Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
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	$ 129,765	$ 114,906	$ 255,238	$ 226,114
Net operating income	87,348	75,135	172,656	147,919
Management and other fees from affiliates	2,796	1,420	5,240	2,645
Depreciation	(41,801)	(37,250)	(82,535)	(73,908)
General and administrative	(5,764)	(5,385)	(11,164)	(10,659)
Cost of management and other fees	(1,611)	(1,073)	(3,251)	(1,997)
Interest expense before amortization	(24,659)	(22,710)	(49,316)	(44,518)
Amortization expense	(2,882)	(2,736)	(5,754)	(5,590)
Interest and other income	5,455	2,628	7,868	9,616
Equity income (loss) from co-investments	3,111	726	5,451	(647)
Gain on remeasurement of co-investment	21,947	0	21,947	0
Loss on early retirement of debts	(1,450)	(253)	(1,450)	(253)
Income from continuing operations	42,490	10,502	59,692	22,608
Net reportable operating segment- real estate assets	3,609,262		3,609,262		3,393,038
Real estate for development	55,343		55,343		44,280
Co-investments	436,230		436,230		383,412
Cash and cash equivalents	27,810		27,810		35,463
Marketable securities	114,166		114,166		74,275
Notes and other receivables	50,895		50,895		66,369
Other non-segment assets	45,410		45,410		40,127
Total assets	4,339,116		4,339,116		4,036,964
Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	61,356	55,330	119,916	109,193
Net operating income	41,107	36,173	80,811	71,559
Net reportable operating segment- real estate assets	1,682,453		1,682,453		1,478,018
Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	42,570	36,550	84,192	71,867
Net operating income	29,225	24,099	58,168	47,438
Net reportable operating segment- real estate assets	1,247,579		1,247,579		1,241,320
Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	22,668	20,210	44,977	39,966
Net operating income	14,841	12,774	29,673	25,331
Net reportable operating segment- real estate assets	589,653		589,653		579,612
Other Real Estate Assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	3,171	2,816	6,153	5,088
Net operating income	2,175	2,089	3,878	3,591
Net reportable operating segment- real estate assets	$ 89,577		$ 89,577		$ 94,088