Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013 Segment		Dec. 31, 2012	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	$ 155,986	$ 152,177	$ 148,783	$ 145,057	$ 140,294	$ 133,760	$ 128,465	$ 124,177	$ 602,003		$ 526,696	$ 460,660
Net operating income									405,991		354,529	303,150
Depreciation									(192,420)		(169,173)	(150,009)
Interest expense before amortization									(104,600)		(100,244)	(91,694)
Amortization expense									(11,924)		(11,644)	(11,474)
Management and other fees from affiliates									11,700		11,489	6,780
General and administrative									(25,601)		(23,307)	(20,694)
Cost of management and other fees									(6,681)		(6,513)	(4,610)
Merger expenses									(4,284)		0	0
Interest and other income									11,633		13,833	17,139
Loss on early retirement of debt									(300)		(5,009)	(1,163)
Gain on sale of land									1,503		0	0
Equity income (loss) income from co-investments									55,865		41,745	(467)
Gain on remeasurement of co-investment									0		21,947	0
Income before discontinued operations									140,882	[1]	127,653	46,958
Net reportable operating segment - real estate assets	4,188,871				3,952,155				4,188,871		3,952,155
Real estate under development	50,430				66,851				50,430		66,851
Co-investments	677,133				571,345				677,133		571,345
Cash and cash equivalents, including restricted cash	53,766				42,126				53,766		42,126
Marketable securities	90,084				92,713				90,084		92,713
Notes and other receivables	68,255				66,163				68,255		66,163
Other non-segment assets	58,300				55,870				58,300		55,870
Total assets	5,186,839				4,847,223				5,186,839		4,847,223
Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues									265,226		246,534	220,416
Net operating income									176,675		164,092	145,353
Net reportable operating segment - real estate assets	1,746,434				1,675,265				1,746,434		1,675,265
Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues									214,402		175,325	149,457
Net operating income									148,204		120,540	99,047
Net reportable operating segment - real estate assets	1,614,159				1,489,095				1,614,159		1,489,095
Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues									107,553		92,489	79,832
Net operating income									71,407		60,853	51,477
Net reportable operating segment - real estate assets	741,533				699,465				741,533		699,465
Other Real Estate Assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues									14,822		12,348	10,955
Net operating income									9,705		9,044	7,273
Net reportable operating segment - real estate assets	$ 86,745				$ 88,330				$ 86,745		$ 88,330

[1] The land is leased pursuant to a ground lease expiring 2027.