Consolidation and Basis of Presentation and Significant Accounting Policies - Additional Information (Detail) $ in Millions	9 Months Ended
	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