Segment Information (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) segment	Sep. 30, 2014 USD ($)	Aug. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Segment Reporting [Abstract]
Number of reportable operating segments defined by geographical regions | segment			3
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	$ 302,522	$ 268,512	$ 876,852	$ 684,813
Net operating income	208,403	180,450	605,485	460,887
Management and other fees from affiliates	2,104	2,361	6,809	6,856
Depreciation and amortization	(116,308)	(102,286)	(336,946)	(253,890)
General and administrative	(11,129)	(11,479)	(31,223)	(28,621)
Merger and integration expenses	0	(3,857)	(3,798)	(46,413)
Acquisition and investment related costs	(381)	(51)	(1,357)	(768)
Interest expense	(50,053)	(45,830)	(148,401)	(117,021)
Interest and other income	7,367	2,992	14,820	8,685
Equity income from co-investments	7,179	4,910	15,962	21,065
Gains on sale of real estate and land	0	31,372	7,112	38,853
Gain on remeasurement of co-investment	0	0	34,014	0
Net income	47,182	58,582	162,477	89,633
Net reportable operating segment - real estate assets	10,462,434		10,462,434			$ 9,679,875
Real estate under development	226,690		226,690			429,096
Co-investments	1,036,043		1,036,043		$ 20,400	1,042,423
Real estate held for sale, net	8,742		8,742			56,300
Cash and cash equivalents, including restricted cash	72,306		72,306			95,749
Marketable securities and other investments	133,058		133,058			117,240
Notes and other receivables	22,668		22,668			24,923
Other non-segment assets	50,776		50,776			81,126
Total assets	12,012,717		12,012,717			11,526,732
Reportable Geographical Components [Member] | Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	135,900	122,089	394,375	311,873
Net operating income	90,377	78,860	263,715	204,177
Net reportable operating segment - real estate assets	4,813,844		4,813,844			4,241,277
Reportable Geographical Components [Member] | Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	110,483	94,807	319,834	239,018
Net operating income	79,223	66,313	228,190	167,034
Net reportable operating segment - real estate assets	3,888,700		3,888,700			3,641,720
Reportable Geographical Components [Member] | Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	51,181	47,132	149,662	120,281
Net operating income	34,484	31,751	101,288	80,198
Net reportable operating segment - real estate assets	1,616,932		1,616,932			1,647,058
Other Real Estate Assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	4,958	4,484	12,981	13,641
Net operating income	4,319	$ 3,526	12,292	$ 9,478
Net reportable operating segment - real estate assets	$ 142,958		$ 142,958			$ 149,820