Organization and Basis of Presentation - Narrative (Details)	3 Months Ended				6 Months Ended
	Jun. 30, 2015 USD ($) community building partnership apartment project shares	Mar. 31, 2015 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($) property	Jun. 30, 2015 USD ($) community building partnership apartment project shares	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($) shares	Apr. 01, 2014 USD ($)
Real Estate Properties [Line Items]
Ownership interest in partnership					96.80%
Apartment communities owned | community	245				245
Apartment units owned | apartment	58,768				58,768
Ownership interest, number of commercial buildings | building	4				4
Ownership interest, number of active development projects | project	9				9
Sales and maturities of marketable securities					$ 1,968,000	$ 5,192,000
Available-for-sale securities, gross realized gain (loss)					$ 0	900,000
Downreit limited partnerships consolidated by company | partnership	19				19
Communities within Downreit partnerships | community	12				12
Units Of limited partnership interest, amount | shares	963,789				963,789		974,790
Redemption value of variable interest entities	$ 204,800,000				$ 204,800,000		$ 201,400,000
Assets related to variable interest entities net of intercompany eliminations	236,500,000				236,500,000		235,100,000
Liabilities related to variable interest entities net of intercompany eliminations	207,900,000				207,900,000		209,100,000
Fixed rate debt carrying amount	4,800,000,000				4,800,000,000
Fixed rate debt fair value	4,900,000,000				4,900,000,000
Investments in mortgage back securities, fair value	101,900,000				101,900,000		$ 96,000,000
Capitalized internal costs related to development and redevelopment projects	$ 3,400,000		$ 3,000,000		$ 5,400,000	4,700,000
BRE Properties, Inc. [Member]
Real Estate Properties [Line Items]
Number of properties acquired | property				14
Value of acquired properties				$ 1,400,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net		$ 4,331,000,000						$ 4,300,000,000
Number of properties contributed | property				14
Decrease in preliminary fair value of real property		13,100,000
Increase in preliminary fair value of investment in joint ventures		6,000,000
Decrease in preliminary estimate for liabilities assumed		$ 7,100,000
Essex Portfolio, L.P. [Member]
Real Estate Properties [Line Items]
Operating Partnership units outstanding | shares	2,181,076				2,181,076		2,168,158
Redemption value of operating partnership units outstanding	$ 463,500,000				$ 463,500,000		$ 447,900,000
Sales and maturities of marketable securities					1,968,000	$ 5,192,000
Collateralized Mortgage Backed Securities [Member]
Real Estate Properties [Line Items]
Held-to-maturity securities	$ 73,743,000				$ 73,743,000		$ 67,996,000
Other Assets [Member] | BRE Properties, Inc. [Member]
Real Estate Properties [Line Items]
Acquired assets with in-place lease value				$ 19,000,000