Segment Reporting (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Segment Reporting
Gross real estate assets held for sale	$ 64,000	$ 62,230
Segment net operating income
Segment Reporting
Total revenue	252,454	228,530
NOI	171,740	149,502
% NOI change from prior year	14.90%	11.30%
Gross real estate	9,400,092	8,558,665
Established
Segment Reporting
Total revenue	179,559	165,919
NOI	125,396	110,586
% NOI change from prior year	10.20%	5.60%
Gross real estate	6,067,540	5,403,304
Established | New England
Segment Reporting
Total revenue	33,881	41,027
NOI	22,065	25,482
% NOI change from prior year	7.50%	6.70%
Gross real estate	1,100,828	1,293,907
Established | Metro NY/NJ
Segment Reporting
Total revenue	57,218	47,691
NOI	39,591	31,559
% NOI change from prior year	9.80%	5.50%
Gross real estate	1,962,197	1,527,764
Established | Mid-Atlantic
Segment Reporting
Total revenue	25,696	26,852
NOI	18,816	19,242
% NOI change from prior year	6.40%	8.80%
Gross real estate	591,140	658,249
Established | Pacific Northwest
Segment Reporting
Total revenue	7,905	9,129
NOI	5,572	6,140
% NOI change from prior year	11.90%	0.80%
Gross real estate	301,793	361,623
Established | Northern California
Segment Reporting
Total revenue	31,112	22,826
NOI	22,793	16,208
% NOI change from prior year	15.50%	5.40%
Gross real estate	1,180,070	866,694
Established | Southern California
Segment Reporting
Total revenue	23,747	18,394
NOI	16,559	11,955
% NOI change from prior year	11.90%	1.90%
Gross real estate	931,512	695,067
Other Stabilized
Segment Reporting
Total revenue	36,674	31,570
NOI	23,932	20,027
Gross real estate	1,289,507	1,557,988
Development / Redevelopment
Segment Reporting
Total revenue	33,672	28,721
NOI	22,412	18,889
Gross real estate	1,641,636	1,324,743
Land Held for Future Development
Segment Reporting
Gross real estate	297,127	193,593
Non-allocated
Segment Reporting
Total revenue	2,549	2,320
Gross real estate	$ 104,282	$ 79,037