Significant Transactions During the Nine Months Ended 2017 and Subsequent Events (Details)	1 Months Ended											3 Months Ended		9 Months Ended
	Oct. 31, 2017 USD ($)	Oct. 30, 2017 USD ($) $ / shares shares	Aug. 31, 2017 USD ($) apartment	Jul. 31, 2017 USD ($)	May 31, 2017 USD ($)	Apr. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2017 USD ($) apartment	Mar. 31, 2017 USD ($) unit	Jan. 31, 2017 USD ($) apartment	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) investment $ / shares shares	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Other Commitments [Line Items]
Payments to acquire preferred equity investments														$ 200,028,000	$ 124,054,000
Gain on remeasurement of co-investment												$ 0	$ 0	88,641,000	0
Co-investments												1,124,577,000		1,124,577,000		$ 1,161,275,000
Land and land improvements												2,719,064,000		2,719,064,000		2,559,743,000
Buildings and improvements												10,585,742,000		10,585,742,000		10,116,563,000
Prepaid expenses and other assets												51,453,000		51,453,000		48,450,000
Commitment to fund preferred equity investment in a multifamily development project located in Seattle, WA														$ 89,300,000
Number of preferred equity investments in development with commitments to fund | investment														5
Preferred equity investment, funded amount												38,000,000		$ 38,000,000
Proceeds from partial redemption of preferred equity investment in a joint venture														112,572,000	34,564,000
Gain on early redemption of preferred equity investment														10,058,000	13,046,000
Proceeds from full redemption of preferred equity investment class			$ 11,700,000
Co-investments												$ 1,088,332,000		$ 1,088,332,000		1,161,275,000
Repaid amount of private placement				$ 40,000,000
Private placement repurchase, coupon rate				4.50%
Common stock issued under equity distribution program (in shares) | shares														311,873
Common stock issued under equity distribution program average price (in dollars per share) | $ / shares												$ 260.30		$ 260.30
Net proceeds from issuance of common stock														$ 80,377,000	$ (382,000)
Subsequent Event [Member]
Other Commitments [Line Items]
Common stock issued under equity distribution program (in shares) | shares		33,571
Common stock issued under equity distribution program average price (in dollars per share) | $ / shares		$ 261.19
Net proceeds from issuance of common stock		$ 8,700,000
Senior Notes [Member] | Senior Unsecured Notes at 5.500% [Member]
Other Commitments [Line Items]
Repurchased face amount of debt							$ 300,000,000	$ 300,000,000	$ 300,000,000	$ 300,000,000
Debt, stated interest rate							5.50%	5.50%	5.50%	5.50%
Senior Notes [Member] | Senior Unsecured Notes At 3.625% [Member]
Other Commitments [Line Items]
Debt, stated interest rate						3.625%
Debt, face amount						$ 350,000,000
Debt, term (in years)						10 years
Minimum [Member]
Other Commitments [Line Items]
Preferred returns rate														9.50%
Maximum [Member]
Other Commitments [Line Items]
Preferred returns rate														11.00%
Secured Note Receivable, 3.5% Interest Rate, Due December 16, 2017 [Member] | Wesco V Property [Member] | Affiliated Entity [Member]
Other Commitments [Line Items]
Notes and other receivables (includes related party receivables of $65.6 million and $11.3 million as of September 30, 2017 and December 31, 2016, respectively)			$ 55,000,000									$ 55,000,000		$ 55,000,000		0
Stated interest rate			3.50%
Limited Liability Company With Development Projects In The Jefferson At Stadium Park Apartments [Member] | Notes and Other Receivables from Affiliates [Member]
Other Commitments [Line Items]
Co-investments					$ 13,200,000
Preferred equity investment, dividend rate, first 30 months					10.00%
Membership Interest In Palm Valley [Member]
Other Commitments [Line Items]
Equity method investment, ownership percentage acquired											50.00%
Payments to acquire preferred equity investments											$ 183,000,000
Mortgage encumbrance																220,000,000
Number of units acquired | apartment											1,098
Gain on remeasurement of co-investment											$ 88,600,000
Weighted Average Essex Ownership Percentage														50.00%
Co-investments												0		$ 0		$ 68,396,000
Membership Interest In Sage At Cupertino [Member]
Other Commitments [Line Items]
Equity method investment, ownership percentage acquired								40.50%
Mortgage encumbrance							$ 52,000,000	$ 52,000,000	$ 52,000,000	$ 52,000,000
Number of units acquired									230	230
Co-investments							15,300,000	$ 15,300,000	$ 15,300,000	$ 15,300,000
Contract price							$ 90,000,000
Membership Interest In Palm Valley And Sage At Cupertino [Member]
Other Commitments [Line Items]
Land and land improvements												169,500,000		169,500,000
Buildings and improvements												365,700,000		365,700,000
Prepaid expenses and other assets												3,200,000		3,200,000
Eighth And Republican Avenue Property [Member]
Other Commitments [Line Items]
Payments to acquire preferred equity investments			$ 101,300,000
Number of units acquired | apartment			211
Wesco V, San Jose Apartment Community [Member] | Wesco V Property [Member]
Other Commitments [Line Items]
Number of units acquired | apartment			213
Consideration transferred in asset acquisition			$ 133,500,000
Mortgage debt assumed in asset acquisition			$ 57,900,000
Mortgage debt assumed in asset acquisition, effective interest rate			3.40%
Jefferson at Hollywood [Member]
Other Commitments [Line Items]
Number of units sold | apartment											270
Sales of real estate											$ 132,500,000
Gain on sale of real estate											$ 26,200,000
Madrid In Mission Viejo [Member] | Wesco I [Member]
Other Commitments [Line Items]
Number of units sold | apartment			230
Sales of real estate			$ 83,000,000
Gain on sale of real estate			10,100,000
Repayments of loan on property			30,100,000
Wesco I [Member] | Subsequent Event [Member]
Other Commitments [Line Items]
Weighted Average Essex Ownership Percentage	58.00%
Expected promote income	$ 38,000,000
Preferred Equity Investment in a Joint Venture that Holds a Property in Seattle, Washington [Member]
Other Commitments [Line Items]
Proceeds from partial redemption of preferred equity investment in a joint venture						$ 12,600,000
Reduction in preferred equity investment						12,400,000
Gain on early redemption of preferred equity investment						$ 300,000
Preferred Equity Investment In Joint Venture That Holds A Property In San Jose, CA [Member]
Other Commitments [Line Items]
Proceeds from partial redemption of preferred equity investment			$ 6,900,000
Preferred equity investment in joint venture												$ 13,400,000		$ 13,400,000
Wesco V Property [Member]
Other Commitments [Line Items]
Weighted Average Essex Ownership Percentage			50.00%
Co-investments			$ 150,000,000
Preferred Equity Investment In Multifamily Development Project In Los Angeles, CA [Member] | Subsequent Event [Member]
Other Commitments [Line Items]
Commitment to fund preferred equity investment in a multifamily development project located in Seattle, WA	$ 40,000,000
Preferred returns rate	11.30%