Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 Segment	Mar. 31, 2012	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	$ 146,394	$ 125,474
Net operating income	99,630	85,310
Management and other fees from affiliates	2,948	2,444
Depreciation	(47,144)	(40,734)
General and administrative	(6,626)	(5,400)
Cost of management and other fees	(1,701)	(1,640)
Interest expense before amortization	(25,211)	(24,658)
Amortization expense	(2,930)	(2,871)
Interest and other income	5,023	2,413
Equity income from co-investments	4,211	2,340
Gain on sale of land	1,503	0
Income from continuing operations	29,703	17,204
Net reportable operating segment - real estate assets	4,094,699		3,952,155
Real estate under development	65,568		66,851
Co-investments	579,654		571,345
Cash and cash equivalents, including restricted cash	55,634		42,126
Marketable securities	80,432		92,713
Notes and other receivables	11,359		66,163
Other non-segment assets	49,113		55,870
Total assets	4,936,459		4,847,223
Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	65,830	58,560
Net operating income	44,434	39,830
Net reportable operating segment - real estate assets	1,662,493		1,675,265
Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	50,770	41,622
Net operating income	35,283	28,942
Net reportable operating segment - real estate assets	1,632,994		1,489,095
Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	26,328	22,309
Net operating income	17,373	14,831
Net reportable operating segment - real estate assets	711,458		699,465
Other Real Estate Assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	3,466	2,983
Net operating income	2,540	1,707
Net reportable operating segment - real estate assets	$ 87,754		$ 88,330