Organization and Basis of Presentation (Details) (USD $)	3 Months Ended		9 Months Ended												1 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010	Sep. 30, 2011 Equity Method Investment [Member] Essex Apartment Value Fund II, L.P. [Member]	Sep. 30, 2011 Equity Method Investment [Member] Wesco I LLC [Member]	Sep. 30, 2011 Collateralized Mortgage Backed Securities [Member]	Sep. 30, 2011 Investment-grade unsecured bonds [Member]	Dec. 31, 2010 Investment-grade unsecured bonds [Member]	Sep. 30, 2011 Investment funds-US treasuries [Member]	Dec. 31, 2010 Investment funds-US treasuries [Member]	Sep. 30, 2011 Common Stock [Member]	Dec. 31, 2010 Common Stock [Member]	Apr. 30, 2011 7.125% Series H Cumulative Redeemable Preferred Stock [Member]	Apr. 30, 2011 Cumulative Redeemable Preferred Units 7.875% Series B [Member]	Jun. 30, 2011 7.8125% Series F Cumulative Redeemable Preferred Stock [Member]
Organization and Basis of Presentation [Abstract]
General partnership interest (in hundredths)			93.70%
Operating Partnership units outstanding (in shares)	2,227,730		2,227,730		2,200,907
Redemption value of operating partnership units outstanding	$ 267,400,000		$ 267,400,000		$ 251,400,000
Schedule Of Equity And Cost Method Investments [Line Items]
Numbers of apartment communities owned	155		155			14	4
Apartment units owned (in units)	32,076		32,076			2,971	1,693
Ownership interests, number of commercial buildings	6		6
Ownership interests, number of active development projects	5		5
Fund Activities [Abstract]
Number of institutional investors						8
Total combined partnership's equity						265,900,000	200,000,000
Equity commitment per partner							100,000,000
Capital contribution to fund						75,000,000	56,800,000
General partner and limited partner interest in fund (in hundredths)						28.20%
Utilized leverage equal to the initial acquisition of the underlying real estate (in hundredths)						55.00%
Joint venture partner's leverage percentage - minimum (in hundredths)							50.00%
Joint venture partner's leverage percentage - maximum (in hundredths)							60.00%
Programmatic joint venture investment period (in years)							2 Y
Schedule of Held-to-maturity Securities [Line Items]
Marketable securities								45,500,000
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29,913,000		29,913,000		45,226,000				3,611,000	22,243,000	11,789,000	14,345,000	14,513,000	8,638,000
Unrealized Gain	199,000		199,000		5,097,000				373,000	4,403,000	145,000	582,000	(319,000)	112,000
Fair Value	30,112,000		30,112,000		50,323,000				3,984,000	26,646,000	11,934,000	14,927,000	14,194,000	8,750,000
Proceeds from sales of available for sale securities			26,800,000	64,700,000
Realized gain from sales of available for sale securities			4,500,000	9,000,000
Variable Interest Entities [Abstract]
Number of DownREIT limited partnerships the company consolidates	19		19
Number of communities within the DownREIT partnership	12		12
Total DownREIT Partnership's Outstanding units (shares)	1,068,693		1,068,693		1,096,871
Redemption value of the variable interest entities	128,300,000		128,300,000		125,300,000
Assets related to variable interest entities, net intercompany eliminations	215,900,000		215,900,000		217,300,000
Liabilities related to variable interest entities, net of intercompany eliminations	172,400,000		172,400,000		168,000,000
Stock-Based Compensation [Abstract]
Stock-based compensation expense for options and restricted stock	300,000	300,000	1,000,000	700,000
Intrinsic value of the stock options exercised	600,000	400,000	3,600,000	1,900,000
Intrinsic value of the stock options outstanding and fully vested	6,900,000		6,900,000
Unrecognized compensation cost related to unvested share-based compensation granted under the stock option and restricted stock plans	3,700,000		3,700,000
Compensation cost recognized for stock options over a weighted-average period	1 to 6 years		1 to 6 years
Compensation cost for restricted stock awards expected to be recognized straight-line	7		7
Stock-based compensation expense for the Series Z Units	200,000	400,000	700,000	1,800,000
Series Z units converted into common units of the operating partnership			131,409
Capitalization of stock based compensation for Z units	100,000	100,000	300,000	600,000
Intrinsic value of Series Z Units outstanding	15,600,000		15,600,000
Unamortized cost recognition period range (in years)	14 Y		14 Y
Total unrecognized compensation cost related to Z-1 Units subject to future vesting	6,300,000		6,300,000
Fair Value of Financial Instruments [Abstract]
Fixed rate debt and unsecured bonds	1,770,000,000		1,770,000,000
Fair value of fixed rate debt and unsecured bonds	1,880,000,000		1,880,000,000
Variable rate debt instruments excluding borrowings under the lines of credit	242,200,000		242,200,000
Fair value of variable rate debt excluding borrowing under lines of credit	224,900,000		224,900,000
Preferred Equity Transactions [Abstract]
Preferred stock issued (in shares)	647,787		1,959,411												2,950,000
Dividend interest rate on preferred stock (in hundredths)															7.125%	7.875%	7.8125%
Price per share of stock issued (in dollars per share)															$ 25.00
Proceeds from stock issuance, net of costs and original issuance discount															71,400,000
Liquidation value of previously issued preferred stock																80,000,000	25,000,000
Amount paid in excess of carrying value to redeem preferred units																$ 1,000,000	$ 900,000