Significant Transactions During the Third Quarter of 2011 and Subsequent Events (Details) (USD $)	3 Months Ended		9 Months Ended		1 Months Ended				1 Months Ended		3 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Sep. 30, 2011	Oct. 31, 2011	Sep. 30, 2011 Bernard [Member]	Sep. 30, 2011 Fountain at La Brea [Member]	Sep. 30, 2011 Joint Venture with Canada Pension Plan Investment Board [Member]	Oct. 31, 2011 Joint venture to develop land in Santa Clara [Member]	Jul. 31, 2011 Reveal Community in Canoga Park of Los Angeles [Member]	Sep. 30, 2011 Redmond Hill [Member]	Sep. 30, 2011 Preferred stock investment in Multifamily Technology Solutions [Member]	Sep. 30, 2011 View Pointe [Member]
Acquisitions [Abstract]
Number of units					63	187	309	494	438	882
Cost of acquired entity					$ 13,800,000				$ 132,900,000	$ 151,300,000
Debt instrument face amount					9,400,000	54,500,000			78,700,000	97,100,000
Mortgage loan interest rate					6.00%					4.06%
Maturity date					Jan. 31, 2019	Oct. 31, 2046
Loan premium to reflect the debt at fair value					1,100,000
Effective interest rate					4.00%
Dispositions [Abstract]
Net proceeds on sale of investment											1,600,000	1,400,000
Gain of sale of investment											900,000	200,000
Development [Abstract]
Number of square feet of retail space						18,200
Equity investment, contribution						9,000,000		800,000
Equity investment, ownership percentage						50.00%	55.00%	15.00%
Debt instrument, amount outstanding						13,500,000
Previous ownership percentage								85.00%
Co-investments [Abstract]
Interest calculation basis			LIBOR						LIBOR
Interest spread over basis		1.25%	1.25%						1.90%
Maturity term (in years)		3	3						2	10
Cost of apartment interior upgrades										11,700,000
Common Stock, Preferred Stock and Preferred Units [Abstract]
Number of common shares sold (in shares)		647,787	1,959,411
Proceeds from sale of common shares		88,200,000	256,790,000
Average price per share of common shares sold (dollars per share)		$ 137.83	$ 132.65
Bonds [Abstract]
Loan pay-off amount		9,800,000
Number of apartments released from an affordable rent bond program (in units)		35	35
Amount of deferred charges written off		200,000
Line of Credit [Abstract]
Amount of Secured line of credit facility with Freddie Mac, notice to terminate				250,000,000
Amount outstanding under secured line of credit with Freddie Mac		174,000,000	174,000,000
Number of properties securing the credit facility with Freddie Mac				11
Amount of accelerated amortization of deferred charges due to termination of credit facility	300,000	300,000
Line of credit amount		425,000,000	425,000,000
Line of credit, previous capacity		275,000,000	275,000,000
Line of credit, accordion feature to expand		$ 500,000,000	$ 500,000,000
Annual facility fee			0.25%