Segment Reporting (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Segment Reporting
Total revenue	$ 413,806		$ 378,207		$ 813,881		$ 679,564
NOI	281,758		258,971		548,319		464,786
Gross real estate assets held for sale			566,344				566,344
Established
Segment Reporting
Total revenue	329,993	[1]	205,999		479,922	[1]	408,322
NOI	230,003	[1]	144,466		333,880	[1]	285,232
% NOI change from prior year	2.80%	[1]	6.50%		2.50%	[1]	6.10%
Gross real estate	7,556,637	[1],[2]	6,561,385	[2]	7,556,637	[1],[2]	6,561,385	[2]
Established | New England
Segment Reporting
Total revenue	47,903	[1]	42,113		92,845	[1]	83,386
NOI	30,759	[1]	28,080		58,590	[1]	54,660
% NOI change from prior year	(0.90%)	[1]	4.80%		(1.20%)	[1]	3.60%
Gross real estate	1,420,125	[1],[2]	1,280,776	[2]	1,420,125	[1],[2]	1,280,776	[2]
Established | Metro NY/NJ
Segment Reporting
Total revenue	94,471	[1]	62,549		157,171	[1]	123,794
NOI	66,054	[1]	43,449		109,353	[1]	85,888
% NOI change from prior year	0.90%	[1]	5.10%		1.60%	[1]	5.30%
Gross real estate	2,373,214	[1],[2]	1,917,740	[2]	2,373,214	[1],[2]	1,917,740	[2]
Established | Mid-Atlantic
Segment Reporting
Total revenue	46,990	[1]	25,312		49,260	[1]	50,346
NOI	32,531	[1]	18,330		34,784	[1]	36,518
% NOI change from prior year	(6.70%)	[1]	2.80%		(3.90%)	[1]	2.20%
Gross real estate	645,172	[1],[2]	631,578	[2]	645,172	[1],[2]	631,578	[2]
Established | Pacific Northwest
Segment Reporting
Total revenue	16,458	[1]	11,603		26,683	[1]	22,979
NOI	11,554	[1]	7,937		18,591	[1]	15,787
% NOI change from prior year	5.90%	[1]	10.50%		4.80%	[1]	10.50%
Gross real estate	499,383	[1],[2]	443,641	[2]	499,383	[1],[2]	443,641	[2]
Established | Northern California
Segment Reporting
Total revenue	62,319	[1]	34,880		85,305	[1]	68,945
NOI	47,498	[1]	26,295		65,364	[1]	51,904
% NOI change from prior year	13.70%	[1]	12.50%		9.60%	[1]	12.00%
Gross real estate	1,400,573	[1],[2]	1,232,282	[2]	1,400,573	[1],[2]	1,232,282	[2]
Established | Southern California
Segment Reporting
Total revenue	61,852	[1]	29,542		68,658	[1]	58,872
NOI	41,607	[1]	20,375		47,198	[1]	40,475
% NOI change from prior year	5.00%	[1]	6.90%		4.50%	[1]	6.20%
Gross real estate	1,218,170	[1],[2]	1,055,368	[2]	1,218,170	[1],[2]	1,055,368	[2]
Other Stabilized
Segment Reporting
Total revenue	46,001	[1]	139,141		248,887	[1]	211,909
NOI	32,589	[1]	96,169		170,159	[1]	146,866
Gross real estate	6,091,177	[1],[2]	6,670,671	[2]	6,091,177	[1],[2]	6,670,671	[2]
Development / Redevelopment
Segment Reporting
Total revenue	31,740	[1]	26,580		72,310	[1]	47,335
NOI	19,166	[1]	18,336		44,280	[1]	32,688
Gross real estate	3,318,362	[1],[2]	2,183,928	[2]	3,318,362	[1],[2]	2,183,928	[2]
Land Held for Future Development
Segment Reporting
Gross real estate	195,673	[1],[2]	409,930	[2]	195,673	[1],[2]	409,930	[2]
Non-allocated
Segment Reporting
Total revenue	2,672	[1],[3]	2,913	[3]	5,750	[1],[3]	5,185	[3]
Gross real estate	43,382	[1],[2],[3]	50,120	[2],[3]	43,382	[1],[2],[3]	50,120	[2],[3]
Continuing Operations [Member]
Segment Reporting
Total revenue	410,406	[1]	374,633		806,869	[1]	672,751
NOI	281,758	[1]	258,971		548,319	[1]	464,786
% NOI change from prior year	8.80%	[1]	58.10%		18.00%	[1]	44.00%
Gross real estate	$ 17,205,231	[1],[2]	$ 15,876,034	[2]	$ 17,205,231	[1],[2]	$ 15,876,034	[2]

[1] Results for the three months ended June 30, 2014 reflect the operating segments updated as of April 1, 2014, which include most stabilized communities acquired as part of the Archstone Acquisition in the Established Communities segment. Results for the six months ended June 30, 2014 reflect the operating segments determined as of January 1, 2014, which include stabilized communities acquired as part of the Archstone Acquisition in the Other Stabilized segment.

[2] Does not include gross real estate assets held for sale of $566,344 as of June 30, 2013.

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