Segment Reporting (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting
Total revenue	$ 389,234	$ 252,445	$ 699,600	$ 496,820
NOI	269,126	171,282	483,141	337,459
% NOI change from prior year	57.10%	10.80%	43.20%	13.00%
Gross real estate	16,442,378	9,335,226	16,442,378	9,335,226
Gross real estate assets held for sale	0	218,377	0	218,377
Established
Segment Reporting
Total revenue	212,037	188,552	420,278	374,263
NOI	148,629	131,347	293,287	260,601
% NOI change from prior year	6.60%	7.10%	6.10%	8.70%
Gross real estate	6,753,376	6,184,986	6,753,376	6,184,986
Established | New England
Segment Reporting
Total revenue	45,555	41,736	90,209	82,812
NOI	30,320	27,263	58,897	53,894
% NOI change from prior year	4.90%	5.70%	3.30%	7.20%
Gross real estate	1,391,807	1,285,318	1,391,807	1,285,318
Established | Metro NY/NJ
Segment Reporting
Total revenue	62,549	58,169	123,794	115,388
NOI	43,449	40,637	85,888	80,229
% NOI change from prior year	5.10%	6.30%	5.30%	8.00%
Gross real estate	1,917,740	1,962,950	1,917,740	1,962,950
Established | Mid-Atlantic
Segment Reporting
Total revenue	25,312	25,829	50,346	51,525
NOI	18,330	18,722	36,518	37,538
% NOI change from prior year	2.80%	2.40%	2.20%	4.40%
Gross real estate	631,578	591,284	631,578	591,284
Established | Pacific Northwest
Segment Reporting
Total revenue	11,603	8,119	22,979	16,024
NOI	7,937	5,651	15,787	11,223
% NOI change from prior year	10.50%	9.50%	10.50%	10.70%
Gross real estate	443,641	303,343	443,641	303,343
Established | Northern California
Segment Reporting
Total revenue	37,476	30,191	74,078	59,645
NOI	28,218	22,051	55,722	43,715
% NOI change from prior year	12.40%	13.00%	12.00%	14.20%
Gross real estate	1,313,242	1,095,715	1,313,242	1,095,715
Established | Southern California
Segment Reporting
Total revenue	29,542	24,508	58,872	48,869
NOI	20,375	17,023	40,475	34,002
% NOI change from prior year	6.90%	8.50%	6.20%	10.20%
Gross real estate	1,055,368	946,376	1,055,368	946,376
Other Stabilized
Segment Reporting
Total revenue	147,704	31,561	226,802	61,813
NOI	102,161	20,042	157,166	39,538
Gross real estate	7,045,024	1,140,536	7,045,024	1,140,536
Development / Redevelopment
Segment Reporting
Total revenue	26,580	29,562	47,335	55,425
NOI	18,336	19,893	32,688	37,320
Gross real estate	2,183,928	1,656,545	2,183,928	1,656,545
Land Held for Future Development
Segment Reporting
Gross real estate	409,930	294,116	409,930	294,116
Non-allocated
Segment Reporting
Total revenue	2,913	2,770	5,185	5,319
Gross real estate	$ 50,120	$ 59,043	$ 50,120	$ 59,043