Organization and Basis of Presentation - Narrative (Details)		3 Months Ended			9 Months Ended
	Mar. 31, 2014 USD ($) property	Sep. 30, 2015 USD ($) community building partnership apartment project shares	Mar. 31, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) community building partnership apartment project shares	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($) shares	Apr. 01, 2014 USD ($)
Real Estate Properties [Line Items]
Stockholders' Equity Attributable to Parent		$ 6,221,041,000			$ 6,221,041,000		$ 6,022,672,000
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
Reclassification adjustment reducing associated debt liabilities		$ 5,255,937,000			$ 5,255,937,000
Sales and maturities of marketable securities					2,000,000	$ 6,300,000
Available-for-sale securities, gross realized gain (loss)					0	900,000
Proceeds from sale of other investments					5,600,000	0
Realized gain on sale of other investments					$ 600,000	0
Downreit limited partnerships consolidated by company | partnership		19			19
Communities within Downreit partnerships | community		12			12
Units Of limited partnership interest, amount | shares		963,172			963,172		974,790
Redemption value of variable interest entities		$ 215,200,000			$ 215,200,000		$ 201,400,000
Assets related to variable interest entities net of intercompany eliminations		239,500,000			239,500,000		235,100,000
Liabilities related to variable interest entities net of intercompany eliminations		208,100,000			208,100,000		209,100,000
Fixed rate debt carrying amount		4,700,000,000			4,700,000,000
Fixed rate debt fair value		4,900,000,000			4,900,000,000
Investments in mortgage back securities, fair value		107,400,000			107,400,000		$ 96,000,000
Capitalized internal costs related to development and redevelopment projects		$ 2,900,000		$ 2,900,000	$ 8,300,000	$ 7,600,000
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
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		2,176,563			2,176,563		2,168,158
Redemption value of operating partnership units outstanding		$ 486,300,000			$ 486,300,000		$ 447,900,000
Collateralized Mortgage Backed Securities [Member]
Real Estate Properties [Line Items]
Held-to-maturity securities, carrying value		76,993,000			76,993,000		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
Adjustments for New Accounting Principle, Early Adoption [Member]
Real Estate Properties [Line Items]
Reclassification adjustment reducing associated debt liabilities		$ (27,100,000)			$ (27,100,000)		$ (29,400,000)