Mortgage Notes Payable, Unsecured Notes and Credit Facility (Details 2) - USD ($)	1 Months Ended						12 Months Ended
	Dec. 31, 2018	Aug. 31, 2018	Jun. 30, 2018	Apr. 30, 2018	Mar. 31, 2018	Feb. 28, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Gains (losses) on extinguishment of debt							$ (17,492,000)	$ (25,472,000)	$ (7,075,000)
Repayment of unsecured debt							$ 258,579,000	$ 300,000,000	$ 504,403,000
Mortgage notes payable | Notes Payable Maturities 2043
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured debt						$ 15,174,000
Stated interest rate (as a percent)						6.60%
Gains (losses) on extinguishment of debt						$ 426,000
Prepayment penalty						(152,000)
Write off of deferred debt issuance cost						(274,000)
Mortgage notes payable | Notes Payable Maturities 2018
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured debt		$ 51,449,000		$ 13,380,000		$ 11,038,000
Stated interest rate (as a percent)				3.06%		4.61%
Mortgage notes payable | Notes Payable Maturities 2033
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured debt			$ 15,295,000
Stated interest rate (as a percent)			6.90%
Gains (losses) on extinguishment of debt			$ 635,000
Prepayment penalty			(282,000)
Write off of deferred debt issuance cost			$ (353,000)
Mortgage notes payable | Notes Payable Maturities 2028
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured debt		44,410,000
Gains (losses) on extinguishment of debt		1,690,000
Mortgage notes payable | Notes Payable Maturities 2018 and 2028
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured debt		$ 95,859,000
Mortgage notes payable | Notes Payable Maturities 2037
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured debt	$ 93,800,000
Gains (losses) on extinguishment of debt	5,837,000
Prepayment penalty	(2,874,000)
Write off of deferred debt issuance cost	$ (2,963,000)
Unsecured Notes 4.35 Percent
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Net proceeds from issuance of debt					$ 296,210,000
Principal amount of notes issued					$ 300,000,000
Unsecured notes | Unsecured Notes 4.35 Percent
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)					4.35%
Unsecured notes | Unsecured Notes 3.97 Percent First 10 Years
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, term					10 years
Debt instrument, interest rate, effective percentage					3.97%
Unsecured notes | Unsecured Notes 4.39 Percent Remaining 20 Years
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, interest rate, effective percentage					4.39%
Unsecured notes | Notes Payable Maturities 2028
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)	3.20%						3.20%
Unsecured notes | Notes Payable 6.100 Maturities 2020
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)	6.10%						6.10%
Gains (losses) on extinguishment of debt	$ 8,926,000
Prepayment penalty	(8,579,000)
Write off of deferred debt issuance cost	(347,000)
Repayment of unsecured debt	$ 250,000,000
Avalon Bowery Place I | Mortgage notes payable | Notes Payable Maturities 2029
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)	4.01%						4.01%
Principal amount of notes issued	$ 93,800,000						$ 93,800,000
Avalon Bowery Place II | Mortgage notes payable | Notes Payable Maturities 2029
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)	4.01%						4.01%
Principal amount of notes issued	$ 39,639,000						$ 39,639,000
Avalon Morningside Park | Mortgage notes payable | Notes Payable 3.95 Maturities 2029
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)	3.95%						3.95%
Principal amount of notes issued	$ 12,500,000						$ 12,500,000
Avalon Morningside Park | Mortgage notes payable | Notes Payable 3.50 Maturities 2029
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)	3.50%						3.50%
Principal amount of notes issued	$ 100,000,000						$ 100,000,000
Avalon West Chelsea and AVA High Line | Mortgage notes payable | Notes Payable Maturities 2029
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)	4.01%						4.01%
Principal amount of notes issued	$ 150,000,000						$ 150,000,000
NYC JV | Mortgage notes payable | Notes Payable Maturities 2029
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Principal amount of notes issued	$ 295,939,000						$ 295,939,000