Commitments And Contingencies (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2020 USD ($) ft²	Dec. 31, 2009 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jan. 10, 2014 USD ($)
Commitments And Contingencies [Line Items]
Letter of credit and performance obligations	$ 20,400,000
Notes Receivable, Related Parties	78,520,000		$ 80,000,000
Property insurance program per occurrence limits	1,000,000,000.0
Per occurrence limit for NBCR Coverage	1,000,000,000.0
Value of program trigger	$ 200,000,000
Coinsurance of program trigger	20.00%
Program trigger deductible	20.00%
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	$ 240,000,000
Annual aggregate limit of the earthquake insurance which covers San Francisco and Los Angeles regions	240,000,000
Amount of earthquake insurance provided by IXP, LLC as direct insurer San Francisco and Los Angeles	20,000,000
Notes receivable, net	25,480,000		15,920,000
Affiliate of Bernstein Companies [Member]
Commitments And Contingencies [Line Items]
Notes receivable, net	$ 10,000,000.0
Note receivable fixed interest rate	8.00%
Boston Properties Limited Partnership
Commitments And Contingencies [Line Items]
Notes Receivable, Related Parties	$ 78,520,000		80,000,000
Operating partnership guarantee to cover liabilities of IXP	20,000,000.0
Notes receivable, net	25,480,000		$ 15,920,000
767 Venture, LLC [Member]
Commitments And Contingencies [Line Items]
Maximum funding obligation	48,900,000
Property insurance program per occurrence limits	1,625,000,000
7750 Wisconsin Avenue [Member]
Commitments And Contingencies [Line Items]
Liabilities for Contingent Guarantees	0
Lehman [Member]
Commitments And Contingencies [Line Items]
Bankruptcy claim, amount filed by general creditor		$ 45,300,000
Bankruptcy claim amount allowed by court to creditor					$ 45,200,000
Bankruptcy Claims, Amount of Claims Settled				$ 18,000,000.0
Bankruptcy remaining claim amount allowed by court to creditor	27,200,000
6595 Springfield Center Drive [Member]
Commitments And Contingencies [Line Items]
Liabilities for Contingent Guarantees	$ 0
325 Main Street
Commitments And Contingencies [Line Items]
Net Rentable Area (in sf) | ft²	115,000
601 Lexington Avenue
Commitments And Contingencies [Line Items]
Coverage For Acts Of Terrorism Under TRIA Covered in Excess of Amount Covered by IXP	$ 250,000,000
Future Residential Building [Member] | 325 Main Street
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
Commitments And Contingencies [Line Items]
Net Rentable Area (in sf) | ft²	420,000
Future Retail Building [Member] | 325 Main Street
Commitments And Contingencies [Line Items]
Net Rentable Area (in sf) | ft²	41,000
Home Ownership Units [Member] | Future Residential Building [Member] | 325 Main Street
Commitments And Contingencies [Line Items]
Affordable Housing requirement	20.00%