Organization and Basis of Presentation - Narrative (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) community building investment partnership apartment project shares	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) shares
Real Estate Properties [Line Items]
Ownership interest in partnership (as a percent)	96.70%		96.70%
Apartment communities owned (in communities) | community	247
Apartment units owned (in units) | apartment	60,240
Ownership interest, number of commercial buildings (in commercial buildings) | building	1
Ownership interest, number of active development projects (in projects) | project	7
Sales and maturities of marketable securities	$ 9,600	$ 24,900
Marketable securities, realized gain (loss)	$ 700	1,600
Downreit limited partnerships consolidated by company (in partnerships) | partnership	16
Communities within Downreit partnerships (in communities) | community	8
Number of previously consolidated co-investments considered VIE (in investments) | investment	8
Assets related to variable interest entities net of intercompany eliminations	$ 845,700		$ 837,700
Liabilities related to variable interest entities net of intercompany eliminations	266,400		265,500
Noncontrolling interest in variable interest entity	67,200		66,700
Fixed rate debt carrying amount	5,100,000		4,900,000
Fixed rate debt fair value	5,100,000		5,000,000
Variable rate debt, carrying amount	612,100		792,900
Variable rate debt fair value	615,600		793,900
Investments in mortgage back securities, fair value	122,800		120,700
Capitalized internal costs related to development and redevelopment projects	$ 5,000	$ 4,800
Number of co-investments with financing distributions in excess of the company's investment (in investments) | investment	1
Interest And Other Income [Member]
Real Estate Properties [Line Items]
Gain (loss) on sale of equity securities	$ (900)
Mortgage Backed Securities [Member]
Real Estate Properties [Line Items]
Carrying Value	$ 113,710		109,548
ASU 2017-05 [Member]
Real Estate Properties [Line Items]
Cumulative effect upon adoption			$ 123,708
Essex Portfolio, L.P. [Member]
Real Estate Properties [Line Items]
Operating Partnership units outstanding (in units) | shares	2,273,413		2,268,114
Redemption value of operating partnership units outstanding	$ 547,200		$ 547,500
Essex Portfolio, L.P. [Member] | ASU 2017-05 [Member]
Real Estate Properties [Line Items]
Cumulative effect upon adoption			$ 123,708