Segment Information (Details) $ in Thousands	1 Months Ended		3 Months Ended		6 Months Ended
	Mar. 31, 2019 USD ($)	Jan. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) segment	Jun. 30, 2018 USD ($)	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			$ 361,635	$ 348,723	$ 717,858	$ 695,978
Net operating income			263,928	252,380	519,500	502,364
Corporate-level property management expenses			(8,212)	(7,782)	(16,365)	(15,552)
Depreciation and amortization			(119,465)	(119,330)	(240,033)	(238,435)
General and administrative			(13,927)	(11,125)	(27,386)	(25,938)
Expensed acquisition and investment related costs			(24)	(68)	(56)	(125)
Gain on sale of real estate and land			0	22,244	0	22,244
Interest expense			(54,112)	(56,278)	(107,755)	(111,139)
Total return swap income			1,975	2,228	4,020	4,498
Interest and other income			8,347	6,895	20,608	12,804
Equity income from co-investments			16,959	15,049	33,235	47,823
Gain on early retirement of debt, net		$ 1,400	332	0	1,668	0
Gain on remeasurement of co-investment	$ 31,500		0	0	31,535	0
Net income			98,061	106,410	223,566	203,049
Net reportable operating segment - real estate assets			10,338,298		10,338,298		$ 10,156,553
Real estate under development			542,207		542,207		454,629
Co-investments			1,324,081		1,324,081		1,300,140
Cash and cash equivalents, including restricted cash			54,971		54,971		151,395
Marketable securities			215,434		215,434		209,545
Notes and other receivables			74,416		74,416		71,895
Operating lease right-of-use assets			76,205		76,205		0
Prepaid expenses and other assets			45,533		45,533		39,439
Total assets			12,671,145		12,671,145		12,383,596
Rental and Other Property Revenues [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues			359,375	346,526	713,263	691,473
Management and Other Fees From Affiliates Income [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues			2,260	2,197	4,595	4,505
Operating Segments [Member] | Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net operating income			108,693	105,882	216,122	210,245
Net reportable operating segment - real estate assets			4,275,854		4,275,854		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			151,966	147,249	302,990	293,807
Operating Segments [Member] | Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net operating income			104,321	96,380	205,026	190,978
Net reportable operating segment - real estate assets			4,549,598		4,549,598		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			140,095	129,590	276,420	258,212
Operating Segments [Member] | Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net operating income			43,845	40,452	85,312	82,433
Net reportable operating segment - real estate assets			1,451,764		1,451,764		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			60,841	58,796	121,081	117,509
Other real estate assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net operating income			7,069	9,666	13,040	18,708
Net reportable operating segment - real estate assets			61,082		61,082		$ 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			$ 6,473	$ 10,891	$ 12,772	$ 21,945