Organization and Basis of Presentation - Narrative (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) community building partnership project apartment investment shares	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) community building partnership project apartment investment shares	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) investment community partnership shares
Real Estate Properties [Line Items]
Apartment communities owned (in communities) | community	246		246
Apartment units owned (in apartments) | apartment	60,799		60,799
Ownership interest, number of commercial buildings (in commercial buildings) | building	3		3
Ownership interest, number of active development projects (in projects) | project	2		2
Ownership interest, number of unconsolidated joint venture projects (in projects) | project	1		1
Equity securities	$ 2,500,000		$ 2,500,000		$ 2,500,000
Sales and maturities of marketable securities	100,000	$ 52,600,000	15,000,000	$ 56,900,000
Marketable securities, realized gain (loss)	$ 0	91,000	2,500,000	100,000
Unrealized gains on equity securities recognized through income			$ 23,772,000	2,215,000
DownREIT limited partnerships consolidated by company (in partnerships) | partnership	18		18		17
Communities within DownREIT partnerships (in communities) | community	9		9		9
Number of previously consolidated co-investments considered VIE (in investments) | investment	6		6		5
Assets related to variable interest entities net of intercompany eliminations	$ 909,100,000		$ 909,100,000		$ 898,500,000
Liabilities related to variable interest entities net of intercompany eliminations	323,300,000		323,300,000		326,800,000
Noncontrolling interest in variable interest entity	122,400,000		122,400,000		120,800,000
Fixed rate debt carrying amount	5,700,000,000		5,700,000,000		5,500,000,000
Fixed rate debt fair value	6,100,000,000		6,100,000,000		6,000,000,000
Variable rate debt, carrying amount	366,400,000		366,400,000		775,100,000
Variable rate debt fair value	364,300,000		364,300,000		770,100,000
Capitalized internal costs related to development and redevelopment projects	5,600,000	6,100,000	17,500,000	24,900,000
Redeemable noncontrolling interest	$ 32,698,000		$ 32,698,000		$ 32,239,000
Maximum
Real Estate Properties [Line Items]
Apartment lease term	12 months		12 months
Minimum
Real Estate Properties [Line Items]
Apartment lease term	9 months		9 months
Interest And Other Income (Expense)
Real Estate Properties [Line Items]
Unrealized gains on equity securities recognized through income	$ 7,100,000	$ 3,300,000	$ 23,800,000	2,200,000
Essex Portfolio, L.P.
Real Estate Properties [Line Items]
Operating Partnership units outstanding (in shares) | shares	2,282,464		2,282,464		2,294,760
Redemption value of operating partnership units outstanding	$ 729,800,000		$ 729,800,000		$ 544,800,000
Unrealized gains on equity securities recognized through income			23,772,000	$ 2,215,000
Redeemable noncontrolling interest	$ 32,698,000		$ 32,698,000		$ 32,239,000
Partnership Interest
Real Estate Properties [Line Items]
Ownership interest in partnership			96.60%		96.60%