Mortgage Notes Payable, Unsecured Notes and Credit Facility (Details 2) - USD ($)	1 Months Ended						12 Months Ended
	Nov. 30, 2016	Oct. 31, 2016	Sep. 30, 2016	May 31, 2016	Apr. 30, 2016	Feb. 29, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jan. 31, 2016
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of Unsecured Debt							$ 504,403,000	$ 0	$ 150,000,000
Gains (losses) on extinguishment of debt							$ 7,075,000	(26,736,000)	$ 412,000
Variable rate unsecured credit facility | LIBOR
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Interest Rate, Effective Percentage							1.60%
Debt instrument, basis spread on variable rate (as a percent)							0.825%
Notes payable maturing in 2017
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Gains (losses) on extinguishment of debt	$ 4,614,000
Notes Payable Maturities 2046 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Gains (losses) on extinguishment of debt					$ 2,461,000
Unsecured notes
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of Unsecured Debt	$ 250,000,000		$ 250,000,000
Fixed rate (as a percent)	5.70%		5.75%
Unsecured notes | Term loan | LIBOR
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)							1.45%
Unsecured notes | Notes Payable 2.900 Maturities 2026 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)		2.90%					2.90%
Unsecured notes | Notes Payable Maturities 2046 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)		3.90%					3.90%
Unsecured notes | Notes Payable 2.950 Maturities 2026 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)				2.95%			2.95%
Mortgage notes payable
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Amount drawn							$ 908,262,000	$ 1,045,182,000
Mortgage notes payable | Notes payable maturing in 2017
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Prepayment Penalty							4,403,000
Write off of Deferred Debt Issuance Cost							$ 211,000
Mortgage notes payable | Notes payable maturing in 2016
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Interest Rate, Effective Percentage						3.32%
Repayments of secured mortgages						$ 16,212,000
Mortgage notes payable | Notes Payable Maturities 2046 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured mortgages					$ 134,500,000
Unsecured Notes 2.95 Percent [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Principal amount of notes issued				$ 475,000,000
Net proceeds from issuance of debt				$ 471,751,000
Unsecured Notes 2.90 Percent [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Principal amount of notes issued		$ 300,000,000
Net proceeds from issuance of debt		297,117,000
Unsecured Notes 3.90 Percent [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Principal amount of notes issued		350,000,000
Net proceeds from issuance of debt		$ 345,520,000
Avalon Hoboken [Member] | Mortgage notes payable | Notes Payable Maturities 2020 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Assumed principal amount of indebtedness										$ 67,904,000
Debt Instrument, Interest Rate, Effective Percentage										4.18%
Avalon Columbia Pike [Member] | Mortgage notes payable | Notes Payable Maturities 2019 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Assumed principal amount of indebtedness			$ 70,507,000
Debt Instrument, Interest Rate, Effective Percentage			3.38%