Mortgage Notes Payable, Net, Mezzanine Notes Payable and Outside Members’ Notes Payable (Narrative) (Details) $ in Thousands		12 Months Ended
	Jun. 07, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2008 USD ($)	Jun. 06, 2017 USD ($)	Apr. 24, 2017 USD ($)
Mortgage Notes Payable [Line Items]
Mortgage notes payable, net (amounts related to VIEs of $2,939,183 and $2,018,483 at December 31, 2017 and 2016, respectively)		$ 2,979,281	$ 2,063,087
Proceeds from Issuance of Secured Debt		2,300,000	0	$ 0
Repayments of Secured Debt		1,317,653	1,326,865	54,801
Repayments of Subordinated Debt		306,000	0	0
Other Notes Payable		0	180,000
Interest Payable		83,646	243,933
Gains (losses) from early extinguishments of debt		$ 496	$ (371)	(22,040)
Number Of Mortgage Loans Accounted For Fair Value		0	1
Mortgage Loans Accounted For At Fair Value Upon Assumption			$ 1,300,000
Interest Expense Decrease Due To Recording Mortgage Loans At Fair Value		$ 19,600	46,400	$ 55,000
Cumulative Liability Related to the Fair Value of Debt Adjustment			$ 33,800
Minimum [Member]
Mortgage Notes Payable [Line Items]
Coupon/Stated Rate		3.43%	4.75%
Number Real Estate Assets Used For Mortgage Loan Collateral		1
Maximum [Member]
Mortgage Notes Payable [Line Items]
Coupon/Stated Rate		7.69%	7.69%
Fixed Rate Mortgages [Member]
Mortgage Notes Payable [Line Items]
Mortgage notes payable, net (amounts related to VIEs of $2,939,183 and $2,018,483 at December 31, 2017 and 2016, respectively)		$ 3,000,000	$ 2,100,000
Debt, Weighted Average Interest Rate		3.78%	5.59%
Variable Rate Mortgages [Member]
Mortgage Notes Payable [Line Items]
Number of variable rate mortgage loans		0	0
767 Fifth Avenue (the general motors building) [Member]
Mortgage Notes Payable [Line Items]
Gains (losses) from early extinguishments of debt	$ 14,600
767 Fifth Avenue (the general motors building) [Member] | Notes Payable, Other Payables [Member]
Mortgage Notes Payable [Line Items]
Repayment of a portion of members loans accrued interest and principal balance	176,100
767 Fifth Avenue (the general motors building) [Member] | Secured Debt [Member]
Mortgage Notes Payable [Line Items]
Mortgage notes payable, net (amounts related to VIEs of $2,939,183 and $2,018,483 at December 31, 2017 and 2016, respectively)							$ 2,300,000
Coupon/Stated Rate							3.43%
Consolidated Entities [Member] | 767 Fifth Avenue (the general motors building) [Member]
Mortgage Notes Payable [Line Items]
Unpaid principal balance of the members' notes payable contributed to equity	$ 273,900
Consolidated Entities [Member] | 767 Fifth Avenue (the general motors building) [Member] | Junior Subordinated Debt [Member]
Mortgage Notes Payable [Line Items]
Coupon/Stated Rate	6.02%
Other Long-term Debt	$ 294,000
Consolidated Entities [Member] | 767 Fifth Avenue (the general motors building) [Member] | Notes Payable, Other Payables [Member]
Mortgage Notes Payable [Line Items]
Coupon/Stated Rate	11.00%
Other Notes Payable	$ 450,000
Interest Payable	$ 425,000
Consolidated Entities [Member] | 767 Fifth Avenue (the general motors building) [Member] | Secured Debt [Member]
Mortgage Notes Payable [Line Items]
Ownership Percentage by Parent	60.00%
Proceeds from Issuance of Secured Debt	$ 2,300,000
Coupon/Stated Rate	3.43%					5.95%
Repayments of Secured Debt	$ 1,300,000
Consolidated Entities [Member] | 767 Fifth Avenue (the general motors building) [Member] | Subordinated Debt [Member]
Mortgage Notes Payable [Line Items]
Coupon/Stated Rate	6.02%
Repayments of Subordinated Debt	$ 306,000
Consolidated Properties [Member] | 767 Fifth Avenue (the general motors building) [Member]
Mortgage Notes Payable [Line Items]
Unpaid principal balance of the members' notes payable contributed to equity						$ 273,900
Ownership Percentage by Parent						60.00%
Consolidation, Eliminations [Member] | 767 Fifth Avenue (the general motors building) [Member] | Junior Subordinated Debt [Member]
Mortgage Notes Payable [Line Items]
Repayments of Other Long-term Debt					$ 263,100
Consolidation, Eliminations [Member] | Consolidated Entities [Member] | 767 Fifth Avenue (the general motors building) [Member]
Mortgage Notes Payable [Line Items]
Unpaid principal balance of the members' notes payable contributed to equity	164,400
Consolidation, Eliminations [Member] | Consolidated Entities [Member] | 767 Fifth Avenue (the general motors building) [Member] | Notes Payable, Other Payables [Member]
Mortgage Notes Payable [Line Items]
Other Notes Payable	270,000
Interest Payable	255,000
Consolidation, Eliminations [Member] | Consolidated Properties [Member] | 767 Fifth Avenue (the general motors building) [Member]
Mortgage Notes Payable [Line Items]
Unpaid principal balance of the members' notes payable contributed to equity						$ 164,400
Boston Properties, Inc. [Member] | Consolidated Entities [Member] | 767 Fifth Avenue (the general motors building) [Member] | Notes Payable, Other Payables [Member]
Mortgage Notes Payable [Line Items]
Other Notes Payable	180,000
Interest Payable	$ 170,000