Segment Reporting (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Sep. 30, 2013
Segment Reporting
Total revenue	$ 442,367		$ 400,075
NOI	297,997		263,874
Gross real estate assets held for sale	60,686		344,068
Continuing Operations
Segment Reporting
Total revenue	440,658		391,809
NOI	297,997		263,874
% NOI change from prior year	12.90%		29.50%
Gross real estate	18,314,655	[1]			16,716,465	[1]
Continuing Operations | Operating Segments | Established
Segment Reporting
Total revenue	342,325		232,788
NOI	237,001		161,308
% NOI change from prior year	5.30%		2.70%
Gross real estate	12,533,563				7,374,940
Continuing Operations | Operating Segments | Established | New England [Member]
Segment Reporting
Total revenue	47,760		46,290
NOI	27,839		28,513
% NOI change from prior year	(3.80%)		(1.60%)
Gross real estate	1,469,071				1,426,168
Continuing Operations | Operating Segments | Established | Metro NY/NJ
Segment Reporting
Total revenue	96,533		72,354
NOI	66,507		50,019
% NOI change from prior year	3.00%		1.30%
Gross real estate	3,268,669				2,187,554
Continuing Operations | Operating Segments | Established | Mid-Atlantic
Segment Reporting
Total revenue	51,704		24,647
NOI	36,031		17,455
% NOI change from prior year	(0.80%)		(3.00%)
Gross real estate	2,170,104				644,657
Continuing Operations | Operating Segments | Established | Pacific Northwest
Segment Reporting
Total revenue	18,489		13,129
NOI	13,373		9,134
% NOI change from prior year	9.00%		3.20%
Gross real estate	718,884				498,710
Continuing Operations | Operating Segments | Established | Northern California
Segment Reporting
Total revenue	65,515		42,219
NOI	49,734		32,834
% NOI change from prior year	11.50%		11.40%
Gross real estate	2,405,670				1,400,087
Continuing Operations | Operating Segments | Established | Southern California
Segment Reporting
Total revenue	62,324		34,149
NOI	43,517		23,353
% NOI change from prior year	13.10%		4.20%
Gross real estate	2,501,165				1,217,764
Continuing Operations | Operating Segments | Other Stabilized
Segment Reporting
Total revenue	52,696		128,565
NOI	34,008		86,332
Gross real estate	2,053,692				6,315,469
Continuing Operations | Operating Segments | Development / Redevelopment
Segment Reporting
Total revenue	43,026		27,379
NOI	26,988		16,234
Gross real estate	3,147,624				2,743,725
Continuing Operations | Land Held for Future Development
Segment Reporting
Gross real estate	536,538				250,204
Continuing Operations | Non-allocated
Segment Reporting
Total revenue	2,611	[2]	3,077	[2]
Gross real estate	$ 43,238				$ 32,127

[1] Does not include gross real estate assets held for sale of $60,686 and $344,068 as of March 31, 2015 and 2014, respectively.

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