Revenues - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenues	$ 361,635	$ 348,723	$ 717,858	$ 695,978
Occupancy threshold for classification as stabilized	95.00%		95.00%
Deferred revenue	$ 4,800		$ 4,800		$ 6,200
Deferred revenue, revenue recognized			1,400
Deferred revenue balance from contracts with remaining performance obligations	4,800		4,800
Rental [Member]
Disaggregation of Revenue [Line Items]
Total revenues	353,167	340,481	700,972	679,496
Other Property [Member]
Disaggregation of Revenue [Line Items]
Total revenues	6,208	6,045	12,291	11,977
Management and Other Fees From Affiliates Income [Member]
Disaggregation of Revenue [Line Items]
Total revenues	2,260	2,197	4,595	4,505
Rental and Other Property Revenues [Member]
Disaggregation of Revenue [Line Items]
Total revenues	359,375	346,526	713,263	691,473
Rental and Other Property Revenues [Member] | Same Property [Member]
Disaggregation of Revenue [Line Items]
Total revenues	340,042	328,602	677,394	655,869
Rental and Other Property Revenues [Member] | Acquisitions [Member]
Disaggregation of Revenue [Line Items]
Total revenues	4,836	0	7,107	0
Rental and Other Property Revenues [Member] | Development [Member]
Disaggregation of Revenue [Line Items]
Total revenues	1,192	450	2,325	469
Rental and Other Property Revenues [Member] | Redevelopment [Member]
Disaggregation of Revenue [Line Items]
Total revenues	5,223	5,036	10,435	10,060
Rental and Other Property Revenues [Member] | Non-Residential/Other, Net [Member]
Disaggregation of Revenue [Line Items]
Total revenues	8,082	12,438	16,002	25,075
Rental and Other Property Revenues [Member] | Operating Segments [Member] | Southern California [Member]
Disaggregation of Revenue [Line Items]
Total revenues	151,966	147,249	302,990	293,807
Rental and Other Property Revenues [Member] | Operating Segments [Member] | Northern California [Member]
Disaggregation of Revenue [Line Items]
Total revenues	140,095	129,590	276,420	258,212
Rental and Other Property Revenues [Member] | Operating Segments [Member] | Seattle Metro [Member]
Disaggregation of Revenue [Line Items]
Total revenues	60,841	58,796	121,081	117,509
Rental and Other Property Revenues [Member] | Other real estate assets [Member]
Disaggregation of Revenue [Line Items]
Total revenues	$ 6,473	$ 10,891	$ 12,772	$ 21,945