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Subsequent Events (Details) |  |  |  | 6 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Jul. 27, 2018 USD ($) ft² yr | Jul. 19, 2018 USD ($) ft² a yr Buildings | Jul. 13, 2018 USD ($) ft² | Jun. 30, 2018 USD ($) | Jun. 30, 2017 USD ($) | Aug. 07, 2018 ft² yr | Dec. 31, 2017 USD ($)
Subsequent Event [Line Items] |  |  |  |  |  |  |
mortgage notes |  |  |  | $ 2,972,052,000 |  |  | $ 2,979,281,000
Payments to Acquire Equity Method Investments |  |  |  | $ 65,250,000 | $ 41,491,000 |  |
Three Hudson Boulevard [Member] | Subsequent Event [Member] |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
Ownership Percentage |  |  | 25.00% |  |  |  |
mortgage notes |  |  | $ 80,000,000 |  |  |  |
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 |  |  | $ 45,600,000 |  |  |  |
Future payments to acquire equity method investments |  |  | $ 62,200,000 |  |  |  |
Santa Monica Business Park [Member] | Subsequent Event [Member] |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
mortgage notes |  | $ 300,000,000 |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  | 1.28% |  |  |  |  |
Derivative, Notional Amount |  | $ 300,000,000 |  |  |  |  |
Derivative, Average Fixed Interest Rate |  | 4.063% |  |  |  |  |
100 Causeway Street [Member] | Subsequent Event [Member] |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
Ownership Percentage | 50.00% |  |  |  |  |  |
Payment for improvements for equity method investment | $ 3,900,000 |  |  |  |  |  |
Net Rentable Area (in sf) | ft² | 627,000 |  |  |  |  |  |
Term of Lease Signed (in years) | yr | 20 |  |  |  |  |  |
Payments to Acquire Equity Method Investments | $ 37,400,000 |  |  |  |  |  |
Project estimated budget | $ 270,000,000 |  |  |  |  |  |
343 Madison Avenue [Member] | Subsequent Event [Member] |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  | 900,000 |
Term of Lease Signed (in years) | yr |  |  |  |  |  | 99 |
Ownership Percentage |  |  |  |  |  | 55.00% |
Santa Monica Business Park [Member] | Santa Monica Business Park [Member] | Subsequent Event [Member] |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
Net purchase price |  | $ 627,500,000.0 |  |  |  |  |
seller funded leasing costs |  | $ 11,500,000 |  |  |  |  |
Area of Land | a |  | 47 |  |  |  |  |
number of buildings | Buildings |  | 21 |  |  |  |  |
Net Rentable Area (in sf) | 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 |  | $ 180,100,000 |  |  |  |  |
Joint venture partner [Member] | Santa Monica Business Park [Member] | Subsequent Event [Member] |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
Ownership Percentage |  | 45.00% |  |  |  |  |
Payments to Acquire Equity Method Investments |  | $ 147,400,000 |  |  |  |  |
Joint venture partner [Member] | 100 Causeway Street [Member] | Subsequent Event [Member] |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
Ownership Percentage | 50.00% |  |  |  |  |  |
Payments to Acquire Equity Method Investments | $ 41,300,000 |  |  |  |  |  |

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