Organization and Basis of Presentation - Narrative (Details) (USD $)	3 Months Ended
	Mar. 31, 2015 project partnership community apartment building	Mar. 31, 2014	Dec. 31, 2014	Apr. 01, 2014
Real Estate Properties [Line Items]
Ownership interest in partnership	96.70%
Apartment communities owned	240
Apartment units owned	57,203
Ownership interest, number of commercial buildings	4
Ownership interest, number of active development projects	12
Sales and maturities of marketable securities	$ 717,000	$ 4,016,000
Proceeds from sale of available-for-sale securities		4,000,000
Available-for-sale securities, gross realized gain (loss)	0	400,000
Downreit limited partnerships consolidated by company	19
Communities within Downreit partnerships	12
Units Of limited partnership interest, amount	965,289		974,790
Redemption value of variable interest entities	221,900,000		201,400,000
Assets related to variable interest entities net of intercompany eliminations	235,600,000		235,100,000
Liabilities related to variable interest entities net of intercompany eliminations	209,300,000		209,100,000
Share-based compensation	1,370,000	1,233,000
Options exercised during period	114,833	11,242
Fixed rate debt carrying amount	4,800,000,000
Fixed rate debt fair value	5,100,000,000
Investments in mortgage back securities, fair value	99,000,000		96,000,000
Capitalized internal costs related to development and redevelopment projects	2,000,000	1,700,000
BRE Properties, Inc. [Member]
Real Estate Properties [Line Items]
Number of properties acquired		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		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
Employee Stock Option [Member]
Real Estate Properties [Line Items]
Share-based compensation	600,000	600,000
Options exercised during period, intrinsic value	10,300,000	600,000
Options outstanding and vested, aggregate intrinsic value	27,200,000
Share based compensation, cost not yet recognized	6,400,000
Unrecognized compensation cost, recognition period minimum	1 year
Period for recognition of unrecognized compensation costs	5 years
Restricted Stock [Member]
Real Estate Properties [Line Items]
Unrecognized compensation cost, recognition period minimum	1 year
Period for recognition of unrecognized compensation costs	7 years
Long Term Incentive Plan Z1 Units and 2014 LTIP Units [Member]
Real Estate Properties [Line Items]
Share-based compensation	800,000	600,000
Period for recognition of unrecognized compensation costs	10 years 3 months 18 days	4 years
Options vested and non-vested, outstanding intrinsic value	59,900,000
Options outstanding, intrinsic value	10,200,000
Essex Portfolio, L.P. [Member]
Real Estate Properties [Line Items]
Operating Partnership units outstanding	2,182,676		2,168,158
Redemption value of operating partnership units outstanding	501,800,000		447,900,000
Sales and maturities of marketable securities	717,000	4,016,000
Share-based compensation	1,370,000	1,233,000
Collateralized Mortgage Backed Securities [Member]
Real Estate Properties [Line Items]
Held-to-maturity securities	70,810,000		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