Significant Transactions During the Third Quarter of 2012 and Subsequent Events (Details) (USD $)	3 Months Ended	9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended	8 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Unsecured Debt [Member]	Sep. 30, 2012 Unsecured Debt [Member]	Sep. 30, 2012 Mortgages [Member]	Aug. 31, 2012 Riley Square formerly Waterstone Santa Clara [Member]	Sep. 30, 2012 Park West [Member]	Sep. 30, 2012 Domaine [Member]	Oct. 31, 2012 Kirkland, Washington [Member]	Oct. 31, 2012 Willow Lake Apartments [Member]	Sep. 30, 2012 Wesco I [Member]
Acquisitions [Abstract]
Cost of acquired entity							$ 38,300,000	$ 31,600,000	$ 34,000,000	$ 15,900,000	$ 148,000,000
Number of units							156	126	92	90	508
Joint venture ownership percentage (in hundredths)												50.00%
Secured Debt	1,571,821,000	1,571,821,000	1,745,858,000				17,500,000		14,600,000
Fixed Rate							5.20%		5.70%
Term of debt							8 years		8 years
Loan premium debt at fair value							2,300,000		2,400,000
Expected exterior renovation costs								8,000,000			4,500,000
Effective interest rate percentage (in hundredths)							3.10%		3.00%
Common stock [Abstract]
Number of common shares sold (in shares)	633,636	1,775,748
Proceeds from sale of common shares	97,900,000	268,858,000
Average price per share of common shares sold (dollars per share)	$ 155.97	$ 152.97
Debt Disclosure [Abstract]
Repayments of loans						86,900,000
Losses from early retirement of debt						1,200,000
Additional borrowings				150,000,000
Unsecured Term Loan				350,000,000	350,000,000
Variable rate basis					LIBOR
Basis spread on variable rate (in hundredths)				1.30%	1.30%
Proceeds from issuance of unsecureddDebt				$ 300,000,000
Interest rate (in hundredths)				3.625%	3.625%