Mortgage Notes Payable, Unsecured Notes and Credit Facility (Details) - USD ($)	1 Months Ended			3 Months Ended	12 Months Ended
	Aug. 31, 2020	May 31, 2020	Feb. 29, 2020	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Total mortgage notes payable and unsecured notes and Term Loans					$ 7,629,814,000	$ 7,355,371,000
Variable rate unsecured credit facility					0	0
Total mortgage notes payable, unsecured notes, Term Loans and Credit Facility					7,629,814,000	7,355,371,000
Repayment of unsecured debt					958,680,000	0	$ 258,579,000
Gain (Loss) on Extinguishment of Debt					(9,333,000)	(602,000)	$ (17,492,000)
Unsecured notes
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Fixed rate notes					6,500,000,000	5,850,000,000
Variable rate notes					0	300,000,000
Total mortgage notes payable and unsecured notes and Term Loans					6,750,000,000
Amount of debt discount					10,380,000	8,610,000
Deferred financing costs, net					37,615,000	32,742,000
Term loan
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Variable rate notes					250,000,000	250,000,000
Secured notes
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Fixed rate notes					408,964,000	479,221,000
Variable rate notes					470,850,000	476,150,000
Total mortgage notes payable and unsecured notes and Term Loans					575,532,000
Amount of debt discount					14,478,000	14,464,000
Deferred financing costs, net					3,004,000	$ 3,265,000
Unsecured Notes 2.30 Percent [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Proceeds from Issuance of Debt			$ 694,701,000
Unsecured Notes 2.45 Percent [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Proceeds from Issuance of Debt		$ 593,430,000
Unsecured Notes 2.30 Percent [Member] | Unsecured notes
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)			2.30%
Notes Payable Maturities 2020
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)	4.18%		3.08%
Notes Payable Maturities 2027
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)			2.38%
Notes Payable Maturities 2027 | Unsecured notes
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Total mortgage notes payable and unsecured notes and Term Loans					$ 400,000,000
Stated interest rate (as a percent)					3.35%
Notes Payable Maturities 2027 | Secured notes
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Total mortgage notes payable and unsecured notes and Term Loans					$ 236,100,000
Notes Payable Maturities 2027 | Secured notes | Avalon San Bruno III [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Fixed rate notes			$ 51,000,000
Notes Payable 3.625 Maturities 2020 [Member] | Unsecured notes
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)				3.625%
Repayment of unsecured debt				$ 400,000,000
Notes Payable Maturities 2021 [Member] | Unsecured notes
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Total mortgage notes payable and unsecured notes and Term Loans					0
Stated interest rate (as a percent)				3.95%
Repayment of unsecured debt				$ 250,000,000
Notes Payable Maturities 2021 [Member] | Secured notes
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Total mortgage notes payable and unsecured notes and Term Loans					$ 27,844,000
Notes Payable 3.625 Maturities 2020 and 2021 [Member] | Unsecured notes
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Gain (Loss) on Extinguishment of Debt				$ 9,170,000
Unsecured Notes 2.45 Percent [Member] | Unsecured notes
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)		2.45%
Floating Rate Unsecured Notes 300 Million | Unsecured notes
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured debt		$ 300,000,000
Gain (Loss) on Extinguishment of Debt		$ 268,000
Notes Payable Maturities 2020, Fixed Rate [Member] | Secured notes | Avalon Hoboken [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured debt	$ 67,904,000