Notes Payable, Unsecured Notes and Credit Facility (Details 2)	1 Months Ended				3 Months Ended		9 Months Ended
	Jul. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)	May. 31, 2015 USD ($)	Apr. 30, 2015 USD ($) community loan	Sep. 30, 2015 USD ($) community	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) community extension	Sep. 30, 2014 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Notes Payable, Unsecured Notes and Credit Facility
Number of operating apartment communities | community					255		255
Gains (Losses) on Extinguishment of Debt					$ 18,987,000	$ 0	$ 26,736,000	$ (412,000)
Debt Instrument, Interest Rate, Stated Percentage			3.45%
Variable rate unsecured credit facility					0		0			$ 0
Mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable held by wholly owned subsidiaries guaranteed by the Company					234,500,000		234,500,000
Unsecured Notes 3.45 Percent [Member]
Notes Payable, Unsecured Notes and Credit Facility
Principal amount of notes issued			$ 525,000,000
Proceeds from Issuance of Debt			$ 520,653,000
Variable Rate Unsecured Term Loan [Member]
Notes Payable, Unsecured Notes and Credit Facility
Additional amount with option to draw									$ 50,000,000
Principal amount of notes issued									$ 300,000,000
Notes payable maturing in 2015 | Mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured mortgages				$ 481,582,000
Mortgage Loans on Real Estate, Number of Loans | loan				8
Number of operating apartment communities | community				8
Current interest rate (as a percent)				3.12%
Gains (Losses) on Extinguishment of Debt				$ 8,724,000
Notes Payable Maturities 2041 [Member] | Mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Current interest rate (as a percent)		7.50%
Gains (Losses) on Extinguishment of Debt		$ (455,000)
Repayments of Debt		$ 15,778,000
Notes Payable Maturities 2027 [Member] | Mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Current interest rate (as a percent)		7.84%
Gains (Losses) on Extinguishment of Debt		$ (263,000)
Repayments of Debt		$ 7,805,000
Notes payable maturing in 2019 | Mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Current interest rate (as a percent)		5.95%
Gains (Losses) on Extinguishment of Debt		$ (259,000)
Repayments of Debt		$ 74,531,000
Notes Payable Maturities 2053 [Member] | Mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured mortgages	$ 140,346,000
Current interest rate (as a percent)	5.56%
Gains (Losses) on Extinguishment of Debt	$ 18,987,000
Prepayment Penalty	11,228,000
Debt Instrument, Unamortized Premium, Write-off	$ 30,215,000
Variable rate unsecured credit facility
Notes Payable, Unsecured Notes and Credit Facility
Available borrowing capacity					1,300,000,000		$ 1,300,000,000
Line of credit facility, extension period							1 year
Number of extension options | extension							2
Extension period under each option							6 months
Extension fee							$ 1,950,000
Period of borrowing rate assumed							1 month
Annual facility fee							$ 1,950,000
Outstanding balance of letters of credit					43,580,000		43,580,000			49,407,000
Variable rate unsecured credit facility					0		0			$ 0
Net carrying value of apartment communities and improved land parcels securing debt					$ 3,363,527,000		$ 3,363,527,000
Fixed rate mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Weighted average interest rate, debt (as a percent)					4.70%		4.70%			4.50%
Variable rate mortgage notes payable, unsecured term loan and Credit Facility
Notes Payable, Unsecured Notes and Credit Facility
Weighted average interest rate, debt (as a percent)					1.70%		1.70%			1.80%
LIBOR | Variable Rate Unsecured Term Loan [Member] | Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Debt instrument variable rate							LIBOR
Debt instrument, basis spread on variable rate (as a percent)							1.45%
LIBOR | Secured Mortgage
Notes Payable, Unsecured Notes and Credit Facility
Debt instrument variable rate							LIBOR
LIBOR | Variable rate unsecured credit facility
Notes Payable, Unsecured Notes and Credit Facility
Current interest rate (as a percent)					1.14%		1.14%
Debt instrument, basis spread on variable rate (as a percent)							0.95%