Organization and Basis of Presentation - Narrative (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) community building investment partnership apartment project shares	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) community building investment partnership apartment project shares	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) investment shares	Jan. 01, 2018 USD ($)	Dec. 31, 2016 USD ($)
Real Estate Properties [Line Items]
Buildings and improvements	$ 10,693,682		$ 10,693,682		$ 10,639,108
Prepaid expenses and other assets	$ 56,338		$ 56,338		$ 43,056
Ownership interest in partnership (as a percent)			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 | investment	3		3
Sales and maturities of marketable securities	$ 9,400	$ 3,900	$ 19,000	$ 28,800
Marketable securities, realized gain (loss)	$ (200)	13	$ 500	1,600
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	$ 835,200		$ 835,200		$ 837,700
Liabilities related to variable interest entities net of intercompany eliminations	262,900		262,900		265,500
Noncontrolling interest in variable interest entity	64,900		64,900		66,700
Fixed rate debt carrying amount	5,100,000		5,100,000		4,900,000
Fixed rate debt fair value	5,100,000		5,100,000		5,000,000
Variable rate debt, carrying amount	615,100		615,100		792,900
Variable rate debt fair value	618,200		618,200		793,900
Investments in mortgage back securities, fair value	127,200		127,200		$ 120,700
Capitalized internal costs related to development and redevelopment projects	4,500	$ 4,600	9,500	$ 9,400
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]
Carrying Value	118,032		118,032		$ 109,548
Interest And Other Income [Member]
Real Estate Properties [Line Items]
Gain (loss) on sale of equity securities	100		(800)
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	10,693,682		10,693,682		10,639,108
Prepaid expenses and other assets	$ 56,338		$ 56,338		$ 43,056
Operating Partnership units outstanding (in units) | shares	2,272,983		2,272,983		2,268,114
Redemption value of operating partnership units outstanding	$ 543,400		$ 543,400		$ 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	10,000		10,000		9,300
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