Segment Reporting (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting
Accounts Receivable, Credit Loss Expense (Reversal)	$ 66,763		$ 14,072
Total revenue	2,301,261	$ 2,324,626	2,284,535
NOI	1,547,190	1,594,174	1,485,132
Gross real estate	23,917,282	23,157,308	21,544,772
Gross real estate assets held for sale	44,940
Established Communities
Segment Reporting
Real estate capitalized additions	126,548	128,324	78,469
Operating segment | Established Communities
Segment Reporting
Total revenue	2,028,317	2,107,288	1,731,570
NOI	1,406,905	1,503,300	1,244,621
Gross real estate	19,045,458	18,923,614	15,239,192
Operating segment | Established Communities | New England
Segment Reporting
Total revenue	297,674	303,816	223,594
NOI	193,053	202,812	148,310
Gross real estate	2,617,725	2,595,907	1,890,304
Operating segment | Established Communities | Metro NY/NJ
Segment Reporting
Total revenue	445,939	466,135	379,968
NOI	305,408	327,356	271,767
Gross real estate	4,235,524	4,214,565	3,367,198
Operating segment | Established Communities | Mid-Atlantic
Segment Reporting
Total revenue	341,311	351,680	284,381
NOI	237,063	250,142	200,381
Gross real estate	3,511,960	3,484,610	2,669,040
Operating segment | Established Communities | Pacific Northwest
Segment Reporting
Total revenue	109,321	113,021	108,861
NOI	76,093	82,186	78,313
Gross real estate	996,317	990,563	985,102
Operating segment | Established Communities | Northern California
Segment Reporting
Total revenue	378,362	397,593	340,247
NOI	283,012	305,450	262,055
Gross real estate	3,201,926	3,186,075	2,753,596
Operating segment | Established Communities | Southern California
Segment Reporting
Total revenue	432,441	451,640	394,519
NOI	298,900	321,776	283,795
Gross real estate	4,160,754	4,131,539	3,573,952
Operating segment | Established Communities | Expansion Markets
Segment Reporting
Total revenue	23,269	23,403
NOI	13,376	13,578
Gross real estate	321,252	320,355
Operating segment | Other Stabilized Communities
Segment Reporting
Total revenue	140,173	110,434	238,584
NOI	92,040	74,814	159,745
Gross real estate	1,596,656	1,587,398	3,063,670
Operating segment | Development / Redevelopment
Segment Reporting
Total revenue	84,001	28,776	120,822
NOI	48,245	16,060	80,766
Gross real estate	2,789,062	2,086,519	2,652,968
Operating segment | Disposals [Member]
Segment Reporting
Gross real estate		401,152	732,397
Segment Reconciling Items
Segment Reporting
Gross real estate	110,142	0	84,712
Non-allocated
Segment Reporting
Total revenue	3,819	4,960	3,572
Gross real estate	375,964	559,777	504,230
Continuing Operations
Segment Reporting
Total revenue	2,256,310	2,251,458	2,094,548
Continuing Operations | Operating segment | Established Communities
Segment Reporting
Total revenue	2,028,317	2,107,288	1,731,570
Continuing Operations | Operating segment | Other Stabilized Communities
Segment Reporting
Total revenue	140,173	110,434	238,584
Continuing Operations | Non-allocated
Segment Reporting
Total revenue	$ 3,819	$ 4,960	$ 3,572