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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) |  |  |  |  |
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