Segment Information (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) segment	Jun. 30, 2015 USD ($)	Dec. 31, 2015 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	$ 319,562,000	$ 294,101,000	$ 631,740,000	$ 574,330,000
Net operating income	223,483,000	204,024,000	441,171,000	397,082,000
Depreciation and amortization	(109,673,000)	(113,731,000)	(219,380,000)	(220,638,000)
Interest expense	(55,568,000)	(50,802,000)	(108,034,000)	(98,348,000)
Total return swap income	2,814,000	0	5,937,000	0
Management and other fees from affiliates	2,028,000	2,061,000	4,052,000	4,705,000
General and administrative	(9,698,000)	(9,549,000)	(18,880,000)	(20,094,000)
Merger and integration expenses	0	(1,410,000)	0	(3,798,000)
Acquisition and investment related costs	(267,000)	(429,000)	(1,095,000)	(976,000)
Interest and other income	9,409,000	3,254,000	14,617,000	7,453,000
Gain on sale of real estate and land	0	0	20,258,000	7,112,000
Deferred tax expense on gain on sale of real estate and land	0	0	(4,279,000)	0
Equity income in co-investments	14,296,000	4,472,000	29,364,000	8,783,000
Gain on remeasurement of co-investment	0	12,652,000	0	34,014,000
Net income	76,824,000	50,542,000	163,731,000	115,295,000
Net reportable operating segment - real estate assets	10,510,567,000		10,510,567,000		$ 10,381,577,000
Real estate under development	157,659,000		157,659,000		242,326,000
Co-investments	1,103,272,000		1,103,272,000		1,036,047,000
Real estate held for sale, net	0		0		26,879,000
Cash and cash equivalents, including restricted cash	215,376,000		215,376,000		123,055,000
Marketable securities	152,263,000		152,263,000		137,485,000
Notes and other receivables	20,448,000		20,448,000		19,285,000
Other non-segment assets	47,106,000		47,106,000		38,437,000
Total assets	12,206,691,000		12,206,691,000		12,005,091,000
Reportable Geographical Components [Member] | Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	144,772,000	131,026,000	286,264,000	254,481,000
Net operating income	98,934,000	87,810,000	195,387,000	173,340,000
Net reportable operating segment - real estate assets	4,847,701,000		4,847,701,000		4,912,264,000
Reportable Geographical Components [Member] | Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	115,361,000	103,041,000	228,094,000	201,692,000
Net operating income	83,510,000	73,682,000	164,221,000	143,579,000
Net reportable operating segment - real estate assets	3,866,145,000		3,866,145,000		3,749,072,000
Reportable Geographical Components [Member] | Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	53,575,000	49,826,000	105,649,000	98,480,000
Net operating income	35,975,000	33,674,000	71,664,000	66,803,000
Net reportable operating segment - real estate assets	1,686,923,000		1,686,923,000		1,613,175,000
Segment Reconciling Items [Member] | Other Real Estate Assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	5,854,000	10,208,000	11,733,000	19,677,000
Net operating income	5,064,000	$ 8,858,000	9,899,000	$ 13,360,000
Net reportable operating segment - real estate assets	$ 109,798,000		$ 109,798,000		$ 107,066,000