Segment Reporting (Details 3) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting
Total revenue	$ 578,522	$ 575,982	$ 569,239	$ 560,792	$ 555,292	$ 550,500	$ 530,512	$ 522,326	$ 2,284,535	$ 2,158,628	$ 2,045,255
NOI									1,539,586	1,419,991	1,313,987
Gross real estate	22,277,169				21,935,936				22,277,169	21,935,936	20,755,780
Gross real estate assets held for sale	65,408								65,408		20,846
Business interruption insurance proceeds									26	3,498	20,564
Established Communities
Segment Reporting
Real estate capitalized additions	78,469				78,241				78,469	78,241	85,676
Operating segment | Established Communities
Segment Reporting
Total revenue									1,632,426	1,591,665	1,432,109
NOI									1,165,509	1,139,261	1,018,919
Gross real estate	13,961,942				13,884,082				13,961,942	13,884,082	12,326,913
Operating segment | Established Communities | New England
Segment Reporting
Total revenue									239,638	232,688	209,935
NOI									157,109	152,514	136,019
Gross real estate	2,014,158				1,993,653				2,014,158	1,993,653	1,667,171
Operating segment | Established Communities | Metro NY/NJ
Segment Reporting
Total revenue									360,430	354,444	294,199
NOI									254,132	251,760	204,882
Gross real estate	3,086,133				3,071,563				3,086,133	3,071,563	2,412,742
Operating segment | Established Communities | Mid-Atlantic
Segment Reporting
Total revenue									237,113	232,987	203,003
NOI									165,724	161,546	141,624
Gross real estate	2,226,315				2,216,292				2,226,315	2,216,292	1,862,091
Operating segment | Established Communities | Pacific Northwest
Segment Reporting
Total revenue									86,571	84,313	79,958
NOI									62,194	61,705	57,857
Gross real estate	727,652				724,751				727,652	724,751	731,277
Operating segment | Established Communities | Northern California
Segment Reporting
Total revenue									366,834	357,209	331,610
NOI									280,994	273,940	253,582
Gross real estate	2,986,068				2,972,311				2,986,068	2,972,311	2,812,859
Operating segment | Established Communities | Southern California
Segment Reporting
Total revenue									341,840	330,024	313,404
NOI									245,356	237,796	224,955
Gross real estate	2,921,616				2,905,512				2,921,616	2,905,512	2,840,773
Operating segment | Other Stabilized Communities
Segment Reporting
Total revenue									262,053	193,594	221,272
NOI									178,172	127,678	151,475
Gross real estate	2,934,711				2,571,356				2,934,711	2,571,356	2,650,966
Operating segment | Development / Redevelopment
Segment Reporting
Total revenue									297,619	235,266	207,049
NOI									195,905	153,052	143,593
Gross real estate	5,201,454				4,104,956				5,201,454	4,104,956	4,154,778
Operating segment | Disposals [Member]
Segment Reporting
Gross real estate					1,228,314					1,228,314	1,458,130
Segment Reconciling Items
Segment Reporting
Gross real estate	84,712				68,364				84,712	68,364	84,293
Non-allocated
Segment Reporting
Total revenue									3,572	4,147	5,599
Gross real estate	$ 94,350				$ 78,864				94,350	78,864	80,700
Continuing Operations
Segment Reporting
Total revenue									2,195,670	2,024,672	1,866,029
Business interruption insurance proceeds									26	3,498	20,529
Continuing Operations | Operating segment | Established Communities
Segment Reporting
Total revenue									1,632,426	1,591,665	1,432,109
Business interruption insurance proceeds									26	3	152
Continuing Operations | Operating segment | Other Stabilized Communities
Segment Reporting
Total revenue									262,053	193,594	221,272
Business interruption insurance proceeds									0	0	65
Continuing Operations | Non-allocated
Segment Reporting
Total revenue									3,572	4,147	5,599
Business interruption insurance proceeds									$ 0	0	0
Avalon Maplewood
Segment Reporting
Business interruption insurance proceeds										$ 3,495
Avalon at Edgewater
Segment Reporting
Business interruption insurance proceeds											$ 20,306