Notes Payable, Unsecured Notes and Credit Facility (Details 2) (USD $)	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Net carrying value of apartment communities and improved land parcels securing debt	$ 4,082,329,000
Mortgage notes payable held by wholly owned subsidiaries guaranteed by the Company	474,916,000
Variable rate unsecured credit facility
Notes Payable, Unsecured Notes and Credit Facility
Available borrowing capacity	1,300,000,000
Line of credit facility, extension period	1 year
Extension fee	1,950,000
Debt instrument, basis spread on variable rate (as a percent)	1.25%
Debt instrument variable rate	LIBOR
Current interest rate (as a percent)	1.05%
Annual facility fee	1,950,000
Borrowings outstanding under the credit facility	0
Outstanding balance of letters of credit	$ 55,491,000	$ 44,883,000
Fixed rate mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Weighted average interest rate, debt (as a percent)	5.90%	5.80%
Variable rate mortgage notes payable and Credit Facility
Notes Payable, Unsecured Notes and Credit Facility
Weighted average interest rate, debt (as a percent)	1.90%	2.70%