Investment in unconsolidated Joint Ventures (Narrative) (Details) $ in Thousands								3 Months Ended		12 Months Ended
	Nov. 16, 2018 USD ($) ft² yr	Jul. 27, 2018 USD ($) 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	Dec. 31, 2018 USD ($) ft²		Dec. 31, 2018 USD ($) ft²		Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Equity Method Investments [Line Items]
Gains on sales of real estate										$ 182,356		$ 7,663	$ 80,606
Proceeds from Issuance of Secured Debt										0		2,300,000	0
Payments to Acquire Equity Method Investments										345,717		109,015	575,795
Mortgage notes payable, net								$ 2,964,572		2,964,572		2,979,281
Capital distributions from unconsolidated joint ventures										0		251,000	20,440
Restricted Cash and Cash Equivalents, Current								95,832		95,832		70,602	$ 63,174	$ 70,602	$ 73,790
Accumulated Other Comprehensive Income (Loss), Net of Tax								$ (47,741)		$ (47,741)		$ (50,429)
Santa Monica Business Park [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage								55.00%		55.00%
100 Causeway Street [Member]
Schedule of Equity Method Investments [Line Items]
Net Rentable Area | ft²		627,000
Term of Lease Signed (in years) | yr		20
Ownership Percentage		50.00%						50.00%	[1]	50.00%	[1]
Payment for improvements for equity method investment		$ 3,900
Payments to Acquire Equity Method Investments		$ 37,400
Podium Developer LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage								50.00%		50.00%
The Hub on Causeway - Residential [Member]
Schedule of Equity Method Investments [Line Items]
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								$ 40,500		$ 40,500
Net Rentable Area | ft²							320,000
Ownership Percentage							50.00%
Number of apartment units | apartments							440
540 Madison Avenue [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Basis Spread on Variable Rate					1.10%	1.50%
Net Rentable Area | ft²					284,000
Ownership Percentage					60.00%
Mortgage notes payable, net					$ 120,000
3 Hudson Boulevard [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Basis Spread on Variable Rate				3.50%
Net Rentable Area | ft²				2,000,000.0
Ownership Percentage				25.00%				25.00%		25.00%
Payments to Acquire Equity Method Investments				$ 45,600
Future payments to acquire equity method investments				62,200
Mortgage notes payable, net				$ 80,000
Annapolis Junction Building Six [Member]
Schedule of Equity Method Investments [Line Items]
Construction Loan Facility Borrowing Capacity	$ 14,300
Debt Instrument, Basis Spread on Variable Rate	2.00%
Number of extensions	1
Extension Option (in years) | yr	1
Construction Loan	$ 13,100
Net Rentable Area | ft²	119,000
Ownership Percentage	50.00%
Annapolis Junction Building One [Member]
Schedule of Equity Method Investments [Line Items]
Net Rentable Area | ft²								118,000		118,000
Ownership Percentage								50.00%		50.00%
Gains on sales of real estate								$ 8,300
Santa Monica Business Park [Member] | Santa Monica Business Park [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Basis Spread on Variable Rate			1.28%
Net Rentable Area | ft²			1,200,000.0
rentable square feet subject to a ground lease (percetnage)			70.00%
Term of Lease Signed (in years) | yr			80
Payments to Acquire Equity Method Investments			$ 179,700
Mortgage notes payable, net			$ 300,000
number of buildings | Buildings			21
Area of Land | a			47
Aggregate purchase price			$ 626,700
seller funded leasing costs			11,500
Derivative, Notional Amount			$ 300,000
Derivative, Fixed Interest Rate			4.063%
Accumulated Other Comprehensive Income (Loss), Net of Tax								(5,600)		$ (5,600)
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
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
BXP [Member]
Schedule of Equity Method Investments [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax								$ (3,100)		$ (3,100)

[1] This entity is a VIE (See Note 1).