Details of Segment Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Segment Reporting Information [Line Items]
Total revenue	$ 968,711		$ 874,005		$ 829,924
NOI	645,953		564,895		542,181
% NOI change from prior year	14.30%		4.20%		80.00%
Gross real estate	9,288,496	[1]	8,511,586	[1]	8,034,028	[1]
Established
Segment Reporting Information [Line Items]
Total revenue	691,538	[2]	628,133	[2]	567,204	[2]
NOI	468,563	[2]	415,654	[2]	383,235	[2]
% NOI change from prior year	8.40%	[2]	(2.80%)	[2]	(7.10%)	[2]
Gross real estate	5,494,094	[1],[2]	4,926,840	[1],[2]	4,058,830	[1],[2]
Established | New England
Segment Reporting Information [Line Items]
Total revenue	169,939		143,564		121,455
NOI	109,048		89,712		75,766
% NOI change from prior year	9.60%		(0.30%)		(7.55%)
Gross real estate	1,302,368	[1]	1,109,016	[1]	858,858	[1]
Established | Metro NY/NJ
Segment Reporting Information [Line Items]
Total revenue	195,652		181,639		155,468
NOI	131,605		121,033		103,558
% NOI change from prior year	6.60%		(1.50%)		(7.40%)
Gross real estate	1,534,923	[1]	1,386,850	[1]	1,048,636	[1]
Established | Mid-Atlantic/Midwest
Segment Reporting Information [Line Items]
Total revenue	110,215		98,899		100,817
NOI	79,498		69,965		71,091
% NOI change from prior year	6.90%		0.80%		(3.30%)
Gross real estate	660,885	[1]	602,395	[1]	626,559	[1]
Established | Pacific Northwest
Segment Reporting Information [Line Items]
Total revenue	37,652		26,352		28,184
NOI	25,059		16,775		19,101
% NOI change from prior year	6.30%		(11.80%)		(9.30%)
Gross real estate	364,987	[1]	240,093	[1]	239,215	[1]
Established | Northern California
Segment Reporting Information [Line Items]
Total revenue	102,960		118,791		98,529
NOI	72,962		80,466		70,819
% NOI change from prior year	11.30%		(6.20%)		(7.90%)
Gross real estate	933,226	[1]	1,118,324	[1]	857,321	[1]
Established | Southern California
Segment Reporting Information [Line Items]
Total revenue	75,120		58,888		62,751
NOI	50,391		37,703		42,900
% NOI change from prior year	9.80%		(6.70%)		(9.70%)
Gross real estate	697,705	[1]	470,162	[1]	428,241	[1]
Other Stabilized
Segment Reporting Information [Line Items]
Total revenue	137,779		122,404		125,691
NOI	89,949		74,609		81,568
Gross real estate	1,567,875	[1]	1,580,910	[1]	1,411,395	[1]
Development / Redevelopment
Segment Reporting Information [Line Items]
Total revenue	129,738		116,114		129,700
NOI	87,441		74,632		77,378
Gross real estate	1,822,448	[1]	1,736,880	[1]	2,264,590	[1]
Land Held for Future Development
Segment Reporting Information [Line Items]
Gross real estate	325,918	[1]	184,150	[1]	237,095	[1]
Non-allocated
Segment Reporting Information [Line Items]
Total revenue	9,656	[3]	7,354	[3]	7,328	[3]
Gross real estate	$ 78,161	[1],[3]	$ 82,806	[1],[3]	$ 62,118	[1],[3]

[1] Does not include gross real estate assets held for sale of $0, $149,626 and $315,807 as of December 31, 2011, December 31, 2010 and December 31, 2009 respectively.

[2] Gross real estate for the Company's established communities includes capitalized additions of approximately $34,359, $38,670 and $10,783 in 2011, 2010 and 2009, respectively.

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