Mortgage Notes Payable, Unsecured Notes and Credit Facility (Details 2) - USD ($)	1 Months Ended		3 Months Ended
	Mar. 31, 2018	Feb. 28, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Notes Payable, Unsecured Notes and Credit Facility
Loss on extinguishment of debt, net			$ 397,000	$ 0
Credit Facility	$ 0		0		$ 0
Net carrying value of apartment communities and improved land parcels securing debt	2,252,748,000		2,252,748,000
Mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable held by wholly owned subsidiaries guaranteed by the Company	100,000,000		100,000,000
Unsecured Notes 4.35 Percent [Member]
Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, face amount	300,000,000		$ 300,000,000
Proceeds from issuance of debt	$ 296,210,000
Unsecured Notes 3.97 Percent First 10 Years [Member] | Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Term			10 years
Current interest rate (as a percent)	3.97%		3.97%
Unsecured Notes 4.39 Percent Remaining 20 Years [Member] | Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Current interest rate (as a percent)	4.39%		4.39%
Line of Credit [Member]
Notes Payable, Unsecured Notes and Credit Facility
Available borrowing capacity	$ 1,500,000,000		$ 1,500,000,000
Line of credit facility, extension period			9 months
Extension fee			$ 1,500,000
Period of borrowing rate assumed			1 month
Annual facility fee, percentage			0.125%
Annual facility fee			$ 1,875,000
Credit Facility	0		0		0
Outstanding balance of letters of credit	$ 43,352,000		$ 43,352,000		$ 47,315,000
Line of Credit [Member] | LIBOR
Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)			0.825%
Current interest rate (as a percent)	2.71%		2.71%
Notes Payable Maturities 2043 [Member] | Mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured mortgages		$ 15,174,000
Loss on extinguishment of debt, net		(426,000)
Prepayment penalty		152,000
Write off of deferred debt issuance cost		274,000
Notes Payable Maturities 2018 | Mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured mortgages		$ 11,038,000
Unsecured Notes 4.35 Percent [Member] | Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate of unsecured notes (as a percent)	4.35%		4.35%
Fixed Rate Mortgage Notes Payable [Member]
Notes Payable, Unsecured Notes and Credit Facility
Weighted average interest rate, debt (as a percent)	3.90%		3.90%		4.00%
Variable Rate Mortgage Notes Payable Unsecured Term Loan and Credit Facility [Member]
Notes Payable, Unsecured Notes and Credit Facility
Weighted average interest rate, debt (as a percent)	3.30%		3.30%		3.20%