Mortgage Notes Payable, Unsecured Notes and Credit Facility (Details 2) - USD ($)	1 Months Ended			3 Months Ended	9 Months Ended
	Aug. 31, 2019	May 31, 2019	Apr. 30, 2019	Mar. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Notes Payable, Unsecured Notes and Credit Facility
Cash in escrow					$ 88,329,000		$ 126,205,000	$ 225,704,000
Net carrying value of apartment communities and improved land parcels securing debt					1,693,511,000
Unsecured Notes 3.30 Percent [Domain]
Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Face Amount						$ 450,000,000
Proceeds from Issuance of Debt		$ 446,877,000
Notes Payable Maturities 2029 [Member] | Secured Debt [Member]
Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Face Amount					$ 30,250,000
Current interest rate (as a percent)					3.26%
Notes Payable Maturities 2029 [Member] | Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Interest Rate, Stated Percentage					3.30%
Variable Rate Unsecured Term Loan $250 Million [Member] | Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Face Amount				$ 250,000,000
Variable Rate Unsecured Term Loan $100 Million | Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Face Amount				$ 100,000,000
Variable Rate Unsecured Term Loan $100 Million | LIBOR | Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)				0.90%	0.90%
Variable Rate Unsecured Term Loan $150 Million | Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Face Amount				$ 150,000,000
Variable Rate Unsecured Term Loan $150 Million | LIBOR | Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)					0.85%
Unsecured Notes 3.30 Percent [Domain] | Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Interest Rate, Stated Percentage						3.30%
Current interest rate (as a percent)						3.66%
Line of Credit [Member]
Notes Payable, Unsecured Notes and Credit Facility
Available borrowing capacity					$ 1,750,000,000		1,500,000,000
Percent of Credit Facility Available to Competitive Bid Option					65.00%
Annual facility fee, percentage					0.125%
Annual facility fee					$ 2,188,000
Outstanding balance of letters of credit					$ 17,813,000		$ 39,810,000
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)					2.79%
Other Letter of Credit [Member]
Notes Payable, Unsecured Notes and Credit Facility
Outstanding balance of letters of credit					$ 18,964,000
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)					3.20%		3.40%
Minimum | Variable Rate Unsecured Term Loan $150 Million | LIBOR | Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)				0.85%
Minimum | Line of Credit [Member] | LIBOR
Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)					0.70%
Maximum | Variable Rate Unsecured Term Loan $150 Million | LIBOR | Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)				1.50%
Maximum | Line of Credit [Member] | LIBOR
Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)					1.45%
Avalon Natick [Member] | Notes Payable Maturities 2019, Fixed Rate [Member] | Secured Debt [Member]
Notes Payable, Unsecured Notes and Credit Facility
Repayments of Secured Debt			$ 13,363,000
Avalon Natick [Member] | Notes Payable Maturities 2019, Variable Rate [Member] | Secured Debt [Member]
Notes Payable, Unsecured Notes and Credit Facility
Repayments of Secured Debt			$ 33,854,000
Eaves Mission Viejo [Member]
Notes Payable, Unsecured Notes and Credit Facility
Cash in escrow						$ 3,706,000
Eaves Mission Viejo [Member] | Notes Payable Maturities 2025 | Secured Debt [Member]
Notes Payable, Unsecured Notes and Credit Facility
Repayments of Secured Debt		7,635,000
AVA Nob Hill [Member]
Notes Payable, Unsecured Notes and Credit Facility
Cash in escrow						10,584,000
AVA Nob Hill [Member] | Notes Payable Maturities 2025 | Secured Debt [Member]
Notes Payable, Unsecured Notes and Credit Facility
Repayments of Secured Debt		20,800,000
Avalon Campbell [Member]
Notes Payable, Unsecured Notes and Credit Facility
Cash in escrow						22,622,000
Avalon Campbell [Member] | Notes Payable Maturities 2025 | Secured Debt [Member]
Notes Payable, Unsecured Notes and Credit Facility
Repayments of Secured Debt		38,800,000
Eaves Pacifica [Member]
Notes Payable, Unsecured Notes and Credit Facility
Cash in escrow						$ 10,263,000
Eaves Pacifica [Member] | Notes Payable Maturities 2025 | Secured Debt [Member]
Notes Payable, Unsecured Notes and Credit Facility
Repayments of Secured Debt		$ 17,600,000
Archstone Lexington [Member] | Notes Payable Maturities 2020 [Member] | Secured Debt [Member]
Notes Payable, Unsecured Notes and Credit Facility
Repayments of Secured Debt	$ 21,700,000