Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
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	$ 141,628	$ 135,070	$ 129,764	$ 125,474	$ 122,373	$ 117,226	$ 114,906	$ 111,208	$ 531,936	$ 465,713	$ 405,728
Net operating income									357,848	306,479	262,564
Depreciation									(170,592)	(151,428)	(128,221)
Interest expense before amortization									(100,244)	(91,694)	(82,756)
Amortization expense									(11,644)	(11,474)	(4,828)
Management and other fees from affiliates									11,489	6,780	4,551
General and administrative									(23,307)	(20,694)	(23,255)
Cost of management and other fees									(6,513)	(4,610)	(2,707)
Impairment and other charges									0	0	(2,302)
Interest and other income									13,833	17,139	27,841
Loss on early retirement of debt									(5,009)	(1,163)	(10)
Equity (loss) income from co-investments									41,745	(467)	(1,715)
Gain on remeasurement of co-investment									21,947	0	0
Income before discontinued operations									129,553	48,868	49,162
Net reportable operating segment - real estate assets	3,952,155				3,393,038				3,952,155	3,393,038
Real estate under development	66,851				44,280				66,851	44,280
Co-investments	571,345		21,900		383,412				571,345	383,412
Cash and cash equivalents, including restricted cash	42,126				35,463				42,126	35,463
Marketable securities	92,713				74,275				92,713	74,275
Notes and other receivables	66,163				66,369				66,163	66,369
Other non-segment assets	55,870				40,127				55,870	40,127
Total assets	4,847,223				4,036,964				4,847,223	4,036,964
Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues									249,524	223,304	200,541
Net operating income									166,162	146,519	132,150
Net reportable operating segment - real estate assets	1,675,265				1,478,018				1,675,265	1,478,018
Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues									175,325	149,457	127,302
Net operating income									120,540	99,047	82,288
Net reportable operating segment - real estate assets	1,489,095				1,241,320				1,489,095	1,241,320
Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues									94,708	81,967	70,348
Net operating income									62,076	52,173	43,006
Net reportable operating segment - real estate assets	699,465				579,612				699,465	579,612
Other Real Estate Assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues									12,379	10,985	7,537
Net operating income									9,070	8,740	5,120
Net reportable operating segment - real estate assets	$ 88,330				$ 94,088				$ 88,330	$ 94,088