Lines of Credit (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended								1 Months Ended	12 Months Ended
	Dec. 31, 2010	Dec. 31, 2011	Dec. 31, 2011 Unsecured Bonds Average 4.5% [Member]	Dec. 31, 2011 Unsecured Bonds 4.36% [Member]	Dec. 31, 2011 Unsecured Bonds 4.5% [Member]	Dec. 31, 2011 Unsecured Bonds 4.92 % [Member]	Dec. 31, 2011 Unsecured Term Loan [Member]	Dec. 31, 2011 Secured Line of Credit Freddie Mac [Member]	Dec. 31, 2010 Secured Line of Credit Freddie Mac [Member]	Jan. 31, 2012 Line of Credit Working Capital [Member]	Dec. 31, 2011 Line of Credit Working Capital [Member]	Dec. 31, 2010 Unsecured Line of Credit [Member]	Dec. 31, 2011 Unsecured Line of Credit [Member]
Line of Credit Facility [Abstract]
Number of outstanding lines of credit		2
Line of Credit Facility [Line Items]
Line of credit maximum borrowing capacity	$ 207.2	$ 440.0							$ 250.0		$ 15.0
Line of credit with an accordion option													425.0
Unsecured line of credit balance												150
Accordion option to increase maximum borrowing capacity												500.0
Basis spread on variable rate, initial (in hundredths)											1.25%	3.00%
Basis spread on variable rate, amended (in hundredths)							1.40%					2.40%
Increment in borrowing limit										25.0
Line of credit facility maturity date									2013-12		2014-01	2014-12
Number of apartment communities securing the credit facility									11
Number of one year extensions available													2
Early retirement of debt	99.0							0.6
Debt Instrument [Line Items]
Proceeds from issuance of unsecured bonds			265
Average interest rates for bonds issued (in hundredths)			4.50%
Maturity date				Jan. 01, 2016	Jan. 01, 2017	Jan. 01, 2019
Interest rate (in hundredths)				4.36%	4.50%	4.92%
Unsecured term loan amount		200.0					200.0
Debt instrument, basis spread on variable rate (in hundredths)							1.40%					2.40%
Term of loan							5Y
Notional amount of interest rate swap							$ 150
Interest rate after swap contract							2.66%