Notes Payable, Unsecured Notes and Credit Facility (Details 2)		1 Months Ended			3 Months Ended		6 Months Ended
		Jun. 30, 2015 USD ($) community	May. 31, 2015 USD ($)	Apr. 30, 2015 USD ($) community loan	Jun. 30, 2015 USD ($) community	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) community extension	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Mar. 31, 2014 USD ($)
Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Interest Rate, Stated Percentage			3.45%
Gains (Losses) on Extinguishment of Debt					$ 7,749,000	$ (412,000)	$ 7,749,000	$ (412,000)
Number of operating apartment communities | community		257			257		257
Variable rate unsecured credit facility		$ 0			$ 0		$ 0		$ 0
Mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Amount drawn		1,046,332,000			1,046,332,000		1,046,332,000		1,047,461,000
Fixed rate notes	[1]	1,812,557,000			1,812,557,000		1,812,557,000		2,400,677,000
Mortgage notes payable held by wholly owned subsidiaries guaranteed by the Company		234,500,000			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
Amount drawn		300,000,000			300,000,000		300,000,000		250,000,000
Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Fixed rate notes	[2]	3,275,000,000			$ 3,275,000,000		$ 3,275,000,000		2,750,000,000
Variable Rate Unsecured Term Loan [Member]
Notes Payable, Unsecured Notes and Credit Facility
Principal amount of notes issued										$ 300,000,000
Additional amount with option to draw										$ 50,000,000
Notes payable maturing in 2015 | Mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Fixed rate (as a percent)				3.12%
Gains (Losses) on Extinguishment of Debt				$ 8,724,000
Repayments of secured mortgages				$ 481,582,000
Mortgage Loans on Real Estate, Number of Loans | loan				8
Number of operating apartment communities | community				8
Notes Payable Maturities 2041 [Member] | Mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Repayments of Debt		$ 15,778,000
Fixed rate (as a percent)		7.50%			7.50%		7.50%
Gains (Losses) on Extinguishment of Debt		$ (455,000)
Notes Payable Maturities 2027 [Member] | Mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Repayments of Debt		$ 7,805,000
Fixed rate (as a percent)		7.84%			7.84%		7.84%
Gains (Losses) on Extinguishment of Debt		$ (263,000)
Notes payable maturing in 2019 | Mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Repayments of Debt		$ 74,531,000
Fixed rate (as a percent)		5.95%			5.95%		5.95%
Gains (Losses) on Extinguishment of Debt		$ (259,000)
Variable rate unsecured credit facility
Notes Payable, Unsecured Notes and Credit Facility
Available borrowing capacity		1,300,000,000			$ 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
Variable rate unsecured credit facility		0			0		0		0
Outstanding balance of letters of credit		46,119,000			46,119,000		46,119,000		$ 49,407,000
Net carrying value of apartment communities and improved land parcels securing debt		$ 3,549,533,000			$ 3,549,533,000		$ 3,549,533,000
Fixed rate mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Weighted average interest rate, debt (as a percent)		4.70%			4.70%		4.70%		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)		1.80%			1.80%		1.80%		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
Debt instrument, basis spread on variable rate (as a percent)							0.95%
Current interest rate (as a percent)		1.14%			1.14%		1.14%

[1] Balances at June 30, 2015 and December 31, 2014 exclude $60,410 and $84,449 of debt premium, respectively, as reflected in mortgage notes payable on the Company’s Condensed Consolidated Balance Sheets.

[2] Balances at June 30, 2015 and December 31, 2014 exclude $7,169 and $6,735 of debt discount, respectively, as reflected in unsecured notes, net on the Company’s Condensed Consolidated Balance Sheets.