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Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Parenthetical) (Details) | Sep. 30, 2017 property
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Segment Reporting Information [Line Items] |
Number of Properties | 97
Assets Held-for-sale [Member] |
Segment Reporting Information [Line Items] |
Number of Properties | 6
Office Properties [Member] |
Segment Reporting Information [Line Items] |
Number of Properties | 82
Office Properties [Member] | Pennsylvania Suburbs [Member] | Assets Held-for-sale [Member] |
Segment Reporting Information [Line Items] |
Number of Properties | 6

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- Definition The number of real estate properties owned as of the balance sheet date. + References No definition available. + Details
Name: | us-gaap_NumberOfRealEstateProperties
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_SegmentReportingInformationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_DisposalGroupClassificationAxis=us-gaap_DisposalGroupHeldforsaleNotDiscontinuedOperationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=us-gaap_OfficeBuildingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=bdn_PennsylvaniaSuburbsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |