Segment Reporting (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting
Total revenue	$ 393,127	$ 261,321	$ 1,080,471	$ 750,106
NOI	264,622	177,353	739,178	509,185
% NOI change from prior year	49.20%	14.70%	45.20%	14.60%
Gross real estate	16,454,071	9,486,090	16,454,071	9,486,090
Gross real estate assets held for sale	318,406	405,662	318,406	405,662
Established
Segment Reporting
Total revenue	214,949	189,052	635,227	555,280
NOI	148,124	130,286	441,410	385,259
% NOI change from prior year	4.20%	6.90%	5.50%	8.10%
Gross real estate	6,764,465	6,006,428	6,764,465	6,006,428
Established | New England
Segment Reporting
Total revenue	46,319	42,755	136,528	125,568
NOI	29,323	27,374	88,220	81,268
% NOI change from prior year	(0.60%)	2.70%	2.00%	5.60%
Gross real estate	1,398,194	1,287,578	1,398,194	1,287,578
Established | Metro NY/NJ
Segment Reporting
Total revenue	63,103	55,257	186,897	162,608
NOI	43,327	38,155	129,215	112,757
% NOI change from prior year	3.40%	5.50%	4.60%	7.40%
Gross real estate	1,919,151	1,779,699	1,919,151	1,779,699
Established | Mid-Atlantic
Segment Reporting
Total revenue	25,262	26,300	75,608	77,825
NOI	17,652	18,618	54,169	56,156
% NOI change from prior year	(1.00%)	3.70%	1.10%	4.20%
Gross real estate	632,102	591,802	632,102	591,802
Established | Pacific Northwest
Segment Reporting
Total revenue	11,773	8,401	34,752	24,426
NOI	7,752	5,984	23,539	17,207
% NOI change from prior year	1.70%	19.60%	7.40%	13.60%
Gross real estate	443,812	304,381	443,812	304,381
Established | Northern California
Segment Reporting
Total revenue	38,491	31,208	112,569	90,853
NOI	29,905	22,931	85,627	66,646
% NOI change from prior year	15.00%	15.10%	13.10%	14.50%
Gross real estate	1,313,694	1,096,166	1,313,694	1,096,166
Established | Southern California
Segment Reporting
Total revenue	30,001	25,131	88,873	74,000
NOI	20,165	17,224	60,640	51,225
% NOI change from prior year	4.70%	6.20%	5.70%	8.80%
Gross real estate	1,057,512	946,802	1,057,512	946,802
Other Stabilized
Segment Reporting
Total revenue	142,795	34,469	357,342	96,282
NOI	95,044	22,384	243,626	61,924
Gross real estate	6,683,016	1,249,203	6,683,016	1,249,203
Development / Redevelopment
Segment Reporting
Total revenue	32,369	35,267	79,704	90,692
NOI	21,454	24,683	54,142	62,002
Gross real estate	2,676,384	1,871,336	2,676,384	1,871,336
Land Held for Future Development
Segment Reporting
Gross real estate	282,285	304,295	282,285	304,295
Non-allocated
Segment Reporting
Total revenue	3,014	2,533	8,198	7,852
Gross real estate	$ 47,921	$ 54,828	$ 47,921	$ 54,828