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Investment in unconsolidated Joint Ventures (Narrative) (Details) $ in Thousands |  |  |  |  | 12 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Sep. 06, 2017 USD ($) ft² | Aug. 07, 2017 USD ($) ft² yr | Jul. 28, 2017 USD ($) ft² a Vehicles Buildings | Jul. 10, 2017 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 01, 2017 ft² apartments
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Proceeds from Issuance of Secured Debt |  |  |  |  | $ 2,300,000 | $ 0 | $ 0 |
Payments to Acquire Equity Method Investments |  |  |  |  | 109,015 | 575,795 | 38,207 |
Capital distributions from unconsolidated joint ventures |  |  |  |  | 251,000 | 20,440 | 24,527 |
Restricted Cash and Cash Equivalents, Current |  |  |  |  | $ 70,602 | 63,174 |  |
Podium Developer LLC [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Net Rentable Area | ft² | 385,000 |  |  |  |  |  |  |
Ownership Percentage | 50.00% |  |  |  | 50.00% |  |  |
Colorado Center [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  | 1,118,000 |  |  |  |  |
Vehicles In Structured Parking | Vehicles |  |  | 3,100 |  |  |  |  |
Ownership Percentage |  |  |  | 50.00% |  |  |  |
number of buildings | Buildings |  |  | 6 |  |  |  |  |
Area of Land | a |  |  | 15 |  |  |  |  |
7750 Wisconsin Avenue [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  | 733,000 |  |  |  |  |  |
Term of Lease Signed (in years) | yr |  | 20 |  |  |  |  |  |
Ownership Percentage |  | 50.00% |  |  |  |  |  |
Payments and improvements for equity method investments |  | $ 20,800 |  |  |  |  |  |
Restricted Cash and Cash Equivalents, Current |  | 25,000 |  |  |  |  |  |
The Hub on Causeway - Residential [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Net Rentable Area | ft² |  |  |  |  |  |  |  | 320,000
Ownership Percentage |  |  |  |  |  |  |  | 50.00%
Number of apartment units | apartments |  |  |  |  |  |  |  | 440
Boston Properties Limited Partnership |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Proceeds from Issuance of Secured Debt |  |  |  |  | $ 2,300,000 | 0 | 0 |
Payments to Acquire Equity Method Investments |  |  |  |  | 109,015 | 575,795 | 38,207 |
Capital distributions from unconsolidated joint ventures |  |  |  |  | 251,000 | 20,440 | $ 24,527 |
Restricted Cash and Cash Equivalents, Current |  |  |  |  | $ 70,602 | $ 63,174 |  |
Joint venture partner [Member] | 7750 Wisconsin Avenue [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Contribution of Property |  | 72,000 |  |  |  |  |  |
Payments and improvements for equity method investments |  | 4,900 |  |  |  |  |  |
Capital distributions from unconsolidated joint ventures |  | $ 11,000 |  |  |  |  |  |
Secured Debt [Member] | Colorado Center [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Coupon/Stated Rate |  |  | 3.56% |  |  |  |  |
Proceeds from Issuance of Secured Debt |  |  | $ 550,000 |  |  |  |  |
Capital distributions from unconsolidated joint ventures |  |  | 502,000 |  |  |  |  |
Secured Debt [Member] | Boston Properties Limited Partnership | Colorado Center [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Capital distributions from unconsolidated joint ventures |  |  | $ 251,000 |  |  |  |  |
Secured Debt [Member] | Podium Developer LLC [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Construction Loan Facility Borrowing Capacity | $ 204,600 |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | 2.25% |  |  |  |  |  |  |
Number of extensions | 2 |  |  |  |  |  |  |
Extension Option (in years) | 1 |  |  |  |  |  |  |
Colorado Center [Member] | Colorado Center [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  | 0.20% |  |  |  |
Payments to Acquire Equity Method Investments |  |  |  | $ 2,100 |  |  |  |

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