Consolidation and Basis of Presentation and Significant Accounting Policies - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($) shares	Mar. 31, 2018 apartmentunits	Mar. 31, 2018 Property	Mar. 31, 2018 Communities	Mar. 31, 2018 USD ($)	Mar. 31, 2018 shares	Mar. 31, 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		$ 24.5
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		58.3
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		$ 5.5
General Partners' Capital Account, Units Outstanding | shares		113,574,798					113,745,207
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	302
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					2
Number of units under development community | Property				578
Development Properties [Member] | Expected Costs [Member]
Real Estate Properties [Line Items]
Development and capital improvements in progress						$ 125.8
Development Properties [Member] | Costs Incurred to Date [Member]
Real Estate Properties [Line Items]
Development and capital improvements in progress						101.4
Retail
Real Estate Properties [Line Items]
Number of owned or owned interests of apartment communities | Property				29
Square Footage of Real Estate Property | ft²								620,000
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	26.50%
Real Estate Investments, Joint Ventures						2.0
Real Estate Technology Ventures, L.P. [Member] | Limited Partner Capital Contribution, Future Obligation [Member]
Real Estate Properties [Line Items]
Real Estate Investments, Joint Ventures						12.8
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						71.0
Land						14.9
Accrued expenses and other liabilities						2.6
Post Massachusetts Avenue [Member]
Real Estate Properties [Line Items]
Equity Method Investment, Ownership Percentage	35.00%
Real Estate Investments, Joint Ventures						$ 45.0
Accounting Standards Update 2016-15 [Member]
Real Estate Properties [Line Items]
Net Cash Provided by (Used in) Operating Activities		$ 0.2
Net Cash Provided by (Used in) Investing Activities		0.2
Water Utility Revenue [Member]
Real Estate Properties [Line Items]
Tenant Reimbursements	$ 9.8	9.4
Cable Utility Revenue [Member]
Real Estate Properties [Line Items]
Tenant Reimbursements	$ 7.5	$ 7.7