Segment Reporting (Details 3) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting
Accounts Receivable, Credit Loss Expense (Reversal)										$ 14,072
Total revenue	$ 593,566	$ 587,613	$ 577,263	$ 566,184	$ 578,522	$ 575,982	$ 569,239	$ 560,792	$ 2,324,626	2,284,535	$ 2,158,628
NOI									1,627,210	1,518,834	1,398,978
Gross real estate	23,558,460				21,942,927				23,558,460	21,942,927	20,380,549
Gross real estate assets held for sale	48,412								48,412
Business interruption insurance proceeds									1,441	26	3,498
Established Communities
Segment Reporting
Real estate capitalized additions	128,324				78,469				128,324	78,469	78,241
Operating segment | Established Communities
Segment Reporting
Total revenue									1,836,339	1,783,112	1,574,110
NOI									1,317,472	1,278,323	1,128,089
Gross real estate	15,747,408				15,637,350				15,747,408	15,637,350	13,736,121
Operating segment | Established Communities | New England
Segment Reporting
Total revenue									249,301	241,793	215,133
NOI									164,977	159,394	141,342
Gross real estate	2,065,954				2,050,131				2,065,954	2,050,131	1,845,692
Operating segment | Established Communities | Metro NY/NJ
Segment Reporting
Total revenue									411,115	400,422	354,444
NOI									291,662	284,344	251,760
Gross real estate	3,545,753				3,527,098				3,545,753	3,527,098	3,071,563
Operating segment | Established Communities | Mid-Atlantic
Segment Reporting
Total revenue									292,943	284,381	232,987
NOI									207,091	200,381	161,546
Gross real estate	2,685,052				2,669,040				2,685,052	2,669,040	2,216,292
Operating segment | Established Communities | Pacific Northwest
Segment Reporting
Total revenue									113,021	108,861	84,313
NOI									82,186	78,313	61,705
Gross real estate	990,563				985,102				990,563	985,102	724,751
Operating segment | Established Communities | Northern California
Segment Reporting
Total revenue									363,910	353,136	357,209
NOI									280,216	272,096	273,940
Gross real estate	2,850,491				2,832,026				2,850,491	2,832,026	2,972,311
Operating segment | Established Communities | Southern California
Segment Reporting
Total revenue									406,049	394,519	330,024
NOI									291,340	283,795	237,796
Gross real estate	3,609,595				3,573,953				3,609,595	3,573,953	2,905,512
Operating segment | Other Stabilized Communities
Segment Reporting
Total revenue									298,415	238,584	174,933
NOI									202,445	159,745	117,837
Gross real estate	3,551,512				3,063,669				3,551,512	3,063,669	2,392,244
Operating segment | Development / Redevelopment
Segment Reporting
Total revenue									163,471	120,822	235,266
NOI									107,293	80,766	153,052
Gross real estate	3,702,194				2,652,967				3,702,194	2,652,967	4,104,956
Operating segment | Disposals [Member]
Segment Reporting
Gross real estate					334,242					334,242	1,555,387
Segment Reconciling Items
Segment Reporting
Gross real estate	0				84,712				0	84,712	68,364
Non-allocated
Segment Reporting
Total revenue									4,960	3,572	4,147
Gross real estate	$ 557,346				$ 504,229				557,346	504,229	78,864
Continuing Operations
Segment Reporting
Total revenue									2,303,185	2,146,090	1,988,456
Business interruption insurance proceeds									1,441	26	3,498
Continuing Operations | Operating segment | Established Communities
Segment Reporting
Total revenue									1,836,339	1,783,112	1,574,110
Business interruption insurance proceeds									478	26	3
Continuing Operations | Operating segment | Other Stabilized Communities
Segment Reporting
Total revenue									298,415	238,584	174,933
Business interruption insurance proceeds									963	0	0
Continuing Operations | Non-allocated
Segment Reporting
Total revenue									4,960	3,572	4,147
Business interruption insurance proceeds									$ 0	0	$ 0
Avalon Maplewood
Segment Reporting
Business interruption insurance proceeds										$ 3,495