Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014 segment	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting [Abstract]
Number of reportable operating segments defined by geographical regions									3
Revenues from External Customers and Long-Lived Assets [Line Items]
Total property revenues	$ 276,209	$ 268,118	$ 256,614	$ 159,017	$ 155,986	$ 152,177	$ 148,783	$ 145,057	$ 959,958	$ 602,003	$ 526,696
Net operating income									649,045	405,991	354,529
Depreciation and amortization									(360,592)	(192,420)	(169,173)
Interest expense									(164,551)	(116,524)	(111,888)
Management and other fees from affiliates									9,347	7,263	8,457
General and administrative									(40,878)	(26,684)	(24,573)
BRE merger consideration paid									(53,530)	(4,284)	0
Cost of management and other fees									(1,878)	(1,161)	(2,215)
Interest and other income									11,811	11,633	13,833
Loss on early retirement of debt									(268)	(300)	(5,009)
Gain on sale of real estate and land									46,039	1,503	0
Equity income from co-investments									39,893	55,865	41,745
Gain on remeasurement of co-investment									0	0	21,947
Income before discontinued operations									134,438	140,882	127,653
Net reportable operating segment - real estate assets	9,687,705				4,188,871				9,687,705	4,188,871
Real estate under development	434,371				50,430				434,371	50,430
Co-investments	1,036,411				677,133				1,036,411	677,133
Real estate held for sale, net	56,300				0				56,300	0
Cash and cash equivalents, including restricted cash	95,420				53,766				95,420	53,766
Marketable securities and other investments	117,240				90,084				117,240	90,084
Notes and other receivables	24,923				68,255				24,923	68,255
Other non-segment assets	110,504				58,300				110,504	58,300
Total assets	11,562,874				5,186,839				11,562,874	5,186,839
Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total property revenues									422,899	263,064	248,150
Net operating income									279,178	175,557	164,841
Net reportable operating segment - real estate assets	4,249,107				1,732,791				4,249,107	1,732,791
Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total property revenues									338,006	212,167	173,264
Net operating income									236,927	146,699	119,154
Net reportable operating segment - real estate assets	3,641,720				1,605,303				3,641,720	1,605,303
Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total property revenues									168,053	107,553	92,489
Net operating income									112,384	71,407	60,853
Net reportable operating segment - real estate assets	1,647,058				741,533				1,647,058	741,533
Other Real Estate Assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total property revenues									31,000	19,219	12,793
Net operating income									20,556	12,328	9,681
Net reportable operating segment - real estate assets	$ 149,820				$ 109,244				$ 149,820	$ 109,244