Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	$ 135,070	$ 117,226	$ 390,308	$ 343,340
Net operating income	89,761	76,376	262,417	224,295
Management and other fees from affiliates	3,072	1,940	8,312	4,585
Depreciation	(43,041)	(37,881)	(125,575)	(111,786)
General and administrative	(5,276)	(4,966)	(16,440)	(15,626)
Cost of management and other fees	(1,642)	(1,164)	(4,893)	(3,161)
Interest expense before amortization	(25,064)	(22,096)	(74,380)	(66,612)
Amortization expense	(2,927)	(2,936)	(8,681)	(8,527)
Interest and other income	3,003	2,741	10,869	12,357
Equity income (loss) from co-investments	3,547	317	8,998	(330)
Gain on remeasurement of co-investment	0	0	21,947	0
Loss on early retirement of debts	(1,211)	(567)	(2,661)	(820)
Income from continuing operations	20,222	11,764	79,913	34,375
Net reportable operating segment- real estate assets	3,720,001		3,720,001		3,393,038
Real estate for development	60,020		60,020		44,280
Cash and cash equivalents	26,877		26,877		35,463
Marketable securities	141,573		141,573		74,275
Co-investments	477,967		477,967		383,412
Notes and other receivables	50,316		50,316		66,369
Other non-segment assets	57,918		57,918		40,127
Total assets	4,534,672		4,534,672		4,036,964
Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	64,339	56,169	184,255	165,362
Net operating income	41,932	36,239	122,869	107,797
Net reportable operating segment- real estate assets	1,675,708		1,675,708		1,478,018
Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	43,605	37,757	127,797	109,624
Net operating income	29,572	24,689	87,740	72,128
Net reportable operating segment- real estate assets	1,274,901		1,274,901		1,241,320
Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	24,103	20,530	69,080	60,496
Net operating income	15,718	12,964	45,391	38,295
Net reportable operating segment- real estate assets	680,562		680,562		579,612
Other Real Estate Assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	3,023	2,770	9,176	7,858
Net operating income	2,539	2,484	6,417	6,075
Net reportable operating segment- real estate assets	$ 88,830		$ 88,830		$ 94,088