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Commitments And Contingencies (Details) $ in Millions | 9 Months Ended | 12 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2019 USD ($) ft² | Dec. 31, 2009 USD ($) | May 09, 2019 ft² | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Jan. 10, 2014 USD ($)
Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Letter of credit and performance obligations | $ 15.2 |  |  |  |  |  |  |  |
Property insurance program per occurrence limits | 1,000.0 |  |  |  |  |  |  |  |
Per occurrence limit for NBCR Coverage | 1,000.0 |  |  |  |  |  |  |  |
Value of program trigger | $ 180.0 |  |  |  |  |  |  |  |
Coinsurance of program trigger | 19.00% |  |  |  |  |  |  |  |
Deductible in insurance as a percentage of the value of the affected property, San Francisco and Los Angeles | 3.00% |  |  |  |  |  |  |  |
Per occurrence limit of the earthquake insurance which covers San Francisco and Los Angeles regions | $ 240.0 |  |  |  |  |  |  |  |
Annual aggregate limit of the earthquake insurance which covers San Francisco and Los Angeles regions | 240.0 |  |  |  |  |  |  |  |
Amount of earthquake insurance provided by IXP, LLC as direct insurer San Francisco and Los Angeles | 20.0 |  |  |  |  |  |  |  |
767 Venture, LLC [Member] |  |  |  |  |  |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Maximum funding obligation | 71.7 |  |  |  |  |  |  |  |
Property insurance program per occurrence limits | 1,625.0 |  |  |  |  |  |  |  |
Lehman [Member] |  |  |  |  |  |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Bankruptcy claim, amount filed by general creditor |  | $ 45.3 |  |  |  |  |  |  |
Bankruptcy claim amount allowed by court to creditor |  |  |  |  |  |  |  |  | $ 45.2
Bankruptcy Claims, Amount of Claims Settled |  |  |  | $ 0.3 | $ 0.4 | $ 1.4 | $ 8.1 | $ 7.7 |
Bankruptcy remaining claim amount allowed by court to creditor | 27.2 |  |  |  |  |  |  |  |
325 Main Street [Member] |  |  |  |  |  |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  | 115,000 |  |  |  |  |  |
601 Lexington Avenue [Member] |  |  |  |  |  |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Coverage For Acts Of Terrorism Under TRIA Covered in Excess of Amount Covered by IXP | 250.0 |  |  |  |  |  |  |  |
Boston Properties Limited Partnership |  |  |  |  |  |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Operating partnership guarantee to cover liabilities of IXP | $ 20.0 |  |  |  |  |  |  |  |
Residential Real Estate [Member] | 325 Main Street [Member] |  |  |  |  |  |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² | 200,000 |  |  |  |  |  |  |  |
Affordable Housing requirement |  |  | 25.00% |  |  |  |  |  |
Future Office Building [Member] | 325 Main Street [Member] |  |  |  |  |  |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  | 420,000 |  |  |  |  |  |
Future Retail Building [Member] | 325 Main Street [Member] |  |  |  |  |  |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  | 41,000 |  |  |  |  |  |
Home ownership units [Member] | Residential Real Estate [Member] | 325 Main Street [Member] |  |  |  |  |  |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Affordable Housing requirement |  |  | 20.00% |  |  |  |  |  |

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