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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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- Definition 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. + References No definition available. + Details
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- Definition Carrying value as of the balance sheet date of obligations incurred and payable, pertaining to costs that are statutory in nature, are incurred on contractual obligations, or accumulate over time and for which invoices have not yet been received or will not be rendered. Examples include taxes, interest, rent and utilities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.15(5)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.15(a)) -URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910 + Details
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