Segment Reporting (Details 3) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting
Total revenue	$ 555,292	$ 550,500	$ 530,512	$ 522,326	$ 518,240	$ 516,211	$ 502,307	$ 508,498	$ 2,158,628	$ 2,045,255	$ 1,856,028
NOI									$ 1,490,068	$ 1,383,943	$ 1,215,742
% NOI change from prior year (percent)									7.70%	13.80%	13.00%
Gross real estate	21,935,936				20,755,780				$ 21,935,936	$ 20,755,780	$ 19,228,571
Gross real estate assets held for sale					20,846					20,846	39,528
Gain on Business Interruption Insurance Recovery									3,498	20,564	1,509
Established
Segment Reporting
Real estate capitalized additions	78,241				85,676				78,241	85,676	74,982
Operating segment | Established
Segment Reporting
Total revenue									1,575,260	1,502,974	1,315,929
NOI									$ 1,112,472	$ 1,057,997	$ 929,944
% NOI change from prior year (percent)									2.50%	4.80%	6.00%
Gross real estate	13,565,763				13,511,377				$ 13,565,763	$ 13,511,377	$ 12,391,586
Operating segment | Established | New England
Segment Reporting
Total revenue									233,091	226,727	170,287
NOI									$ 150,253	$ 145,671	$ 107,189
% NOI change from prior year (percent)									2.10%	4.80%	3.00%
Gross real estate	1,852,676				1,888,524				$ 1,852,676	$ 1,888,524	$ 1,460,746
Operating segment | Established | Metro NY/NJ
Segment Reporting
Total revenue									362,273	359,373	342,768
NOI									$ 247,720	$ 245,654	$ 244,280
% NOI change from prior year (percent)									2.10%	1.20%	3.60%
Gross real estate	3,069,690				3,212,220				$ 3,069,690	$ 3,212,220	$ 3,152,361
Operating segment | Established | Mid-Atlantic
Segment Reporting
Total revenue									221,064	233,711	209,012
NOI									$ 153,750	$ 162,243	$ 145,497
% NOI change from prior year (percent)									1.50%	1.30%	0.20%
Gross real estate	2,056,066				2,339,395				$ 2,056,066	$ 2,339,395	$ 2,177,823
Operating segment | Established | Pacific Northwest
Segment Reporting
Total revenue									84,189	72,475	67,900
NOI									$ 61,527	$ 52,434	$ 48,833
% NOI change from prior year (percent)									6.30%	7.40%	8.50%
Gross real estate	738,532				737,289				$ 738,532	$ 737,289	$ 721,040
Operating segment | Established | Northern California
Segment Reporting
Total revenue									336,767	319,121	273,432
NOI									$ 257,673	$ 244,458	$ 210,226
% NOI change from prior year (percent)									1.60%	7.00%	11.90%
Gross real estate	2,830,963				2,661,258				$ 2,830,963	$ 2,661,258	$ 2,414,184
Operating segment | Established | Southern California
Segment Reporting
Total revenue									337,876	291,567	252,530
NOI									$ 241,549	$ 207,537	$ 173,919
% NOI change from prior year (percent)									3.90%	9.10%	9.40%
Gross real estate	3,017,836				2,672,691				$ 3,017,836	$ 2,672,691	$ 2,465,432
Operating segment | Other Stabilized
Segment Reporting
Total revenue									278,868	232,977	211,633
NOI									196,733	165,130	140,167
Gross real estate	3,189,393				2,330,503				3,189,393	2,330,503	2,040,269
Operating segment | Development / Redevelopment
Segment Reporting
Total revenue									276,896	233,432	222,222
NOI									180,863	160,816	145,631
Gross real estate	5,015,094				4,755,315				5,015,094	4,755,315	4,238,967
Segment Reconciling Items [Member]
Segment Reporting
Gross real estate	68,364				84,293				68,364	84,293	484,377
Non-allocated
Segment Reporting
Total revenue									4,147	5,599	9,947
Gross real estate	$ 97,322				$ 74,292				97,322	74,292	73,372
Continuing Operations
Segment Reporting
Total revenue									2,135,171	1,974,982	$ 1,759,731
Avalon Maplewood [Member]
Segment Reporting
Gain on Business Interruption Insurance Recovery									$ 3,495
Avalon at Edgewater [Member]
Segment Reporting
Gain on Business Interruption Insurance Recovery										$ 20,306