Real Estate Narrative (Details) $ in Thousands							1 Months Ended			2 Months Ended	3 Months Ended				5 Months Ended	6 Months Ended	9 Months Ended			12 Months Ended
	Sep. 20, 2019 USD ($) ft² a	Jul. 16, 2019 USD ($) ft²	Jun. 28, 2019 USD ($) ft²	Jun. 03, 2019 USD ($)	May 09, 2019 USD ($) ft²	Jan. 24, 2019 USD ($) ft² a	Sep. 30, 2019 USD ($) ft²		Jan. 23, 2019 USD ($)	Jun. 02, 2019 USD ($)	Sep. 30, 2019 USD ($) ft²		Jun. 27, 2019 USD ($)	Sep. 30, 2018 USD ($)	Jun. 02, 2019 USD ($)	Jun. 27, 2019 USD ($)	Sep. 30, 2019 USD ($) ft²		Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Aug. 27, 2019 USD ($) ft² Real_Estate_Properties	Jul. 19, 2019 USD ($)	Jun. 01, 2019 ft²	Apr. 18, 2019 USD ($) ft²	Jan. 10, 2019 USD ($) ft²
Real Estate [Line Items]
Revenues											$ 743,553			$ 686,284			$ 2,203,061		$ 2,011,919
Payments to Acquire Equity Method Investments																	65,499		314,075
Accelerated depreciation											165,862			157,996			507,867		480,210
Right of use assets - finance leases							$ 237,394				237,394						237,394
Lease liabilities - finance leases							221,029				221,029						221,029
Proceeds from sales of real estate																	83,486		175,577
Gains (loss) on sales of real estate											(15)			7,863			766		122,552
Boston Properties Limited Partnership
Real Estate [Line Items]
Revenues											743,553			686,284			2,203,061		2,011,919
Payments to Acquire Equity Method Investments																	65,499		314,075
Accelerated depreciation											164,020			156,056			501,901		474,383
Right of use assets - finance leases							237,394				237,394						237,394
Lease liabilities - finance leases							$ 221,029				221,029						221,029
Proceeds from sales of real estate																	83,486		175,577
Gains (loss) on sales of real estate											$ (15)			9,154			$ 915		126,831
325 Main Street [Member]
Real Estate [Line Items]
Term of Lease Signed (in years)					15-year
Net Rentable Area (in sf) | ft²					115,000
Accelerated depreciation					$ 9,900
325 Main Street [Member] | Boston Properties Limited Partnership
Real Estate [Line Items]
Accelerated depreciation					$ 9,500
20 CityPoint [Member]
Real Estate [Line Items]
Net Rentable Area (in sf) | ft²																							211,000
200 West Street [Member]
Real Estate [Line Items]
Net Rentable Area (in sf) | ft²							261,000				261,000						261,000
Platform 16 [Member]
Real Estate [Line Items]
Net Rentable Area (in sf) | ft²						1,100,000
Finance Lease fixed purchase price						$ 134,800
Right of use assets - finance leases						122,600
Lease liabilities - finance leases						$ 122,563
Lessee, Finance Lease, Term of Contract						65 years
Area of Land | a						5.6
2100 Pennsylvania Avenue [Member]
Real Estate [Line Items]
Term of Lease Signed (in years)		16-year
Net Rentable Area (in sf) | ft²		470,000
Prepaid Rent		$ 15,000
Right of use assets - finance leases		185,100
Lease liabilities - finance leases		165,000																				$ 164,977
Finance lease - right of use asset -prepaid rent		15,000
Finance lease - right of use asset - initial direct costs		$ 5,100
Lesse, finance lease, term of amortization		75 years
Lessee, Finance Lease, Term of Contract		75 years
Square Footage Of Signed Lease | ft²		300,000
Carnegie Center [Member]
Real Estate [Line Items]
Net Rentable Area (in sf) | ft²																									1,700,000
Aggregate purchase price																									$ 51,500
Deferred purchase price																									$ 8,600
880 & 890 Winter Street [Member]
Real Estate [Line Items]
Revenues							$ 1,300
Net Rentable Area (in sf) | ft²																					392,000
Aggregate purchase price																					$ 106,000
Number of real estate properties | Real_Estate_Properties																					2
Net income (loss)							$ (200)
164 Lexington Road [Member]
Real Estate [Line Items]
Net Rentable Area (in sf) | ft²			64,000
Gross Sale Price			$ 4,000
Proceeds from sales of real estate			3,800
Net income (loss)													$ (100)	(50)		$ (100)			(200)
Gains (loss) on sales of real estate			2,500
164 Lexington Road [Member] | Boston Properties Limited Partnership
Real Estate [Line Items]
Gains (loss) on sales of real estate			$ 2,600
2600 Tower Oaks Boulevard [Member]
Real Estate [Line Items]
Net Rentable Area (in sf) | ft²						179,000
Gross Sale Price						$ 22,700
Proceeds from sales of real estate						$ 21,400
Net income (loss)									$ (200)					(200)					(500)
Gains (loss) on sales of real estate																	$ (600)
Impairment loss																				$ 3,100
2600 Tower Oaks Boulevard [Member] | Boston Properties Limited Partnership
Real Estate [Line Items]
Impairment loss																				$ 1,500
One Tower Center [Member]
Real Estate [Line Items]
Net Rentable Area (in sf) | ft²																								410,000
Gross Sale Price																								$ 38,000
Proceeds from sales of real estate				$ 36,600
Net income (loss)										$ (100)				$ (700)	$ (900)				$ (2,000)
Gains (loss) on sales of real estate				$ (800)
Impairment loss																	24,000
One Tower Center [Member] | Boston Properties Limited Partnership
Real Estate [Line Items]
Impairment loss																	$ 22,300
Future Office Building [Member] | 325 Main Street [Member]
Real Estate [Line Items]
Net Rentable Area (in sf) | ft²					420,000
Square Footage Of Signed Lease | ft²					379,000
Future Retail Building [Member] | 325 Main Street [Member]
Real Estate [Line Items]
Net Rentable Area (in sf) | ft²					41,000
Office Properties Under Construction [Member] | 200 West Street [Member]
Real Estate [Line Items]
Net Rentable Area (in sf) | ft²							126,000				126,000						126,000
Platform 16 [Member]
Real Estate [Line Items]
Net Rentable Area (in sf) | ft²	1,100,000
Payments to Acquire Equity Method Investments	$ 28,200
Finance Lease fixed purchase price	$ 134,800
Lessee, Finance Lease, Term of Contract	65 years
Area of Land | a	5.6
Ownership Percentage	55.00%						55.00%	[1]			55.00%	[1]					55.00%	[1]
Joint Venture Partner [Member] | Platform 16 [Member]
Real Estate [Line Items]
Payments to Acquire Equity Method Investments	$ 23,100
Ownership Percentage	45.00%

[1] This entity is a VIE (See Note 2).