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%