Segment Information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) segment	Mar. 31, 2017 USD ($)	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]
Property revenues	$ 344,947	$ 333,168
Net operating income	249,984	241,164
Management and other fees from affiliates	2,308	2,236
Corporate-level property management expenses	(7,770)	(7,509)
Depreciation and amortization	(119,105)	(115,503)
General and administrative	(14,813)	(10,601)
Expensed acquisition and investment related costs	(57)	(556)
Interest expense	(54,861)	(54,583)
Total return swap income	2,270	2,584
Interest and other income	5,909	6,764
Equity income from co-investments	32,774	10,899
Gain on sale of real estate and land	0	26,174
Gain on remeasurement of co-investment	0	86,482
Net income	96,639	187,551
Net reportable operating segment - real estate assets	10,488,605		$ 10,579,534
Real estate under development	395,710		355,735
Co-investments	1,290,957		1,155,984
Cash and cash equivalents, including restricted cash	139,078		61,126
Marketable securities	197,745		190,004
Notes and other receivables (includes related party receivables of $9.9 million and $41.2 million as of March 31, 2018 and December 31, 2017, respectively)	70,525		100,926
Prepaid expenses and other assets	60,047		52,397
Total assets	12,642,667		12,495,706
Operating Segments [Member] | Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	152,060	147,099
Net operating income	107,932	104,501
Net reportable operating segment - real estate assets	4,744,730		4,788,225
Operating Segments [Member] | Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	128,622	123,308
Net operating income	94,717	89,743
Net reportable operating segment - real estate assets	4,182,261		4,215,449
Operating Segments [Member] | Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	58,713	56,192
Net operating income	42,012	40,388
Net reportable operating segment - real estate assets	1,506,405		1,520,372
Other real estate assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	5,552	6,569
Net operating income	5,323	$ 6,532
Net reportable operating segment - real estate assets	$ 55,209		$ 55,488