Segment Reporting (Details 3) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting
Total revenue		$ 475,360	$ 430,525	$ 1,375,187	$ 1,244,407
NOI		323,040	290,425	934,463	830,537
Gross real estate assets held for sale			245,060		245,060
Continuing Operations
Segment Reporting
Total revenue		474,007	423,621	1,369,025	1,216,908
NOI		$ 323,040	$ 290,425	$ 934,463	$ 830,537
% NOI change from prior year		11.20%	13.60%	12.50%	16.50%
Gross real estate	[1]	$ 19,002,878	$ 17,285,687	$ 19,002,878	$ 17,285,687
Continuing Operations | Operating Segments | Established
Segment Reporting
Total revenue		356,163	331,805	1,043,282	712,871
NOI		$ 247,643	$ 230,509	$ 727,387	$ 497,137
% NOI change from prior year		5.10%	5.50%	5.10%	3.00%
Gross real estate		$ 12,495,778	$ 7,432,186	$ 12,495,778	$ 7,432,186
Continuing Operations | Operating Segments | Established | New England [Member]
Segment Reporting
Total revenue		50,377	46,788	147,138	133,732
NOI		$ 32,202	$ 30,258	$ 91,026	$ 85,123
% NOI change from prior year		3.10%	4.60%	0.60%	0.90%
Gross real estate		$ 1,487,944	$ 1,363,271	$ 1,487,944	$ 1,363,271
Continuing Operations | Operating Segments | Established | Metro NY/NJ
Segment Reporting
Total revenue		98,585	93,905	289,288	231,259
NOI		$ 68,973	$ 65,839	$ 201,962	$ 161,879
% NOI change from prior year		3.70%	3.80%	3.20%	2.40%
Gross real estate		$ 3,196,771	$ 2,297,417	$ 3,196,771	$ 2,297,417
Continuing Operations | Operating Segments | Established | Mid-Atlantic
Segment Reporting
Total revenue		52,839	47,122	156,806	73,964
NOI		$ 36,157	$ 32,284	$ 108,125	$ 51,947
% NOI change from prior year		0.30%	(2.20%)	(0.40%)	(3.30%)
Gross real estate		$ 2,172,951	$ 645,872	$ 2,172,951	$ 645,872
Continuing Operations | Operating Segments | Established | Pacific Northwest
Segment Reporting
Total revenue		19,493	16,744	57,029	40,437
NOI		$ 13,502	$ 11,668	$ 40,532	$ 28,104
% NOI change from prior year		5.10%	9.40%	7.30%	6.30%
Gross real estate		$ 720,223	$ 499,611	$ 720,223	$ 499,611
Continuing Operations | Operating Segments | Established | Northern California
Segment Reporting
Total revenue		69,850	64,120	202,508	129,560
NOI		$ 53,095	$ 48,805	$ 155,464	$ 99,030
% NOI change from prior year		9.50%	12.20%	10.80%	7.80%
Gross real estate		$ 2,412,264	$ 1,401,286	$ 2,412,264	$ 1,401,286
Continuing Operations | Operating Segments | Established | Southern California
Segment Reporting
Total revenue		65,019	63,126	190,513	103,919
NOI		$ 43,714	$ 41,655	$ 130,278	$ 71,054
% NOI change from prior year		7.90%	6.80%	9.10%	4.50%
Gross real estate		$ 2,505,625	$ 1,224,729	$ 2,505,625	$ 1,224,729
Continuing Operations | Operating Segments | Other Stabilized
Segment Reporting
Total revenue		56,571	45,003	165,335	370,569
NOI		36,930	31,926	108,283	255,350
Gross real estate		2,106,947	6,003,229	2,106,947	6,003,229
Continuing Operations | Operating Segments | Development / Redevelopment
Segment Reporting
Total revenue		59,112	44,310	152,694	125,215
NOI		38,467	27,990	98,793	78,050
Gross real estate		3,795,868	3,639,770	3,795,868	3,639,770
Continuing Operations | Land Held for Future Development
Segment Reporting
Gross real estate		553,729	176,484	553,729	176,484
Continuing Operations | Non-allocated
Segment Reporting
Total revenue	[2]	2,161	2,503	7,714	8,253
Gross real estate		$ 50,556	$ 34,018	$ 50,556	$ 34,018

[1] Does not include gross real estate assets held for sale of $245,060 as of

[2] (2)Revenue represents third-party management, asset management and developer fees and miscellaneous income which are not allocated to a reportable segment.