Segment Reporting (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting
Total revenue	$ 1,038,660	$ 936,087	$ 842,820
NOI	708,375	624,444	545,510
% NOI change from prior year	14.20%	14.50%	4.60%
Gross real estate	10,071,342	9,288,496	8,661,211	8,360,091
Gross real estate assets held for sale	70,246	255,274	398,679
Established
Segment Reporting
Total revenue	763,405	668,811	606,537
NOI	531,868	454,347	402,973
% NOI change from prior year	7.60%	8.30%	(2.60%)
Gross real estate	6,264,368	5,309,257	4,744,269
Real estate capitalized additions	25,448	34,359	38,670
Established | New England
Segment Reporting
Total revenue	168,260	169,939	143,564
NOI	109,301	109,048	89,712
% NOI change from prior year	4.90%	9.60%	(0.30%)
Gross real estate	1,289,604	1,302,368	1,109,016
Established | Metro NY/NJ
Segment Reporting
Total revenue	229,624	195,652	181,639
NOI	160,026	131,605	121,033
% NOI change from prior year	7.30%	6.60%	(1.50%)
Gross real estate	1,947,668	1,534,923	1,386,850
Established | Mid-Atlantic
Segment Reporting
Total revenue	103,784	102,834	91,927
NOI	75,313	74,756	65,737
% NOI change from prior year	6.90%	6.60%	1.00%
Gross real estate	591,669	603,345	545,190
Established | Pacific Northwest
Segment Reporting
Total revenue	32,942	30,057	19,045
NOI	23,433	20,374	12,426
% NOI change from prior year	15.00%	6.00%	(10.70%)
Gross real estate	306,289	301,662	176,940
Established | Northern California
Segment Reporting
Total revenue	129,493	95,209	111,474
NOI	94,915	68,173	76,362
% NOI change from prior year	14.00%	10.90%	(6.40%)
Gross real estate	1,181,395	869,254	1,056,111
Established | Southern California
Segment Reporting
Total revenue	99,302	75,120	58,888
NOI	68,880	50,391	37,703
% NOI change from prior year	7.00%	9.80%	(6.70%)
Gross real estate	947,743	697,705	470,162
Other Stabilized
Segment Reporting
Total revenue	135,231	137,779	122,403
NOI	86,722	89,949	74,609
Gross real estate	1,314,690	1,567,875	1,580,910
Development / Redevelopment
Segment Reporting
Total revenue	129,767	119,841	106,526
NOI	89,785	80,148	67,928
Gross real estate	2,032,277	1,752,006	1,670,398
Land Held for Future Development
Segment Reporting
Gross real estate	316,037	325,918	184,150
Non-allocated
Segment Reporting
Total revenue	10,257	9,656	7,354
Gross real estate	$ 73,724	$ 78,161	$ 82,806