Segment Reporting (Details 3) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting
Total revenue	$ 480,840	$ 475,360	$ 457,459	$ 442,367	$ 440,656	$ 430,525	$ 413,806	$ 400,075	$ 1,856,028	$ 1,685,061	$ 1,462,921
NOI									$ 1,264,205	$ 1,121,938	$ 966,058
% NOI change from prior year (percent)									12.70%	16.10%	47.20%
Gross real estate	19,228,571				17,603,867				$ 19,228,571	$ 17,603,867	$ 16,310,602
Gross real estate assets held for sale	39,528				245,449				39,528	245,449	489,720
Established
Segment Reporting
Real estate capitalized additions	74,982				52,635				74,982	52,635	33,553
Operating segment | Established
Segment Reporting
Total revenue									1,384,823	944,837	796,894
NOI									$ 973,405	$ 660,644	$ 555,372
% NOI change from prior year (percent)									5.80%	3.60%	5.00%
Gross real estate	12,391,586				7,360,944				$ 12,391,586	$ 7,360,944	$ 6,354,099
Operating segment | Established | New England
Segment Reporting
Total revenue									190,802	172,153	152,800
NOI									$ 120,026	$ 109,745	$ 99,484
% NOI change from prior year (percent)									2.80%	0.80%	2.40%
Gross real estate	1,460,746				1,333,854				$ 1,460,746	$ 1,333,854	$ 1,189,040
Operating segment | Established | Metro NY/NJ
Segment Reporting
Total revenue									382,457	305,496	236,920
NOI									$ 268,986	$ 215,239	$ 164,827
% NOI change from prior year (percent)									3.30%	3.10%	4.60%
Gross real estate	3,152,361				2,251,697				$ 3,152,361	$ 2,251,697	$ 1,793,902
Operating segment | Established | Mid-Atlantic
Segment Reporting
Total revenue									209,013	98,590	100,548
NOI									$ 145,497	$ 69,498	$ 71,851
% NOI change from prior year (percent)									0.20%	(2.50%)	0.10%
Gross real estate	2,177,823				647,374				$ 2,177,823	$ 647,374	$ 633,598
Operating segment | Established | Pacific Northwest
Segment Reporting
Total revenue									76,589	54,230	46,564
NOI									$ 54,751	$ 37,637	$ 31,283
% NOI change from prior year (percent)									8.20%	7.00%	5.30%
Gross real estate	721,040				500,247				$ 721,040	$ 500,247	$ 444,825
Operating segment | Established | Northern California
Segment Reporting
Total revenue									273,432	174,527	141,038
NOI									$ 210,226	$ 132,899	$ 106,745
% NOI change from prior year (percent)									11.90%	8.20%	11.70%
Gross real estate	2,414,184				1,402,444				$ 2,414,184	$ 1,402,444	$ 1,233,851
Operating segment | Established | Southern California
Segment Reporting
Total revenue									252,530	139,841	119,024
NOI									$ 173,919	$ 95,626	$ 81,182
% NOI change from prior year (percent)									9.40%	5.20%	5.10%
Gross real estate	2,465,432				1,225,328				$ 2,465,432	$ 1,225,328	$ 1,058,883
Operating segment | Other Stabilized
Segment Reporting
Total revenue									221,063	497,677	486,701
NOI									145,170	343,415	331,338
Gross real estate	2,040,269				6,057,783				2,040,269	6,057,783	6,621,825
Operating segment | Development / Redevelopment
Segment Reporting
Total revenue									222,222	186,852	117,186
NOI									145,630	117,879	79,348
Gross real estate	4,238,967				3,972,180				4,238,967	3,972,180	3,024,035
Land Held for Development
Segment Reporting
Gross real estate	484,377				180,516				484,377	180,516	300,364
Non-allocated
Segment Reporting
Total revenue									9,947	11,050	11,502
Gross real estate	$ 73,372				$ 32,444				73,372	32,444	10,279
Continuing Operations
Segment Reporting
Total revenue									$ 1,838,055	$ 1,640,416	$ 1,412,283