Segment Information (Details) $ in Thousands	1 Months Ended		3 Months Ended
	Mar. 31, 2019 USD ($)	Jan. 31, 2019 USD ($)	Mar. 31, 2019 USD ($) segment	Mar. 31, 2018 USD ($)	Dec. 31, 2018 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			$ 356,223	$ 347,255
Net operating income			255,572	249,984
Corporate-level property management expenses			(8,153)	(7,770)
Depreciation and amortization			(120,568)	(119,105)
General and administrative			(13,459)	(14,813)
Expensed acquisition and investment related costs			(32)	(57)
Interest expense			(53,643)	(54,861)
Total return swap income			2,045	2,270
Interest and other income			12,261	5,909
Equity income from co-investments			16,276	32,774
Gain on early retirement of debt, net		$ 1,400	1,336	0
Gain on remeasurement of co-investment	$ 31,500		31,535	0
Net income			125,505	96,639
Net reportable operating segment - real estate assets	10,242,457		10,242,457		$ 10,156,553
Real estate under development	497,794		497,794		454,629
Co-investments	1,307,561		1,307,561		1,300,140
Cash and cash equivalents, including restricted cash	124,126		124,126		151,395
Marketable securities	211,030		211,030		209,545
Notes and other receivables	71,154		71,154		71,895
Operating lease right-of-use assets	76,996		76,996		0
Prepaid expenses and other assets	46,883		46,883		39,439
Total assets	12,578,001		12,578,001		12,383,596
Rental and Other Property Revenues [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues			353,888	344,947
Management and Other Fees From Affiliates Income [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues			2,335	2,308
Operating Segments [Member] | Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net operating income			107,429	104,363
Net reportable operating segment - real estate assets	4,309,022		4,309,022		4,350,377
Operating Segments [Member] | Southern California [Member] | Rental and Other Property Revenues [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues			151,024	146,558
Operating Segments [Member] | Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net operating income			100,705	94,598
Net reportable operating segment - real estate assets	4,411,355		4,411,355		4,270,238
Operating Segments [Member] | Northern California [Member] | Rental and Other Property Revenues [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues			136,325	128,622
Operating Segments [Member] | Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net operating income			41,467	41,981
Net reportable operating segment - real estate assets	1,461,068		1,461,068		1,472,916
Operating Segments [Member] | Seattle Metro [Member] | Rental and Other Property Revenues [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues			60,240	58,713
Other real estate assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net operating income			5,971	9,042
Net reportable operating segment - real estate assets	$ 61,012		61,012		$ 63,022
Other real estate assets [Member] | Rental and Other Property Revenues [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues			$ 6,299	$ 11,054