Segment Reporting (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Segment Reporting
Total revenue	$ 312,131	$ 246,032
NOI	215,253	167,306
% NOI change from prior year	28.70%	17.40%
Gross real estate	16,157,761	9,208,659
Gross real estate assets held for sale	0	255,431
Established
Segment Reporting
Total revenue	205,823	179,560
NOI	143,021	125,396
% NOI change from prior year	5.60%	10.20%
Gross real estate	6,678,203	6,067,540
Established | New England
Segment Reporting
Total revenue	44,655	33,881
NOI	28,577	22,065
% NOI change from prior year	1.70%	7.50%
Gross real estate	1,387,495	1,100,828
Established | Metro NY/NJ
Segment Reporting
Total revenue	61,244	57,219
NOI	42,439	39,591
% NOI change from prior year	5.50%	9.80%
Gross real estate	1,915,890	1,962,197
Established | Mid-Atlantic
Segment Reporting
Total revenue	25,035	25,696
NOI	18,188	18,816
% NOI change from prior year	1.60%	6.40%
Gross real estate	631,207	591,140
Established | Pacific Northwest
Segment Reporting
Total revenue	11,376	7,905
NOI	7,850	5,572
% NOI change from prior year	10.50%	11.90%
Gross real estate	443,564	301,793
Established | Northern California
Segment Reporting
Total revenue	36,603	31,112
NOI	27,504	22,793
% NOI change from prior year	11.60%	15.50%
Gross real estate	1,312,838	1,180,070
Established | Southern California
Segment Reporting
Total revenue	26,910	23,747
NOI	18,463	16,559
% NOI change from prior year	5.90%	11.90%
Gross real estate	987,209	931,512
Other Stabilized
Segment Reporting
Total revenue	80,862	30,252
NOI	56,423	19,498
Gross real estate	7,136,065	1,098,074
Development / Redevelopment
Segment Reporting
Total revenue	23,174	33,671
NOI	15,989	22,412
Gross real estate	1,915,146	1,641,636
Land Held for Future Development
Segment Reporting
Gross real estate	359,029	297,127
Non-allocated
Segment Reporting
Total revenue	2,272	2,549
Gross real estate	$ 69,138	$ 104,282