Segment Information (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) segment	Sep. 30, 2015 USD ($)	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	$ 327,078,000	$ 302,522,000	$ 958,818,000	$ 876,852,000
Net operating income	227,717,000	208,403,000	668,888,000	605,485,000
Management and other fees from affiliates	2,093,000	2,104,000	6,145,000	6,809,000
Depreciation and amortization	(110,467,000)	(116,308,000)	(329,847,000)	(336,946,000)
General and administrative	(9,647,000)	(11,129,000)	(28,527,000)	(31,223,000)
Merger and integration expenses	0	0	0	(3,798,000)
Acquisition and investment related costs	(284,000)	(381,000)	(1,379,000)	(1,357,000)
Interest expense	(56,693,000)	(50,053,000)	(164,727,000)	(148,401,000)
Total return swap income	3,143,000	0	9,080,000	0
Interest and other income	4,943,000	7,367,000	19,560,000	14,820,000
Equity income in co-investments	9,568,000	7,179,000	38,932,000	15,962,000
Loss on early retirement of debt	(211,000)	0	(211,000)	0
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
Gain on remeasurement of co-investment	0	0	0	34,014,000
Net income	70,162,000	47,182,000	233,893,000	162,477,000
Net reportable operating segment - real estate assets	10,443,912,000		10,443,912,000		$ 10,381,577,000
Real estate under development	170,972,000		170,972,000		242,326,000
Co-investments	1,122,913,000		1,122,913,000		1,036,047,000
Real estate held for sale, net	0		0		26,879,000
Cash and cash equivalents, including restricted cash	211,055,000		211,055,000		123,055,000
Marketable securities	153,703,000		153,703,000		137,485,000
Notes and other receivables	22,941,000		22,941,000		19,285,000
Prepaid expenses and other assets	51,700,000		51,700,000		38,437,000
Total assets	12,177,196,000		12,177,196,000		12,005,091,000
Reportable Geographical Components [Member] | Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	147,953,000	135,900,000	434,217,000	390,381,000
Net operating income	100,447,000	90,377,000	295,834,000	261,320,000
Net reportable operating segment - real estate assets	4,830,293,000		4,830,293,000		4,912,264,000
Reportable Geographical Components [Member] | Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	117,699,000	106,428,000	345,793,000	308,118,000
Net operating income	84,790,000	76,327,000	249,011,000	219,906,000
Net reportable operating segment - real estate assets	3,844,421,000		3,844,421,000		3,749,072,000
Reportable Geographical Components [Member] | Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	55,544,000	51,181,000	161,192,000	149,661,000
Net operating income	37,688,000	34,484,000	109,352,000	101,288,000
Net reportable operating segment - real estate assets	1,679,303,000		1,679,303,000		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	5,882,000	9,013,000	17,616,000	28,692,000
Net operating income	4,792,000	$ 7,215,000	14,691,000	$ 22,971,000
Net reportable operating segment - real estate assets	$ 89,895,000		$ 89,895,000		$ 107,066,000