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