Segment Reporting (Details 3) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting
Total revenue		$ 457,459	$ 413,806	$ 899,826	$ 813,881
NOI		314,014	277,548	611,423	540,111
Gross real estate assets held for sale		79,128	244,805	79,128	244,805
Continuing Operations
Segment Reporting
Total revenue		455,174	403,580	895,017	793,286
NOI		$ 314,014	$ 277,548	$ 611,423	$ 540,111
% NOI change from prior year		13.10%	8.90%	13.20%	18.10%
Gross real estate	[1]	$ 18,611,288	$ 16,960,426	$ 18,611,288	$ 16,960,426
Continuing Operations | Operating Segments | Established
Segment Reporting
Total revenue		346,995	325,467	687,118	470,937
NOI		$ 244,141	$ 227,093	$ 479,744	$ 328,337
% NOI change from prior year		5.00%	2.90%	5.10%	2.60%
Gross real estate		$ 12,471,582	$ 7,413,467	$ 12,471,582	$ 7,413,467
Continuing Operations | Operating Segments | Established | New England [Member]
Segment Reporting
Total revenue		49,000	45,523	96,760	88,125
NOI		$ 30,985	$ 29,179	$ 58,823	$ 55,620
% NOI change from prior year		2.30%	(0.90%)	(0.70%)	(1.00%)
Gross real estate		$ 1,477,145	$ 1,356,000	$ 1,477,145	$ 1,356,000
Continuing Operations | Operating Segments | Established | Metro NY/NJ
Segment Reporting
Total revenue		96,372	92,325	190,704	152,906
NOI		$ 67,880	$ 64,724	$ 132,989	$ 106,780
% NOI change from prior year		2.90%	0.90%	2.90%	1.70%
Gross real estate		$ 3,191,141	$ 2,294,169	$ 3,191,141	$ 2,294,169
Continuing Operations | Operating Segments | Established | Mid-Atlantic
Segment Reporting
Total revenue		52,263	46,990	103,967	49,260
NOI		$ 35,938	$ 32,531	$ 71,969	$ 34,784
% NOI change from prior year		(0.90%)	(6.70%)	(0.80%)	(3.90%)
Gross real estate		$ 2,170,822	$ 645,172	$ 2,170,822	$ 645,172
Continuing Operations | Operating Segments | Established | Pacific Northwest
Segment Reporting
Total revenue		19,047	16,458	37,536	26,683
NOI		$ 13,657	$ 11,554	$ 27,030	$ 18,591
% NOI change from prior year		7.90%	5.90%	8.40%	4.80%
Gross real estate		$ 719,366	$ 499,383	$ 719,366	$ 499,383
Continuing Operations | Operating Segments | Established | Northern California
Segment Reporting
Total revenue		67,144	62,319	132,658	85,305
NOI		$ 52,635	$ 47,498	$ 102,369	$ 65,364
% NOI change from prior year		11.70%	13.70%	11.60%	9.60%
Gross real estate		$ 2,409,781	$ 1,400,573	$ 2,409,781	$ 1,400,573
Continuing Operations | Operating Segments | Established | Southern California
Segment Reporting
Total revenue		63,169	61,852	125,493	68,658
NOI		$ 43,046	$ 41,607	$ 86,564	$ 47,198
% NOI change from prior year		6.70%	5.00%	9.80%	4.50%
Gross real estate		$ 2,503,327	$ 1,218,170	$ 2,503,327	$ 1,218,170
Continuing Operations | Operating Segments | Other Stabilized
Segment Reporting
Total revenue		54,681	43,701	108,764	244,289
NOI		36,536	31,289	71,353	167,494
Gross real estate		2,028,096	5,998,737	2,028,096	5,998,737
Continuing Operations | Operating Segments | Development / Redevelopment
Segment Reporting
Total revenue		50,556	31,740	93,582	72,310
NOI		33,337	19,166	60,326	44,280
Gross real estate		3,581,408	3,318,362	3,581,408	3,318,362
Continuing Operations | Land Held for Future Development
Segment Reporting
Gross real estate		487,205	195,673	487,205	195,673
Continuing Operations | Non-allocated
Segment Reporting
Total revenue	[2]	2,942	2,672	5,553	5,750
Gross real estate		$ 42,997	$ 34,187	$ 42,997	$ 34,187

[1] Does not include gross real estate assets held for sale of $79,128 and $244,805 as of June 30, 2015 and 2014, respectively.

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