Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Segment	Sep. 30, 2012	Aug. 31, 2013	Dec. 31, 2012
Segment Information [Abstract]
Number of reportable operating segments defined by geographical regions			3
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	$ 152,945	$ 134,518	$ 448,318	$ 388,642
Net operating income	102,382	89,445	302,676	261,488
Management and other fees	2,957	3,072	9,139	8,312
Depreciation	(48,438)	(42,897)	(143,320)	(125,137)
General and administrative	(6,075)	(5,276)	(18,925)	(16,440)
Cost of management and other fees	(1,613)	(1,642)	(5,047)	(4,893)
Interest expense before amortization	(26,187)	(25,064)	(77,724)	(74,380)
Amortization expense	(3,005)	(2,927)	(8,937)	(8,681)
Interest and other income	2,387	3,003	9,326	10,869
Equity income from co-investments	40,802	3,547	52,295	8,998
Gain (loss) on early retirement of debt	(178)	(1,211)	846	(2,661)
Gain on sale of land	0	0	1,503	0
Gain on remeasurement of co-investment	0	0	0	21,947
Income from continuing operations	63,032	20,050	121,832	79,422
Net reportable operating segment - real estate assets	4,080,820		4,080,820			3,952,155
Real estate under development	45,804		45,804			66,851
Co-investments	674,075		674,075		8,500	571,345
Cash and cash equivalents, including restricted cash	55,994		55,994			42,126
Marketable securities	89,899		89,899			92,713
Notes and other receivables	67,628		67,628			66,163
Other non-segment assets	71,382		71,382			55,870
Total assets	5,085,602		5,085,602			4,847,223
Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	67,114	64,339	199,177	184,255
Net operating income	44,084	41,932	132,983	122,868
Net reportable operating segment - real estate assets	1,650,435		1,650,435			1,675,265
Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	54,755	43,605	158,614	127,797
Net operating income	37,822	29,572	109,567	87,740
Net reportable operating segment - real estate assets	1,620,434		1,620,434			1,489,095
Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	27,212	23,559	79,443	67,436
Net operating income	18,047	15,411	52,453	44,481
Net reportable operating segment - real estate assets	723,175		723,175			699,465
Other Real Estate Assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	3,864	3,015	11,084	9,154
Net operating income	2,429	2,530	7,673	6,399
Net reportable operating segment - real estate assets	$ 86,776		$ 86,776			$ 88,330