Mortgage Notes Payable, Unsecured Notes and Credit Facility (Details 2)	1 Months Ended					12 Months Ended
	Nov. 30, 2017 USD ($)	Oct. 31, 2017 USD ($)	Jun. 30, 2017 USD ($) community	May 31, 2017 USD ($) community	Feb. 28, 2017 USD ($)	Dec. 31, 2017 USD ($) community	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Apr. 30, 2017 USD ($)
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Gains (losses) on extinguishment of debt						$ (25,472,000)	$ (7,075,000)	$ 26,736,000
Number of Real Estate Properties | community						267
Repayments of Unsecured Debt						$ 300,000,000	504,403,000	$ 0
Variable rate unsecured credit facility | LIBOR
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt Instrument, Interest Rate, Effective Percentage						2.39%
Debt instrument, basis spread on variable rate (as a percent)						0.825%
Variable Rate Unsecured Term Loan $250 Million [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Principal amount of notes issued									$ 250,000,000
Variable Rate Unsecured Term Loan $100 Million [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Principal amount of notes issued					$ 100,000,000
Variable Rate Unsecured Term Loan $100 Million [Member] | LIBOR
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)					0.90%
Variable Rate Unsecured Term Loan $150 Million [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Principal amount of notes issued					$ 150,000,000
Variable Rate Unsecured Term Loan $150 Million [Member] | LIBOR
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)					1.50%
Unsecured notes
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Amount drawn						$ 300,000,000	0
Unsecured notes | Variable Rate Unsecured Term Loan $300 Million [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Write off of Deferred Debt Issuance Cost	$ (1,367,000)
Repayments of Unsecured Debt	$ 300,000,000
Unsecured notes | Unsecured Notes 3.20 Percent [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)	3.20%
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%
Unsecured notes | Notes Payable Maturities 2027 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)						3.35%
Unsecured notes | Notes Payable Maturities 2046 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)						3.90%
Unsecured notes | Variable Rate Unsecured Term Loan $100 Million [Member] | LIBOR
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)						0.90%
Unsecured notes | Variable Rate Unsecured Term Loan $150 Million [Member] | 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 3.350 Maturities 2027 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)				3.35%
Unsecured notes | Unsecured Notes 4.15 Percent [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Stated interest rate (as a percent)			4.15%
Unsecured Notes 4.15 Percent [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Principal amount of notes issued			$ 300,000,000
Net proceeds from issuance of debt			297,372,000
Mortgage notes payable
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Amount drawn						$ 910,326,000	908,262,000
Mortgage notes payable | Notes Payable Maturities 2019 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured mortgages			556,313,000
Gains (losses) on extinguishment of debt			$ (34,965,000)
Stated interest rate (as a percent)			5.86%
Number of Real Estate Properties | community			12
Prepayment Penalty			$ (33,515,000)
Write off of Deferred Debt Issuance Cost			$ (1,450,000)
Mortgage notes payable | Notes payable maturing in 2017
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured mortgages				$ 670,590,000	$ 17,300,000
Gains (losses) on extinguishment of debt				$ 10,839,000
Stated interest rate (as a percent)				6.256%
Number of Real Estate Properties | community				11
Mortgage notes payable | Notes Payable Maturities 2027 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Principal amount of notes issued				$ 185,100,000
Stated interest rate (as a percent)				3.61%
Number of Real Estate Properties | community				3
Unsecured Notes 3.20 Percent [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Principal amount of notes issued	$ 450,000,000
Net proceeds from issuance of debt	$ 445,271,000
Unsecured Notes 3.35 Percent [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Principal amount of notes issued				$ 400,000,000
Net proceeds from issuance of debt				$ 396,016,000
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
Avalon Columbia Pike [Member] | Mortgage notes payable | Notes Payable Maturities 2020 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Assumed principal amount of indebtedness							$ 70,507,000
Archstone Lexington [Member] | Mortgage notes payable | Notes Payable Maturities 2020 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Amount drawn		$ 21,700,000
Debt instrument, basis spread on variable rate (as a percent)		1.35%