties [Line Items]
Number of apartments included in a community | ApartmentUnit	312
Gain (Loss) on Sale of Properties	$ 20
Assets Held for Sale
Real Estate Properties [Line Items]
Number of owned or owned interests of apartment communities | ApartmentUnit		1
Assets Held for Sale | Subsequent Event [Member]
Real Estate Properties [Line Items]
Number of owned or owned interests of apartment communities | ApartmentUnit									3
Accounting Standards Update 2016-02 [Member]
Real Estate Properties [Line Items]
Operating Lease, Right-of-Use Asset										$ 54,300
Operating Lease, Liability										$ 33,600
Operating Lease, Weighted Average Remaining Lease Term										33 years
Operating Lease, Weighted Average Discount Rate, Percent										4.40%
Post Massachusetts Avenue [Member]
Real Estate Properties [Line Items]
Number of apartments included in a community | Community		1
Real Estate Investments, Joint Ventures		$ 43,800
Post Massachusetts Avenue [Member] | Apartment LLC [Member]
Real Estate Properties [Line Items]
Equity Method Investment, Ownership Percentage		35.00%
Real Estate Technology Ventures, L.P. [Member]
Real Estate Properties [Line Items]
Equity Method Investment, Ownership Percentage		20.40%
Real Estate Investments, Joint Ventures		$ 9,000
Real Estate Technology Ventures, L.P. [Member] | Other Assets [Member]
Real Estate Properties [Line Items]
Real Estate Investments, Joint Ventures		$ 12,300
Post River North [Member]
Real Estate Properties [Line Items]
Percentage Of Ownership Interests		92.50%
Number of apartments included in a community | ApartmentUnit		359
Buildings, Improvements, and Other		$ 68,500
Land		14,900
Accrued expenses and other liabilities		$ 1,000
Percentage of remaining ownership interests		7.50%
Phoenix Midtown [Member]
Real Estate Properties [Line Items]
Percentage Of Ownership Interests		80.00%
Buildings, Improvements, and Other		$ 14,300
Land		9,400
Accrued expenses and other liabilities		$ 2,400
Percentage of remaining ownership interests		20.00%
Minimum [Member]
Real Estate Properties [Line Items]
Percentage Of Ownership Interests		80.00%
Maximum [Member]
Real Estate Properties [Line Items]
Percentage Of Ownership Interests		100.00%
Maximum [Member] | Accounting Standards Update 2016-02 [Member]
Real Estate Properties [Line Items]
Operating lease, term of contract		1 year
Limited Partner [Member]
Real Estate Properties [Line Items]
General Partners' Capital Account, Units Outstanding | shares		114,065,859									113,844,267
Number of owned or owned interests of apartment communities | Property		1
Development and capital improvements in progress		$ 101,469									$ 59,506
Buildings, Improvements, and Other		11,825,934									11,670,216
Land		1,898,509									1,868,828
Accrued expenses and other liabilities		$ 521,374									$ 413,850
Limited Partner [Member] | Minimum [Member]
Real Estate Properties [Line Items]
Percentage Of Ownership Interests		80.00%
Limited Partner [Member] | Maximum [Member]
Real Estate Properties [Line Items]
Percentage Of Ownership Interests		100.00%
Scenario Forecast [Member]
Real Estate Properties [Line Items]
Number of owned or owned interests of apartment communities | Property				2
Retail [Member]
Real Estate Properties [Line Items]
Number of owned or owned interests of apartment communities | Property		31
Square Footage of Real Estate Property | ft²		630,000
Development Properties
Real Estate Properties [Line Items]
Number of owned or owned interests of apartment communities | Community		6
Number of units under development community | Property		1,686
Development and capital improvements in progress		$ 389,500
Development and capital improvements in progress		$ 106,700
Development Properties | Expected Costs [Member]
Real Estate Properties [Line Items]
Number of owned or owned interests of apartment communities | Community		6
Development Properties | Scenario Forecast [Member]
Real Estate Properties [Line Items]
Number of owned or owned interests of apartment communities | Property					1	1	1	1
Commercial Real Estate [Member]
Real Estate Properties [Line Items]
Number of owned or owned interests of apartment communities | Property		4
Square Footage of Real Estate Property | ft²		260,000
M A A L P
Real Estate Properties [Line Items]
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest		96.60%	96.50%

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Amount before accumulated depreciation of building structures held for productive use including addition, improvement, or renovation to the structure, including, but not limited to, interior masonry, interior flooring, electrical, and plumbing, equipment commonly used in offices and stores that have no permanent connection to the structure of a building or utilities. Examples include, but are not limited to, desks, chairs, tables, and bookcases. Also includes amount of investment in certain other real estate investments not specifically identified elsewhere in the existing taxonomy.

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