Details of Segment Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010
Segment Reporting Information [Line Items]
Total revenue	$ 244,859		$ 220,342		$ 480,666		$ 435,790
NOI	162,502		140,794		314,334		277,008
% NOI change from prior year	14.30%		2.80%		12.40%		1.20%
Gross real estate	8,956,246	[1]	8,404,430	[1]	8,956,246	[1]	8,404,430	[1]
Established
Segment Reporting Information [Line Items]
Total revenue	174,728		161,709		345,883		321,488
NOI	119,487		105,479		233,652		208,466
% NOI change from prior year	8.00%		(4.40%)		6.90%		(6.00%)
Gross real estate	5,540,048	[1]	5,051,671	[1]	5,540,048	[1]	5,051,671	[1]
Established | New England
Segment Reporting Information [Line Items]
Total revenue	42,083		35,654		83,110		70,847
NOI	27,006		22,300		52,488		43,944
% NOI change from prior year	10.50%		(2.30%)		8.60%		(3.40%)
Gross real estate	1,297,280	[1]	1,094,054	[1]	1,297,280	[1]	1,094,054	[1]
Established | Metro NY/NJ
Segment Reporting Information [Line Items]
Total revenue	48,501		45,287		96,191		89,677
NOI	33,153		30,589		64,712		60,096
% NOI change from prior year	6.40%		(4.50%)		5.90%		(4.00%)
Gross real estate	1,530,426	[1]	1,389,706	[1]	1,530,426	[1]	1,389,706	[1]
Established | Mid-Atlantic/Midwest
Segment Reporting Information [Line Items]
Total revenue	30,892		29,931		61,100		59,322
NOI	22,404		18,665		44,047		36,211
% NOI change from prior year	6.10%		0.70%		7.10%		(2.40%)
Gross real estate	726,241	[1]	751,070	[1]	726,241	[1]	751,070	[1]
Established | Pacific Northwest
Segment Reporting Information [Line Items]
Total revenue	9,346		6,614		18,475		13,231
NOI	6,349		4,249		12,489		8,675
% NOI change from prior year	6.40%		(14.10%)		3.60%		(14.50%)
Gross real estate	360,995	[1]	239,836	[1]	360,995	[1]	239,836	[1]
Established | Northern California
Segment Reporting Information [Line Items]
Total revenue	25,338		29,537		50,045		58,952
NOI	18,182		20,245		35,568		40,403
% NOI change from prior year	10.30%		(7.20%)		8.40%		(11.00%)
Gross real estate	929,886	[1]	1,109,337	[1]	929,886	[1]	1,109,337	[1]
Established | Southern California
Segment Reporting Information [Line Items]
Total revenue	18,568		14,686		36,962		29,459
NOI	12,393		9,431		24,348		19,137
% NOI change from prior year	8.70%		(7.80%)		5.20%		(8.80%)
Gross real estate	695,220	[1]	467,668	[1]	695,220	[1]	467,668	[1]
Other Stabilized
Segment Reporting Information [Line Items]
Total revenue	36,560		29,880		70,171		58,659
NOI	21,133		18,193		39,910		35,117
Gross real estate	1,661,506	[1]	1,539,112	[1]	1,661,506	[1]	1,539,112	[1]
Development / Redevelopment
Segment Reporting Information [Line Items]
Total revenue	31,239		27,069		59,960		52,110
NOI	21,882		17,122		40,772		33,425
Gross real estate	1,459,969	[1]	1,497,779	[1]	1,459,969	[1]	1,497,779	[1]
Non-allocated
Segment Reporting Information [Line Items]
Total revenue	2,332	[2]	1,684	[2]	4,652	[2]	3,533	[2]
Gross real estate	68,827	[1],[2]	78,339	[1],[2]	68,827	[1],[2]	78,339	[1],[2]
Land Held for Future Development
Segment Reporting Information [Line Items]
Gross real estate	$ 225,896	[1]	$ 237,529	[1]	$ 225,896	[1]	$ 237,529	[1]

[1] Does not include gross real estate assets held for sale of $18,429 and $28,329 as of June 30, 2011 and 2010, respectively.

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