Notes Payable, Unsecured Notes and Credit Facility (Details 2) (USD $)	12 Months Ended		1 Months Ended				1 Months Ended				12 Months Ended			12 Months Ended						1 Months Ended		12 Months Ended	1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Nov. 30, 2012 Unsecured notes	Dec. 31, 2012 Unsecured notes Minimum	Dec. 31, 2012 Unsecured notes Maximum	Dec. 31, 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	Dec. 31, 2012 Variable rate unsecured credit facility option	Dec. 31, 2011 Variable rate unsecured credit facility	Sep. 30, 2011 Variable rate unsecured credit facility	Dec. 31, 2012 Variable rate unsecured credit facility Minimum	Dec. 31, 2012 Variable rate unsecured credit facility Maximum	Dec. 31, 2012 Fixed rate mortgage notes payable	Dec. 31, 2011 Fixed rate mortgage notes payable	Dec. 31, 2012 Variable rate mortgage notes payable and Credit Facility	Dec. 31, 2011 Variable rate mortgage notes payable and Credit Facility	May 31, 2012 Other	Dec. 31, 2012 2.85% unsecured notes maturing in September 2023	Dec. 31, 2012 2.85% unsecured notes maturing in September 2023	Sep. 30, 2012 2.95% unsecured notes maturing in September 2022
Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate, percentage			6.125%				5.50%	4.61%													2.85%	2.85%	2.95%
Principal amount of debt repaid			$ 201,601,000				$ 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	1,940,000								1,179,000
Loss on extinguishment of debt, net	602,000	3,880,000																		602,000
Principal amount of unsecured notes issued																					250,000,000		450,000,000
Effective interest rate, percentage																						3.00%	4.30%
Available borrowing capacity											1,300,000,000		750,000,000
Line of credit facility, extension options											2
Line of credit facility, additional extension period											6 months
Line of credit facility, extension period											1 year
Additional borrowing capacity											550,000,000
Debt instrument variable rate											LIBOR
Debt instrument, basis spread on variable rate (as a percent)				0.25%	0.45%						1.25%			0.95%	1.725%
Current interest rate (as a percent)											1.05%
Maximum borrowing capacity under competitive bid option											65.00%
Annual facility fee rate (as a percent)														0.175%	0.15%
Annual facility fee											1,950,000
Outstanding balance of letters of credit											44,883,000	52,659,000
Net carrying value of apartment communities and improved land parcels securing debt						1,513,526,000
Mortgage notes payable held by wholly owned subsidiaries guaranteed by the Company						245,787,000
Weighted average interest rate, debt (as a percent)																5.80%	5.70%	2.70%	2.30%
Extension fee											$ 975,000