Segment Information (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) segment	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Segment Reporting [Abstract]
Number of reportable operating segments defined by geographical regions | segment			3
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	$ 294,101	$ 256,952	$ 574,330	$ 416,302
Net operating income	204,024	173,394	397,082	280,438
Management and other fees from affiliates	2,061	2,836	4,705	4,495
Depreciation and amortization	(113,731)	(101,292)	(220,638)	(151,604)
General and administrative	(9,549)	(9,558)	(20,094)	(17,141)
Merger and integration expenses	(1,410)	(26,497)	(3,798)	(42,556)
Acquisition costs	(429)	(529)	(976)	(717)
Interest expense	(50,802)	(42,151)	(98,348)	(71,192)
Interest and other income	3,254	2,814	7,453	5,693
Equity income from co-investments	4,472	5,629	8,783	16,155
Gains on sale of real estate and land	0	0	7,112	7,481
Gain on remeasurement of co-investment	12,652	0	34,014	0
Net income	50,542	4,646	115,295	31,052
Net reportable operating segment - real estate assets	10,510,902		10,510,902		$ 9,679,875
Real estate under development	220,911		220,911		429,096
Co-investments	1,044,208		1,044,208		1,042,423
Real estate held for sale, net	0		0		56,300
Cash and cash equivalents, including restricted cash	65,152		65,152		95,749
Marketable securities and other investments	121,244		121,244		117,240
Notes and other receivables	25,676		25,676		24,923
Other non-segment assets	56,911		56,911		81,126
Total assets	12,045,004		12,045,004		11,526,732
Other Real Estate Assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	6,326	10,006	12,019	14,552
Net operating income	6,089	7,904	7,972	9,040
Net reportable operating segment - real estate assets	151,442		151,442		149,820
Reportable Geographical Components [Member] | Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	131,026	114,780	254,481	184,391
Net operating income	87,810	75,292	173,340	122,231
Net reportable operating segment - real estate assets	4,836,313		4,836,313		4,241,277
Reportable Geographical Components [Member] | Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	106,923	87,722	209,350	144,210
Net operating income	76,451	60,797	148,967	100,720
Net reportable operating segment - real estate assets	3,899,013		3,899,013		3,641,720
Reportable Geographical Components [Member] | Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	49,826	44,444	98,480	73,149
Net operating income	33,674	$ 29,401	66,803	$ 48,447
Net reportable operating segment - real estate assets	$ 1,624,134		$ 1,624,134		$ 1,647,058