Organization and Basis of Presentation - Narrative (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) community partnership building investment apartment project shares	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) community partnership building investment apartment project shares	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) investment shares	Jan. 01, 2018 USD ($)	Dec. 31, 2016 USD ($)
Real Estate Properties [Line Items]
Buildings and improvements	$ 10,744,841		$ 10,744,841		$ 10,643,009
Prepaid expenses and other assets	$ 50,924		$ 50,924		$ 39,155
Ownership interest in partnership			96.70%		96.70%
Apartment communities owned (in communities) | community	247		247
Apartment units owned (in units) | apartment	59,982		59,982
Ownership interest, number of commercial buildings (in commercial buildings) | building	1		1
Ownership interest, number of active development projects (in projects) | project	6		6
Ownership interest, number of loan investments (in investments) | investment	3		3
Sales and maturities of marketable securities	$ 3,500	$ 4,600	$ 22,500	$ 33,400
Marketable securities, realized gain (loss)	$ 100	32	$ 700	1,700
Downreit limited partnerships consolidated by company (in partnerships) | partnership	16		16
Communities within Downreit partnerships (in communities) | community	8		8
Number of previously consolidated co-investments considered VIE (in investments) | investment	8		8
Assets related to variable interest entities net of intercompany eliminations	$ 839,100		$ 839,100		$ 837,700
Liabilities related to variable interest entities net of intercompany eliminations	264,100		264,100		265,500
Noncontrolling interest in variable interest entity	64,600		64,600		66,700
Fixed rate debt carrying amount	5,000,000		5,000,000		4,900,000
Fixed rate debt fair value	5,000,000		5,000,000		5,000,000
Variable rate debt, carrying amount	612,700		612,700		792,900
Variable rate debt fair value	615,300		615,300		793,900
Investments in mortgage back securities, fair value	130,600		130,600		$ 120,700
Capitalized internal costs related to development and redevelopment projects	4,700	$ 4,600	14,200	$ 14,000
Number of co-investments with financing distributions in excess of the company's investment (in investments) | investment					1
Mortgage backed securities [Member]
Real Estate Properties [Line Items]
Investments in mortgage backed securities, carrying value	122,523		122,523		$ 109,548
Interest And Other Income [Member]
Real Estate Properties [Line Items]
Gain (loss) on sale of equity securities	1,200		400
ASU 2016-01 [Member]
Real Estate Properties [Line Items]
Cumulative effect upon adoption					0
ASU 2017-05 [Member]
Real Estate Properties [Line Items]
Cumulative effect upon adoption					123,708		$ 123,700
Essex Portfolio, L.P. [Member]
Real Estate Properties [Line Items]
Buildings and improvements	10,744,841		10,744,841		10,643,009
Prepaid expenses and other assets	$ 50,924		$ 50,924		$ 39,155
Operating Partnership units outstanding (in units) | shares	2,272,983		2,272,983		2,268,114
Redemption value of operating partnership units outstanding	$ 560,800		$ 560,800		$ 547,500
Essex Portfolio, L.P. [Member] | ASU 2016-01 [Member]
Real Estate Properties [Line Items]
Cumulative effect upon adoption					0
Essex Portfolio, L.P. [Member] | ASU 2017-05 [Member]
Real Estate Properties [Line Items]
Cumulative effect upon adoption					123,708
Adjustment [Member]
Real Estate Properties [Line Items]
Buildings and improvements					13,200
Prepaid expenses and other assets	$ (10,000)		$ (10,000)		$ (9,300)
Adjustment [Member] | ASU 2016-01 [Member]
Real Estate Properties [Line Items]
Cumulative effect upon adoption						$ 2,200