Segment Information (Details) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Segment Reporting Information [Line Items]
Revenues	$ 115,778	$ 99,163	$ 227,866	$ 198,436
Net operating income	75,508	64,346	148,702	129,602
Depreciation	(37,510)	(31,156)	(74,426)	(61,539)
Interest expense before amortization expense	(22,710)	(20,161)	(44,518)	(39,758)
Amortization expense	(2,736)	(843)	(5,590)	(2,083)
Interest and other income	2,628	7,085	9,616	14,941
General and administrative	(6,371)	(6,219)	(12,486)	(11,837)
Management and other fees from affiliates	1,420	1,022	2,645	2,500
Equity income (loss) from co-investments	726	(360)	(647)	(401)
Loss on early retirement of debt	(253)	(10)	(253)	(10)
Income before discontinued operations	10,702	13,704	23,043	31,415
Real Estate Assets	3,237,158		3,237,158		3,189,008
Real estate under development	191,620		191,620		217,531
Cash and cash equivalents	32,847		32,847		35,694
Marketable securities	75,673		75,673		92,310
Co-investments	197,364		197,364		107,840
Notes and other receivables	49,956		49,956		49,444
Other non-segment assets	41,237		41,237		41,060
Total assets	3,825,855		3,825,855		3,732,887
Southern California [Member]
Segment Reporting Information [Line Items]
Revenues	56,081	50,503	110,704	100,996
Net operating income	36,590	33,688	72,425	67,449
Real Estate Assets	1,445,006		1,445,006		1,428,264
Northern California [Member]
Segment Reporting Information [Line Items]
Revenues	36,548	29,973	71,865	59,961
Net operating income	24,097	19,243	47,436	39,043
Real Estate Assets	1,140,907		1,140,907		1,119,555
Seattle Metro [Member]
Segment Reporting Information [Line Items]
Revenues	20,210	16,717	39,966	33,445
Net operating income	12,774	10,124	25,331	20,698
Real Estate Assets	552,372		552,372		560,463
Other Real Estate Assets [Member]
Segment Reporting Information [Line Items]
Revenues	2,939	1,970	5,331	4,034
Net operating income	2,047	1,291	3,510	2,412
Real Estate Assets	$ 98,873		$ 98,873		$ 80,726