Amount before accumulated depreciation of building structures held for productive use including addition, improvement, or renovation to the structure, including, but not limited to, interior masonry, interior flooring, electrical, and plumbing.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 4
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=121566466&loc=d3e6812-107765

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

+ Details

Name:	us-gaap_BuildingsAndImprovementsGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before accumulated deprecation and depletion of real estate held for productive use and additions or improvements to real estate held for productive use, examples include, but are not limited to, walkways, driveways, fences, and parking lots. Excludes land held for sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=121566466&loc=d3e6812-107765

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(13))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_LandAndLandImprovements
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of real estate investment property which may include the following: (1) land available-for-sale; (2) land available-for-development; (3) investments in building and building improvements; (4) tenant allowances; (5) developments in-process; (6) rental properties; and (7) other real estate investments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(d))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

+ Details

Name:	us-gaap_RealEstateInvestmentPropertyAtCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of real estate owned for income production and capital accretion potential, not otherwise specified in the existing taxonomy. Such real estate excludes that which is occupied or used in the business, for entertainment purposes, or held in inventory for specific purposes.

+ References

+ Details

Name:	us-gaap_RealEstateInvestmentsOther
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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_RealEstatePropertiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_RealEstatePropertiesAxis=bfs_AshbrookMarketplaceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_LeasesAcquiredInPlaceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_AboveMarketLeasesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=bfs_BelowMarketLeasesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RealEstatePropertiesAxis=bfs_A7316WisconsinAvenueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RealEstatePropertiesAxis=bfs_A2018AcquisitionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Real Estate - Allocation of Purchase Prices to Acquired Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Real Estate Properties [Line Items]
Land	$ 511,482
Buildings	1,543,837
Total Purchase Price	$ 2,124,796	$ 2,081,597	$ 1,948,165	$ 1,803,200
Ashbrook Marketplace
Real Estate Properties [Line Items]
Land			8,776
Buildings			0
Total Purchase Price			8,776
Ashbrook Marketplace \| In-place Leases
Real Estate Properties [Line Items]
Intangible assets			0
Ashbrook Marketplace \| Above Market Rent
Real Estate Properties [Line Items]
Intangible assets			0
Ashbrook Marketplace \| Below Market Rent
Real Estate Properties [Line Items]
Intangible assets			0
Hampden House (formerly 7316 Wisconsin Avenue)
Real Estate Properties [Line Items]
Land			38,662
Buildings			979
Total Purchase Price			40,674
Hampden House (formerly 7316 Wisconsin Avenue) \| In-place Leases
Real Estate Properties [Line Items]
Intangible assets			(886)
Hampden House (formerly 7316 Wisconsin Avenue) \| Above Market Rent
Real Estate Properties [Line Items]
Intangible assets			(168)
Hampden House (formerly 7316 Wisconsin Avenue) \| Below Market Rent
Real Estate Properties [Line Items]
Intangible assets			(21)
2018 Acquisitions
Real Estate Properties [Line Items]
Land			47,438
Buildings			979
Total Purchase Price			49,450
2018 Acquisitions \| In-place Leases
Real Estate Properties [Line Items]
Intangible assets			(886)
2018 Acquisitions \| Above Market Rent
Real Estate Properties [Line Items]
Intangible assets			(168)
2018 Acquisitions \| Below Market Rent
Real Estate Properties [Line Items]
Intangible assets			$ (21)