Segment Information (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting [Abstract]
Number of reportable operating segments defined by geographical regions | segment									3
Revenues from External Customers and Long-Lived Assets [Line Items]
Total property revenues	$ 342,417	$ 341,974	$ 336,766	$ 333,168	$ 326,905	$ 327,078	$ 319,562	$ 312,178	$ 1,354,325	$ 1,285,723	$ 1,185,498
Net operating income									948,783	896,796	821,990
Management and other fees from affiliates									9,574	8,278	8,909
Depreciation and amortization									(468,881)	(441,682)	(453,423)
General and administrative									(41,385)	(40,751)	(40,090)
Merger and integration expenses									0	0	(3,798)
Acquisition and investment related costs									(1,569)	(1,841)	(2,414)
Interest expense									(222,894)	(219,654)	(204,827)
Total return swap income									10,098	11,716	5,655
Interest and other income									24,604	27,305	19,143
Equity income from co-investments									86,445	48,698	21,861
Loss on early retirement of debt									(1,796)	(606)	(6,114)
Gain on sale of real estate and land									26,423	154,561	47,333
Deferred tax expense on gain on sale of real estate and land									0	(4,410)	0
Gain on remeasurement of co-investment									88,641	0	34,014
Net income									458,043	438,410	248,239
Net reportable operating segments - real estate assets	10,579,534				10,364,760				10,579,534	10,364,760
Real estate under development	355,735				190,505				355,735	190,505
Co-investments	1,155,984				1,161,275				1,155,984	1,161,275
Real estate held for sale, net	0				101,957				0	101,957
Cash and cash equivalents, including restricted cash	61,126				170,302				61,126	170,302
Marketable securities	190,004				139,189				190,004	139,189
Notes and other receivables (includes related party receivables of $41.2 million and $11.3 million as of December 31, 2017 and December 31, 2016, respectively)	100,926				40,970				100,926	40,970
Prepaid expenses and other assets	52,397				48,450				52,397	48,450
Total assets	12,495,706				12,217,408				12,495,706	12,217,408
Reportable Geographical Components [Member] | Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total property revenues									596,217	561,094	507,536
Net operating income									409,133	382,312	340,797
Net reportable operating segments - real estate assets	4,788,225				4,924,792				4,788,225	4,924,792
Reportable Geographical Components [Member] | Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total property revenues									505,313	453,140	407,590
Net operating income									363,238	325,394	291,168
Net reportable operating segments - real estate assets	4,215,449				3,791,549				4,215,449	3,791,549
Reportable Geographical Components [Member] | Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total property revenues									229,871	217,259	201,417
Net operating income									156,404	148,279	136,579
Net reportable operating segments - real estate assets	1,520,372				1,570,340				1,520,372	1,570,340
Other real estate assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total property revenues									22,924	54,230	68,955
Net operating income									20,008	40,811	$ 53,446
Net reportable operating segments - real estate assets	$ 55,488				$ 78,079				$ 55,488	$ 78,079