Segment Information (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) segment	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting [Abstract]
Number of reportable operating segments defined by geographical regions (in segments) | segment			3
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	$ 348,723	$ 339,062	$ 695,978	$ 674,466
Net operating income	252,380	246,023	502,364	487,187
Corporate-level property management expenses	(7,782)	(7,522)	(15,552)	(15,031)
Depreciation and amortization	(119,330)	(117,939)	(238,435)	(233,442)
General and administrative	(11,125)	(10,337)	(25,938)	(20,938)
Expensed acquisition and investment related costs	(68)	(274)	(125)	(830)
Interest expense	(56,278)	(56,812)	(111,139)	(111,395)
Total return swap income	2,228	2,531	4,498	5,115
Interest and other income	6,895	5,362	12,804	12,126
Equity income from co-investments	15,049	10,308	47,823	21,207
Gain on sale of real estate and land	22,244	0	22,244	26,174
Gain on remeasurement of co-investment	0	2,159	0	88,641
Net income	106,410	75,795	203,049	263,346
Net reportable operating segment - real estate assets	10,411,002		10,411,002		$ 10,588,875
Real estate under development	343,841		343,841		355,735
Co-investments	1,291,320		1,291,320		1,155,984
Cash and cash equivalents, including restricted cash	177,576		177,576		61,126
Marketable securities	207,191		207,191		190,004
Notes and other receivables (includes related party receivables of $10.8 million and $41.2 million as of June 30, 2018 and December 31, 2017, respectively)	68,386		68,386		100,926
Prepaid expenses and other assets	56,338		56,338		43,056
Total assets	12,555,654		12,555,654		12,495,706
Operating Segments [Member] | Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net operating income	108,158	103,522	214,624	206,639
Net reportable operating segment - real estate assets	4,601,755		4,601,755		4,685,358
Operating Segments [Member] | Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net operating income	97,058	94,680	191,775	184,423
Net reportable operating segment - real estate assets	4,253,397		4,253,397		4,219,114
Operating Segments [Member] | Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net operating income	40,496	39,596	82,507	79,984
Net reportable operating segment - real estate assets	1,495,388		1,495,388		1,521,907
Other real estate assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net operating income	6,668	8,225	13,458	16,141
Net reportable operating segment - real estate assets	60,462		60,462		$ 162,496
Rental and Other Property Revenues [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	346,526	336,766	691,473	669,934
Rental and Other Property Revenues [Member] | Operating Segments [Member] | Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	150,431	145,600	300,200	290,504
Rental and Other Property Revenues [Member] | Operating Segments [Member] | Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	129,590	126,550	258,212	249,858
Rental and Other Property Revenues [Member] | Operating Segments [Member] | Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	58,795	57,087	117,509	113,279
Rental and Other Property Revenues [Member] | Other real estate assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	7,710	7,529	15,552	16,293
Management and Other Fees From Affiliates Income [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	$ 2,197	$ 2,296	$ 4,505	$ 4,532