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Consolidation and Basis of Presentation and Significant Accounting Policies - Additional Information (Detail) $ in Millions | 9 Months Ended |
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Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($) shares | Sep. 30, 2018 apartmentunits | Sep. 30, 2018 Property | Sep. 30, 2018 Communities | Sep. 30, 2018 USD ($) | Sep. 30, 2018 shares | Sep. 30, 2018 ft²
Cash and Cash Equivalents [Member] | Accounting Standards Update 2016-18 [Member] |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification |  | $ 80.3 |  |  |  |  |  |
Change in Cash Used in Investing Activities [Member] | Accounting Standards Update 2016-18 [Member] |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification |  | 10.6 |  |  |  |  |  |
Change in Provided by Operating Activities [Member] | Accounting Standards Update 2016-18 [Member] |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification |  | $ 2.6 |  |  |  |  |  |
General Partners' Capital Account, Units Outstanding | shares |  | 113,627,014 |  |  |  |  | 113,838,139 |
Limited Liability (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest | 96.50% |  |  |  |  |  |  |
Number of owned or owned interests of apartment communities |  |  |  | 303 | 303 |  |  |
Percentage Of Rental Revenue And Other Income From Leasing Of Apartment Homes | 93.00% |  |  |  |  |  |  |
Percentage of Rental Revenue and Other Income From Non-leasing Activities | 7.00% |  |  |  |  |  |  |
Minimum [Member] |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |
Percentage Of Ownership Interests | 92.50% |  |  |  |  |  |  |
Maximum [Member] |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |
Percentage Of Ownership Interests | 100.00% |  |  |  |  |  |  |
Development Properties [Member] |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |
Number of owned or owned interests of apartment communities | Communities |  |  |  |  | 4 |  |  |
Number of units under development community | Property |  |  |  | 717 |  |  |  |
Development Properties [Member] | Expected Costs [Member] |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |
Development and capital improvements in progress |  |  |  |  |  | $ 148.0 |  |
Development Properties [Member] | Costs Incurred to Date [Member] |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |
Development and capital improvements in progress |  |  |  |  |  | 45.7 |  |
Retail |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |
Number of owned or owned interests of apartment communities | Property |  |  |  | 30 |  |  |  |
Square Footage of Real Estate Property | ft² |  |  |  |  |  |  |  | 615,000
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
Real Estate Technology Ventures, L.P. [Member] |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |
Equity Method Investment, Ownership Percentage | 21.00% |  |  |  |  |  |  |
Real Estate Investments, Joint Ventures |  |  |  |  |  | 2.5 |  |
Real Estate Technology Ventures, L.P. [Member] | Limited Partner Capital Contribution, Future Obligation [Member] |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |
Real Estate Investments, Joint Ventures |  |  |  |  |  | 12.0 |  |
Post River North [Member] |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |
Percentage Of Ownership Interests | 92.50% |  |  |  |  |  |  |
Number of apartments included in a community | apartmentunits |  |  | 359 |  |  |  |  |
Buildings, Improvements, and Other |  |  |  |  |  | 70.3 |  |
Land |  |  |  |  |  | 14.9 |  |
Accrued expenses and other liabilities |  |  |  |  |  | 1.1 |  |
Post Massachusetts Avenue [Member] |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |
Equity Method Investment, Ownership Percentage | 35.00% |  |  |  |  |  |  |
Real Estate Investments, Joint Ventures |  |  |  |  |  | 44.6 |  |
Accounting Standards Update 2016-15 [Member] |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |
Net Cash Provided by (Used in) Operating Activities |  | $ 1.8 |  |  |  |  |  |
Net Cash Provided by (Used in) Investing Activities |  | 0.2 |  |  |  |  |  |
Net Cash Provided by (Used in) Financing Activities |  | 1.6 |  |  |  |  |  |
Accounting Standards Update 2016-02 [Member] |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |
Operating Lease, Right-of-Use Asset |  |  |  |  |  | $ 40.0 |  |
Water Utility Revenue [Member] |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |
Property Management Fee Revenue | $ 29.2 | 28.5 |  |  |  |  |  |
Cable Utility Revenue [Member] |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |
Property Management Fee Revenue | $ 22.4 | $ 22.9 |  |  |  |  |  |

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