Segment Information - Summary of Certain Balance Sheet Data by Segment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Real estate, at cost	$ 6,025,797	$ 4,155,391
Investments in and advances to unconsolidated real estate ventures	261,811	45,776
Total assets	6,071,807	3,660,640
Operating Segments \| Office
Segment Reporting Information [Line Items]
Real estate, at cost	3,955,013	2,798,946
Investments in and advances to unconsolidated real estate ventures	124,659	45,647
Total assets	3,542,977	2,388,396
Operating Segments \| Multifamily
Segment Reporting Information [Line Items]
Real estate, at cost	1,476,423	959,404
Investments in and advances to unconsolidated real estate ventures	98,835	0
Total assets	1,434,999	873,157
Operating Segments \| Other
Segment Reporting Information [Line Items]
Real estate, at cost	594,361	397,041
Investments in and advances to unconsolidated real estate ventures	38,317	129
Total assets	1,299,085	399,087
Elimination of Intersegment Activity
Segment Reporting Information [Line Items]
Real estate, at cost	0	0
Investments in and advances to unconsolidated real estate ventures	0	0
Total assets	$ (205,254)	$ 0

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(12))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(11))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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- Definition

Amount of real estate investment property which may include the following: (1) land available-for-sale; (2) land available-for-development; (3) investments in building and building improvements; (4) tenant allowances; (5) developments in-process; (6) rental properties; and (7) other real estate investments.

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The carrying amount of investments in unconsolidated real estate and other joint ventures not separately presented. This includes direct and indirect investments.

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