Organization and Basis of Presentation - Narrative (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) community building investment partnership apartment project shares	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) community building investment partnership apartment project shares	Sep. 30, 2016 USD ($)	Dec. 31, 2016 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		247
Apartment units owned (in units) | apartment	60,305		60,305
Ownership interest, number of commercial buildings (in commercial buildings) | building	1		1
Ownership interest, number of active development projects (in projects) | project	7		7
Sales and maturities of marketable securities	$ 4,600	$ 3,500	$ 33,400	$ 14,700
Available-for-sale securities, gross realized gain (loss)	$ 32	1,000	$ 1,700	2,900
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	$ 842,700		$ 842,700		$ 746,100
Liabilities related to variable interest entities net of intercompany eliminations	278,700		278,700		221,300
Noncontrolling interest in variable interest entity	66,100		66,100		45,400
Fixed rate debt carrying amount	5,000,000		5,000,000		5,100,000
Fixed rate debt fair value	5,100,000		5,100,000
Investments in mortgage back securities, fair value	117,600		117,600		108,800
Capitalized internal costs related to development and redevelopment projects	4,900	$ 4,800	$ 15,000	$ 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]
Held-to-maturity securities, carrying value	$ 105,541		$ 105,541		$ 94,393
Essex Portfolio, L.P. [Member]
Real Estate Properties [Line Items]
Operating Partnership units outstanding (in units) | shares	2,251,112		2,251,112		2,237,290
Redemption value of operating partnership units outstanding	$ 571,800		$ 571,800		$ 520,200