Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 Segment	Jun. 30, 2013	Dec. 31, 2013
Segment Information [Abstract]
Number of reportable operating segments defined by geographical regions			3
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	$ 256,614	$ 148,781	$ 415,631	$ 293,839
Net operating income	173,394	100,485	280,439	199,216
Management and other fees	2,836	2,034	4,495	4,041
Depreciation	(97,510)	(47,673)	(147,822)	(94,459)
General and administrative	(9,558)	(6,589)	(17,142)	(13,588)
Merger expenses	(26,497)	0	(42,556)
Acquisition and disposition costs	(529)	(168)	(1,504)	(555)
Interest expense	(42,151)	(29,327)	(71,192)	(57,468)
Interest and other income	2,814	1,917	5,693	6,939
Equity income from co-investments	5,629	7,282	16,155	11,493
Gain on early retirement of debt	0	1,024	0	1,024
Gains on sale of real estate and land	0	0	8,268	1,503
Income from continuing operations	8,428	28,985	34,834	58,146
Net reportable operating segment - real estate assets	9,487,587		9,487,587		4,188,871
Real estate under development	422,372		422,372		50,430
Co-investments	964,970		964,970		677,133
Real Estate Held-for-sale	107,772		107,772		0
Cash and cash equivalents, including restricted cash	47,538		47,538		53,766
Marketable securities	106,358		106,358		90,084
Notes and other receivables	66,522		66,522		68,255
Other non-segment assets	145,244		145,244		58,300
Total assets	11,348,363		11,348,363		5,186,839
Other Real Estate Assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	9,191	4,109	12,935	7,906
Net operating income	7,477	2,917	8,056	5,659
Net reportable operating segment - real estate assets	153,677		153,677		86,745
Reportable Geographical Components [Member] | Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	114,843	65,103	184,530	129,843
Net operating income	75,330	43,696	122,397	87,405
Net reportable operating segment - real estate assets	4,348,186		4,348,186		1,746,434
Reportable Geographical Components [Member] | Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	88,198	53,089	145,143	103,859
Net operating income	61,223	36,462	101,577	71,746
Net reportable operating segment - real estate assets	3,403,553		3,403,553		1,614,159
Reportable Geographical Components [Member] | Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	44,382	26,480	73,023	52,231
Net operating income	29,364	17,410	48,409	34,406
Net reportable operating segment - real estate assets	$ 1,582,171		$ 1,582,171		$ 741,533