Mortgage Notes Payable, Unsecured Notes and Credit Facility (Details 4) - USD ($)	1 Months Ended				3 Months Ended	12 Months Ended
	Nov. 30, 2019	Aug. 31, 2019	May 31, 2019	Apr. 30, 2019	Mar. 31, 2019	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Cash in escrow						$ 87,927,000			$ 126,205,000	$ 134,818,000
Mortgage notes payable and unsecured notes						$ 7,355,371,000			7,102,355,000
Percentage of principle amount at which the entity may redeem some or all of the notes						100.00%
Notes Payable Maturities 2019 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)				2.99%		3.38%
Variable rate unsecured credit facility
Mortgage 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%
Outstanding balance of letters of credit						$ 11,488,000			$ 39,810,000
Variable rate unsecured credit facility | LIBOR
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Interest Rate, Effective Percentage						2.54%
Debt instrument, basis spread on variable rate (as a percent)						0.775%
Variable rate unsecured credit facility | Minimum | LIBOR
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)						0.70%
Variable rate unsecured credit facility | Maximum | LIBOR
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)						1.45%
Other Letter of Credit [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Outstanding balance of letters of credit						$ 24,939,000
Unsecured notes
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes						$ 6,400,000,000
Unsecured notes | Minimum
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)						0.20%
Unsecured notes | Maximum
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)						0.45%
Unsecured notes | Variable Rate Unsecured Term Loan $250 Million [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Face Amount					$ 250,000,000
Unsecured notes | Unsecured Notes 3.30 Percent [Domain]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Interest Rate, Effective Percentage								3.66%
Stated interest rate (as a percent)								3.30%
Unsecured notes | Notes Payable 3.625 Maturities 2020 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes						$ 400,000,000
Stated interest rate (as a percent)						3.625%
Unsecured notes | Notes Payable Maturities 2021 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes						$ 250,000,000
Stated interest rate (as a percent)						3.95%
Unsecured notes | Floating Rate Unsecured Notes 300 Million
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes						$ 300,000,000
Unsecured notes | Floating Rate Unsecured Notes 300 Million | LIBOR
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)						0.43%
Unsecured notes | Notes payable maturing in 2022
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes						$ 450,000,000
Stated interest rate (as a percent)						2.95%
Unsecured notes | Variable Rate Unsecured Term Loan $100 Million
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Face Amount					$ 100,000,000
Mortgage notes payable and unsecured notes						$ 100,000,000
Unsecured notes | Variable Rate Unsecured Term Loan $100 Million | LIBOR
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)					0.90%	0.90%
Unsecured notes | Notes Payable 4.200 Maturities 2023
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes						$ 350,000,000
Stated interest rate (as a percent)						4.20%
Unsecured notes | Notes Payable 2.850 Maturities 2023
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes						$ 250,000,000
Stated interest rate (as a percent)						2.85%
Unsecured notes | Notes Payable 2024
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes						$ 300,000,000
Stated interest rate (as a percent)						3.50%
Unsecured notes | Variable Rate Unsecured Term Loan $150 Million
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Face Amount					$ 150,000,000
Mortgage notes payable and unsecured notes						$ 150,000,000
Unsecured notes | Variable Rate Unsecured Term Loan $150 Million | LIBOR
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)						1.50%
Unsecured notes | Variable Rate Unsecured Term Loan $150 Million | Minimum | LIBOR
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)					0.85%
Unsecured notes | Variable Rate Unsecured Term Loan $150 Million | Maximum | LIBOR
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)					1.50%
Unsecured notes | Notes Payable 3.450 Maturities 2025
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes						$ 525,000,000
Stated interest rate (as a percent)						3.45%
Unsecured notes | Notes Payable 3.500 Maturities 2025
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes						$ 300,000,000
Stated interest rate (as a percent)						3.50%
Unsecured notes | Notes Payable 2.950 Maturities 2026
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes						$ 475,000,000
Stated interest rate (as a percent)						2.95%
Unsecured notes | Notes Payable 2.900 Maturities 2026
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes						$ 300,000,000
Stated interest rate (as a percent)						2.90%
Unsecured notes | Notes Payable Maturities 2027
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes						$ 400,000,000
Stated interest rate (as a percent)						3.35%
Unsecured notes | Notes Payable Maturities 2028
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes						$ 450,000,000
Stated interest rate (as a percent)						3.20%
Unsecured notes | Notes Payable Maturities 2029 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes						$ 450,000,000
Stated interest rate (as a percent)						3.30%
Unsecured notes | Notes Payable Maturities 2046
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes						$ 350,000,000
Stated interest rate (as a percent)						3.90%
Unsecured notes | Notes Payable Maturities 2047
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes						$ 300,000,000
Stated interest rate (as a percent)						4.15%
Unsecured notes | Notes Payable Maturities 2048
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes						$ 300,000,000
Stated interest rate (as a percent)						4.35%
Secured notes
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments						$ 312,864,000
Mortgage notes payable and unsecured notes						642,507,000
Secured notes | Notes Payable Maturities 2020
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments						8,782,000
Mortgage notes payable and unsecured notes						118,729,000
Secured notes | Notes Payable Maturities 2021 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments						9,304,000
Mortgage notes payable and unsecured notes						27,844,000
Secured notes | Notes payable maturing in 2022
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments						9,918,000
Secured notes | Notes Payable Maturities 2023
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments						10,739,000
Secured notes | Notes Payable 2024
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments						11,577,000
Secured notes | Notes Payable Maturities 2025
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments						12,508,000
Mortgage notes payable and unsecured notes						0
Secured notes | Notes Payable Maturities 2026
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments						13,545,000
Secured notes | Notes Payable Maturities 2027
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments						14,980,000
Mortgage notes payable and unsecured notes						185,100,000
Secured notes | Notes Payable Maturities 2028
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments						20,607,000
Mortgage notes payable and unsecured notes						0
Secured notes | Notes Payable Maturities 2029 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Face Amount							$ 30,250,000
Debt Instrument, Interest Rate, Effective Percentage							3.26%
Secured notes payments						11,742,000
Mortgage notes payable and unsecured notes						66,250,000
Secured notes | Notes Payable Maturities Thereafter
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments						189,162,000
Mortgage notes payable and unsecured notes						$ 244,584,000
Unsecured Notes 3.30 Percent [Domain]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Face Amount								$ 450,000,000
Proceeds from Issuance of Debt			$ 446,877,000
Avalon Natick [Member] | Secured notes | Notes Payable Maturities 2019, Fixed Rate [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured debt				$ 13,363,000
Avalon Natick [Member] | Secured notes | Notes Payable Maturities 2019, Variable Rate [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured debt				$ 33,854,000
Eaves Mission Viejo [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Cash in escrow								3,706,000
Eaves Mission Viejo [Member] | Secured notes | Notes Payable Maturities 2025
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured debt			7,635,000
AVA Nob Hill [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Cash in escrow								10,584,000
AVA Nob Hill [Member] | Secured notes | Notes Payable Maturities 2025
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured debt			20,800,000
Avalon Campbell [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Cash in escrow								22,622,000
Avalon Campbell [Member] | Secured notes | Notes Payable Maturities 2025
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured debt			38,800,000
Eaves Pacifica [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Cash in escrow								$ 10,263,000
Eaves Pacifica [Member] | Secured notes | Notes Payable Maturities 2025
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured debt			$ 17,600,000
Archstone Lexington [Member] | Secured notes | Notes Payable Maturities 2020
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured debt		$ 21,700,000
Avalon Columbia Pike [Member] | Secured notes | Notes Payable Maturities 2019 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured debt	$ 65,749,000