Unsecured Line of Credit (Details) (USD $)	12 Months Ended
Dec. 31, 2014 Y	Dec. 31, 2013
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 1,000,000,000
Line of Credit Facility, Expiration Date	Jul. 26, 2018
Line of Credit Facility, Frequency of Commitment Fee Payment	quarterly
Line of Credit Facility, Amount Outstanding	0us-gaap_LineOfCredit	0us-gaap_LineOfCredit
Leverage Ratio, Maximum	60.00%bxp_LeverageRatioMaximum
Leverage Ratio Maximum Increasing Limit	65.00%bxp_LeverageRatioMaximumIncreasingLimit
Leverage Ratio Reduction Limit	60.00%bxp_LeverageRatioReductionLimit
Leverage Ratio Reduction Period, Years	1bxp_LeverageRatioReductionPeriodYears
Maximum Secured Debt Leverage Ratio	55.00%bxp_MaximumSecuredDebtLeverageRatio
Fixed Charge Coverage Ratio Minimum.	1.40bxp_FixedChargeCoverageRatioMinimum
Maximum Unsecured Debt Leverage Ratio	60.00%bxp_MaximumUnsecuredDebtLeverageRatio
Unsecured Debt Leverage Ratio Maximum Increasing Limit	65.00%
Unsecured Debt Leverage Ratio Reduction Limit	60.00%
Unsecured Debt Leverage Ratio Reduction Period, Years	1
Unsecured Debt Interest Coverage Ratio Minimum.	1.75
Alternative Base Interest Rate Calculation [Member]
Line of Credit Facility [Line Items]
Amount added to Federal Funds Rate to Calculate Interest Rate	0.50%
LIBOR Period Used to Calculate Interest Rate (in months)	1
Amount added to LIBOR to Calculate Interest Rate	1.00%
Maximum [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 1,500,000,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.35%
Maximum [Member] \| Interest Rate Based on LIBOR or CDOR [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	1.70%
Maximum [Member] \| Alternative Base Interest Rate Calculation [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	0.70%
Minimum [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.125%
Minimum [Member] \| Interest Rate Based on LIBOR or CDOR [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	0.925%
Minimum [Member] \| Alternative Base Interest Rate Calculation [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	0.00%
Rate Based on Current Credit Rating [Member] \| Interest Rate Based on LIBOR or CDOR [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	1.00%
Rate Based on Current Credit Rating [Member] \| Alternative Base Interest Rate Calculation [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	0.00%
Rate Based on Current Credit Rating [Member] \| Current Rates Based on Credit Ratings [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.15%

Amount added to Federal Funds Rate to Calculate Interest Rate

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- Definition

Amount added to LIBOR to Calculate Interest Rate

+ References

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- Definition

Fixed Charge Coverage Ratio Minimum.

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- Definition

Leverage Ratio Maximum

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- Definition

Leverage Ratio Maximum Increasing Limit

+ References

No definition available.

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- Definition

Leverage Ratio Reduction Limit

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No definition available.

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- Definition

Leverage Ratio Reduction Period, Years

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- Definition

LIBOR Period Used to Calculate Interest Rate (in months)

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- Definition

Margin added to Calculated Interest Rate

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- Definition

Maximum Secured Debt Leverage Ratio

+ References

No definition available.

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- Definition

Maximum Unsecured Debt Leverage Ratio

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No definition available.

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- Definition

Unsecured Debt Interest Coverage Ratio Minimum.

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- Definition

Unsecured Debt Leverage Ratio Maximum Increasing Limit

+ References

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- Definition

Unsecured Debt Leverage Ratio Reduction Limit

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- Definition

Unsecured Debt Leverage Ratio Reduction Period, Years

+ References

No definition available.

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 20, 22

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Date the credit facility terminates, in CCYY-MM-DD format.

+ References

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
Period Type:	duration

- Definition

Description of when the fee for available but unused credit capacity is required to be paid (for example, annually, quarterly, or monthly).

+ References

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- Definition

The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

+ References

No definition available.

+ Details

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Period Type:	duration

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