Significant Transactions During the First Quarter and Subsequent Event (Details) (USD $)	3 Months Ended	4 Months Ended
	Mar. 31, 2012	May 03, 2012	Dec. 31, 2011
Common stock [Abstract]
Sale of Stock, Number of Shares Issued in Transaction	221,072	569,307
Sale of Stock, Consideration Received on Transaction	$ 31,600,000	$ 84,400,000
Sale of Stock, Price Per Share	$ 144.54	$ 149.83
Private Placement Unsecured Notes [Abstract]
Unsecured debt	465,000,000		465,000,000
Mortgage loan interest rate	5.50%
Notes Receivable From Affiliates [Member]
Notes Receivable [Abstract]
The company contributed a note recievable	12,400,000
Interest rate on receivable	5.00%
Unsecured Debt [Member]
Acquisitions [Abstract]
Amount of deferred amortization written off along with applicable prepayment penalties	2,500,000
Private Placement Unsecured Notes [Abstract]
Unsecured debt	200,000,000
All in rate (in hundredths)	4.30%
Term of debt (in years)	9Y
Mortgages [Member]
Notes Receivable [Abstract]
Loan paid in full	7,300,000
Construction Loan Payable [Member]
Acquisitions [Abstract]
Rate Margin added to LIBOR Rate Construction Loan (in basis points)	350
Notes Receivable [Abstract]
Loan paid in full	5,600,000
Reed Square [Member]
Acquisitions [Abstract]
Number of units	100
Cost of acquired entity	23,000,000
Bon Terra [Member]
Acquisitions [Abstract]
Number of units	60
Cost of acquired entity	16,000,000
Essex Skyline at MacArthrur Place [Member]
Acquisitions [Abstract]
Number of units	349
Cost of acquired entity	85,000,000
Promote booked for performance acheivement	2,300,000
Secured debt paid off early	10,800,000
Amount of deferred amortization written off along with applicable prepayment penalties	800,000
CPPIB [Member]
Acquisitions [Abstract]
Number of units	197
Notes Receivable [Abstract]
Equity Method Investment, Ownership Percentage	55.00%
Equity Method Investments	$ 5,500,000