Segment Information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2016 USD ($) segment	Mar. 31, 2015 USD ($)	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]
Property revenues	$ 312,178	$ 280,229
Net operating income	217,688	193,058
Depreciation and amortization	(109,707)	(106,907)
Interest expense	(52,466)	(47,546)
Total return swap income	3,123	0
Management and other fees from affiliates	2,024	2,644
General and administrative	(9,182)	(10,545)
Merger and integration expenses	0	(2,388)
Acquisition and investment related costs	(828)	(547)
Interest and other income	5,208	4,199
Gain on sale of real estate and land	20,258	7,112
Deferred tax expense on gain on sale of real estate and land	(4,279)	0
Equity income in co-investments	15,068	4,311
Gain on remeasurement of co-investment	0	21,362
Net income	86,907	64,753
Net reportable operating segment - real estate assets	10,574,875		$ 10,381,577
Real estate under development	145,711		242,326
Co-investments	1,069,684		1,036,047
Real estate held for sale, net	0		26,879
Cash and cash equivalents, including restricted cash	80,483		123,055
Marketable securities	138,597		137,485
Notes and other receivables	18,198		19,285
Other non-segment assets	39,936		38,437
Total assets	12,067,484		12,005,091
Reportable Geographical Components [Member] | Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	141,492	123,455
Net operating income	96,453	83,088
Net reportable operating segment - real estate assets	4,875,525		4,912,264
Reportable Geographical Components [Member] | Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	112,733	102,427
Net operating income	80,711	72,516
Net reportable operating segment - real estate assets	3,892,121		3,749,072
Reportable Geographical Components [Member] | Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	52,074	48,654
Net operating income	35,689	33,129
Net reportable operating segment - real estate assets	1,698,550		1,613,175
Other Real Estate Assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	5,879	5,693
Net operating income	4,835	$ 4,325
Net reportable operating segment - real estate assets	$ 108,679		$ 107,066