Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014 Segment	Mar. 31, 2013	Dec. 31, 2013
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	$ 159,017	$ 145,057
Net operating income	107,044	98,731
Management and other fees	2,628	2,948
Depreciation	(50,312)	(46,787)
General and administrative	(7,075)	(6,239)
Merger expenses	(16,059)	0
Cost of management and other fees	(1,477)	(1,701)
Interest expense before amortization	(26,055)	(25,211)
Amortization expense	(2,986)	(2,930)
Interest and other income	2,879	5,023
Equity income from co-investments	10,526	4,211
Gains on sale of real estate and land	8,268	1,503
Income from continuing operations	27,381	29,548
Net reportable operating segment - real estate assets	5,339,988		4,188,871
Real estate under development	308,266		50,430
Co-investments	716,443		677,133
Cash and cash equivalents, including restricted cash	48,671		53,766
Marketable securities	100,348		90,084
Notes and other receivables	36,105		68,255
Other non-segment assets	83,221		58,300
Total assets	6,633,042		5,186,839
Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	69,687	64,740
Net operating income	47,066	43,708
Net reportable operating segment - real estate assets	2,551,297		1,746,434
Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	56,944	50,770
Net operating income	40,353	35,283
Net reportable operating segment - real estate assets	1,967,133		1,614,159
Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	28,641	25,751
Net operating income	19,045	16,995
Net reportable operating segment - real estate assets	734,972		741,533
Other Real Estate Assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	3,745	3,796
Net operating income	580	2,745
Net reportable operating segment - real estate assets	$ 86,586		$ 86,745