Notes Payable, Unsecured Notes and Credit Facility (Details 2) (USD $)	6 Months Ended		1 Months Ended				6 Months Ended						1 Months Ended
	Jun. 30, 2012	Jun. 30, 2012 Mortgage notes payable	Jan. 31, 2012 5.5% notes payable maturing in 2012	Feb. 29, 2012 4.61% fixed-rate mortgage note	May 31, 2012 Variable rate secured mortgage note	Feb. 29, 2012 Variable rate secured mortgage note	Jun. 30, 2012 Variable rate unsecured credit facility	Dec. 31, 2011 Variable rate unsecured credit facility	Jun. 30, 2012 Fixed rate mortgage notes payable	Dec. 31, 2011 Fixed rate mortgage notes payable	Jun. 30, 2012 Variable rate mortgage notes payable and Credit Facility	Dec. 31, 2011 Variable rate mortgage notes payable and Credit Facility	May 31, 2012 Other
Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate, percentage			5.50%	4.61%
Principal amount of debt repaid			$ 179,400,000		$ 14,566,000	$ 48,500,000							$ 33,100,000
Debt assumed				11,958,000
Loss on extinguishment of debt, net	1,179,000					1,179,000							602,000
Available borrowing capacity							750,000,000
Term of debt instrument							4 years
Line of credit facility, extension period							1 year
Debt instrument variable rate							LIBOR
Debt instrument, basis spread on variable rate (as a percent)							1.075%
Current interest rate (as a percent)							1.32%
Outstanding balance of letters of credit							45,975,000	52,659,000
Net carrying value of apartment communities and improved land parcels securing debt		1,535,159,000
Mortgage notes payable held by wholly owned subsidiaries guaranteed by the Company		$ 179,777,000
Weighted average interest rate, debt (as a percent)									5.70%	5.70%	2.60%	2.30%