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Long Term Lease Obligations - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2018 USD ($) property | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Leases Disclosure [Line Items] |  |  |
Number of properties subject to noncancelable long-term lease | property | 0 |  |
Minimum rental obligations |  | $ 10,500 | $ 159,000
Rent expense | $ 779,800 | $ 774,700 | $ 843,300
90-year Ground Leasehold Interest |  |  |
Leases Disclosure [Line Items] |  |  |
Ground leasehold interest period | 90 years |  |
Minimum annual lease rent over lease term | $ 1 |  |
99-year Ground Lease |  |  |
Leases Disclosure [Line Items] |  |  |
Ground leasehold interest period | 99 years |  |
Minimum annual lease rent over lease term | $ 1 |  |
Percent of underlying land held under a 99-year ground lease | 3.40% |  |
Flagship Center |  |  |
Leases Disclosure [Line Items] |  |  |
Number of developed parcels | property | 2 |  |

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- Definition Annual Rental Payments Over Lease Term + References No definition available. + Details
Name: | bfs_AnnualRentalPaymentsOverLeaseTerm
Namespace Prefix: | bfs_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Lease Expiration Period + References No definition available. + Details
Name: | bfs_LeaseExpirationPeriod
Namespace Prefix: | bfs_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Leases Disclosure [Line Items] + References No definition available. + Details
Name: | bfs_LeasesDisclosureLineItems
Namespace Prefix: | bfs_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Lessee Leasing Arrangements, Number of Properties Under Lease + References No definition available. + Details
Name: | bfs_LesseeLeasingArrangementsNumberofPropertiesUnderLease
Namespace Prefix: | bfs_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Number of Developed Parcels + References No definition available. + Details
Name: | bfs_NumberofDevelopedParcels
Namespace Prefix: | bfs_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage of Lease Commitment + References No definition available. + Details
Name: | bfs_PercentageOfLeaseCommitment
Namespace Prefix: | bfs_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition This element represents the payments that the lessee is obligated to make or can be required to make in connection with a property under the terms of an agreement classified as an operating lease, excluding contingent rentals and a guarantee by the lessee of the lessor's debt and the lessee's obligation to pay (apart from the rental payments) executory costs such as insurance, maintenance, and taxes. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41499-112717 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 10 -Section 25 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=77910348&loc=d3e34039-112682 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 10 -Section 55 -Paragraph 40 -Subparagraph (Note 3) -URI http://asc.fasb.org/extlink&oid=82846649&loc=d3e38371-112697 + Details
Name: | us-gaap_OperatingLeasesRentExpenseMinimumRentals
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41499-112717 + Details
Name: | us-gaap_OperatingLeasesRentExpenseNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | bfs_CapitalizedCostByTypeAxis=bfs_NinetyYearGroundLeaseMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | bfs_CapitalizedCostByTypeAxis=bfs_NinetyNineYearGroundLeaseMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | bfs_RealEstatePropertyNameAxis=bfs_FlagshipCenterMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |