Segment Information (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)	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	$ 350,917	$ 344,369	$ 1,046,895	$ 1,018,835
Net operating income	250,835	245,410	753,199	732,597
Corporate-level property management expenses	(7,761)	(7,573)	(23,313)	(22,604)
Depreciation and amortization	(120,852)	(117,451)	(359,287)	(350,893)
General and administrative	(10,601)	(9,788)	(36,539)	(30,726)
Expensed acquisition and investment related costs	(31)	(324)	(156)	(1,154)
Interest expense	(55,196)	(55,938)	(166,335)	(167,333)
Total return swap income	2,184	2,538	6,682	7,653
Interest and other income	8,437	5,790	21,241	17,916
Equity income from co-investments	16,788	19,727	64,611	40,934
Gain on sale of real estate and land	0	249	22,244	26,423
Gain on remeasurement of co-investment	0	0	0	88,641
Net income	86,110	85,035	289,159	348,381
Net reportable operating segment - real estate assets	10,341,594		10,341,594		$ 10,592,776
Real estate under development	403,644		403,644		355,735
Co-investments	1,267,593		1,267,593		1,155,984
Cash and cash equivalents, including restricted cash	174,626		174,626		61,126
Marketable securities	210,596		210,596		190,004
Notes and other receivables	69,166		69,166		100,926
Prepaid expenses and other assets	50,924		50,924		39,155
Total assets	12,518,143		12,518,143		12,495,706
Rental and Other Property Revenues [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	348,610	341,974	1,040,083	1,011,908
Management and Other Fees From Affiliates Income [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	2,307	2,395	6,812	6,927
Operating Segments [Member] | Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net operating income	106,853	104,378	321,477	311,017
Net reportable operating segment - real estate assets	4,566,713		4,566,713		4,687,277
Operating Segments [Member] | Southern California [Member] | Rental and Other Property Revenues [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	151,847	148,150	452,047	438,654
Operating Segments [Member] | Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net operating income	97,306	93,212	289,081	277,635
Net reportable operating segment - real estate assets	4,229,777		4,229,777		4,220,551
Operating Segments [Member] | Northern California [Member] | Rental and Other Property Revenues [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	131,538	127,673	389,750	377,531
Operating Segments [Member] | Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net operating income	41,040	41,137	123,547	121,121
Net reportable operating segment - real estate assets	1,484,544		1,484,544		1,522,452
Operating Segments [Member] | Seattle Metro [Member] | Rental and Other Property Revenues [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	59,231	58,285	176,740	171,564
Other real estate assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net operating income	5,636	6,683	19,094	22,824
Net reportable operating segment - real estate assets	60,560		60,560		$ 162,496
Other real estate assets [Member] | Rental and Other Property Revenues [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	$ 5,994	$ 7,866	$ 21,546	$ 24,159