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Investments in Unconsolidated Joint Ventures Narrative (Details) $ in Thousands							9 Months Ended
	Jul. 27, 2018 USD ($) ft² yr	Jul. 19, 2018 USD ($) ft² a yr Buildings	Jul. 13, 2018 USD ($) ft²	Apr. 27, 2018 USD ($) ft²	Apr. 26, 2018	Apr. 19, 2018 USD ($) ft² yr apartments	Sep. 30, 2018 USD ($)		Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Equity Method Investments [Line Items]
Mortgage notes payable, net							$ 2,967,548			$ 2,979,281
Payments to Acquire Equity Method Investments							314,075		$ 89,874
Accumulated other comprehensive loss							$ (45,137)			$ (50,429)
100 Causeway Street [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage	50.00%						50.00%	[1]
Payment for improvements for equity method investment	$ 3,900
Net Rentable Area (in sf) | ft²	627,000
Payments to Acquire Equity Method Investments	$ 37,400
Term of Lease Signed (in years) | yr	20
The Hub on Causeway - Residential [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage						50.00%
Construction Loan Facility Borrowing Capacity						$ 180,000
Debt Instrument, Basis Spread on Variable Rate						2.00%
Number of extensions						2
Extension Option (in years) | yr						1
Construction Loan						$ 8,200
Net Rentable Area (in sf) | ft²						320,000
Number of apartment units | apartments						440
540 Madison Avenue [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage				60.00%
Mortgage notes payable, net				$ 120,000
Debt Instrument, Basis Spread on Variable Rate				1.10%	1.50%
Net Rentable Area (in sf) | ft²				284,000
Three Hudson Boulevard [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage			25.00%				25.00%
Mortgage notes payable, net			$ 80,000
Debt Instrument, Basis Spread on Variable Rate			3.50%
Net Rentable Area (in sf) | ft²			2,000,000.0
Payments to Acquire Equity Method Investments			$ 45,600
Future payments to acquire equity method investments			$ 62,200
Santa Monica Business Park [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage							55.00%
Accumulated other comprehensive loss							$ 2,400
Santa Monica Business Park [Member] | Santa Monica Business Park [Member]
Schedule of Equity Method Investments [Line Items]
Mortgage notes payable, net		$ 300,000
Debt Instrument, Basis Spread on Variable Rate		1.28%
Net Rentable Area (in sf) | ft²		1,200,000.0
Payments to Acquire Equity Method Investments		$ 179,700
Aggregate purchase price		626,700
seller funded leasing costs		$ 11,500
Area of Land | a		47
number of buildings | Buildings		21
rentable square feet subject to a ground lease (percetnage)		70.00%
Term of Lease Signed (in years) | yr		80
Derivative, Notional Amount		$ 300,000
Derivative, Fixed Interest Rate		4.063%
Joint venture partner [Member] | 100 Causeway Street [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage	50.00%
Payments to Acquire Equity Method Investments	$ 41,300
Joint venture partner [Member] | Santa Monica Business Park [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		45.00%
Payments to Acquire Equity Method Investments		$ 147,000
BXP [Member]
Schedule of Equity Method Investments [Line Items]
Accumulated other comprehensive loss							$ 1,300
[1] This entity is a VIE (See Note 2 to the Consolidated Financial Statements).

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