Segment Reporting (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Segment Reporting
Total revenue	$ 430,525		$ 389,189		$ 1,244,407		$ 1,068,752
NOI	293,353		258,436		838,830		721,347
Gross real estate assets held for sale	87,393		654,729		87,393		654,729
Established
Segment Reporting
Total revenue	336,451	[1]	208,842		726,503	[1]	617,163
NOI	233,525	[1]	144,126		505,695	[1]	429,359
% NOI change from prior year	5.50%	[1]	4.20%		3.00%	[1]	5.40%
Gross real estate	7,575,596	[1],[2]	6,572,177	[2]	7,575,596	[1],[2]	6,572,177	[2]
Established | New England
Segment Reporting
Total revenue	49,227	[1]	42,853		140,891	[1]	126,239
NOI	31,858	[1]	27,221		89,693	[1]	81,882
% NOI change from prior year	4.60%	[1]	(0.60%)		0.70%	[1]	2.10%
Gross real estate	1,427,605	[1],[2]	1,286,876	[2]	1,427,605	[1],[2]	1,286,876	[2]
Established | Metro NY/NJ
Segment Reporting
Total revenue	96,112	[1]	63,103		237,732	[1]	186,897
NOI	67,255	[1]	43,327		165,867	[1]	129,215
% NOI change from prior year	3.90%	[1]	3.40%		2.40%	[1]	4.60%
Gross real estate	2,376,493	[1],[2]	1,919,151	[2]	2,376,493	[1],[2]	1,919,151	[2]
Established | Mid-Atlantic
Segment Reporting
Total revenue	47,122	[1]	25,262		73,964	[1]	75,608
NOI	32,284	[1]	17,652		51,947	[1]	54,169
% NOI change from prior year	(2.20%)	[1]	(1.00%)		(3.30%)	[1]	1.10%
Gross real estate	645,872	[1],[2]	632,102	[2]	645,872	[1],[2]	632,102	[2]
Established | Pacific Northwest
Segment Reporting
Total revenue	16,744	[1]	11,773		40,437	[1]	34,752
NOI	11,668	[1]	7,752		28,104	[1]	23,539
% NOI change from prior year	9.40%	[1]	1.70%		6.30%	[1]	7.40%
Gross real estate	499,611	[1],[2]	443,812	[2]	499,611	[1],[2]	443,812	[2]
Established | Northern California
Segment Reporting
Total revenue	64,120	[1]	35,850		129,560	[1]	104,794
NOI	48,805	[1]	28,009		99,030	[1]	79,914
% NOI change from prior year	12.20%	[1]	15.30%		7.80%	[1]	13.10%
Gross real estate	1,401,286	[1],[2]	1,232,724	[2]	1,401,286	[1],[2]	1,232,724	[2]
Established | Southern California
Segment Reporting
Total revenue	63,126	[1]	30,001		103,919	[1]	88,873
NOI	41,655	[1]	20,165		71,054	[1]	60,640
% NOI change from prior year	6.80%	[1]	4.70%		4.50%	[1]	5.70%
Gross real estate	1,224,729	[1],[2]	1,057,512	[2]	1,224,729	[1],[2]	1,057,512	[2]
Other Stabilized
Segment Reporting
Total revenue	45,023	[1]	139,093		370,627	[1]	348,028
NOI	31,838	[1]	92,856		255,085	[1]	237,846
Gross real estate	6,008,289	[1],[2]	6,539,176	[2]	6,008,289	[1],[2]	6,539,176	[2]
Development / Redevelopment
Segment Reporting
Total revenue	44,310	[1]	32,369		125,215	[1]	79,704
NOI	27,990	[1]	21,454		78,050	[1]	54,142
Gross real estate	3,639,770	[1],[2]	2,676,384	[2]	3,639,770	[1],[2]	2,676,384	[2]
Land Held for Future Development
Segment Reporting
Gross real estate	176,484	[1],[2]	282,285	[2]	176,484	[1],[2]	282,285	[2]
Non-allocated
Segment Reporting
Total revenue	2,503	[1],[3]	3,014	[3]	8,253	[1],[3]	8,198	[3]
Gross real estate	43,215	[1],[2],[3]	47,921	[2],[3]	43,215	[1],[2],[3]	47,921	[2],[3]
Continuing Operations [Member]
Segment Reporting
Total revenue	428,287	[1]	383,318		1,230,598	[1]	1,053,093
NOI	293,353	[1]	258,436		838,830	[1]	721,347
% NOI change from prior year	13.50%	[1]	49.30%		16.30%	[1]	45.40%
Gross real estate	$ 17,443,354	[1],[2]	$ 16,117,943	[2]	$ 17,443,354	[1],[2]	$ 16,117,943	[2]

[1] Results for the three months ended September 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 nine months ended September 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 $87,393 and $654,729 as of September 30, 2014 and 2013, respectively.

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