Mortgage Notes Payable, Unsecured Notes and Credit Facility (Details 2) - USD ($)	1 Months Ended	6 Months Ended
	Jan. 31, 2021	Jun. 30, 2021	Dec. 31, 2020
Notes Payable, Unsecured Notes and Credit Facility
Credit Facility		$ 0	$ 0
Net carrying value of apartment communities and improved land parcels securing debt		1,373,062,000
Secured Debt [Member]
Notes Payable, Unsecured Notes and Credit Facility
Long-term Debt, Percentage Bearing Fixed Interest, Amount		380,030,000	408,964,000
Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Long-term Debt, Percentage Bearing Fixed Interest, Amount		6,500,000,000	6,500,000,000
Notes Payable Maturities 2021 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Interest Rate, Stated Percentage	5.37%
Line of Credit [Member]
Notes Payable, Unsecured Notes and Credit Facility
Available borrowing capacity		$ 1,750,000,000
Annual facility fee, percentage		0.125%
Annual facility fee		$ 2,188,000
Credit Facility		0	0
Outstanding balance of letters of credit		$ 2,599,000	2,900,000
Line of Credit [Member] | LIBOR
Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)		0.775%
Current interest rate (as a percent)		0.88%
Other Letter of Credit [Member]
Notes Payable, Unsecured Notes and Credit Facility
Outstanding balance of letters of credit		$ 36,682,000	$ 32,079,000
Fixed Rate Mortgage Notes Payable [Member]
Notes Payable, Unsecured Notes and Credit Facility
Weighted average interest rate, debt (as a percent)		3.80%	3.80%
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)		1.60%	1.70%
Avalon San Bruno II | Notes Payable Maturities 2021 [Member] | Secured Debt [Member]
Notes Payable, Unsecured Notes and Credit Facility
Repayments of Secured Debt	$ 27,795,000