Lessee, Leases, Number Of Options To Extend

No definition available.

Name:	bxc_LesseeLeasesNumberOfOptionsToExtend
Namespace Prefix:	bxc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Lessee, Leases, Renewal Term

+ References

No definition available.

+ Details

Name:	bxc_LesseeLeasesRenewalTerm
Namespace Prefix:	bxc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Lessee, Leases, Term Of Contract

+ References

No definition available.

+ Details

Name:	bxc_LesseeLeasesTermOfContract
Namespace Prefix:	bxc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Number Of Real Estate Financing Transactions

+ References

No definition available.

+ Details

Name:	bxc_NumberOfRealEstateFinancingTransactions
Namespace Prefix:	bxc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Proceeds From Financing Lease Obligation

+ References

No definition available.

+ Details

Name:	bxc_ProceedsFromFinancingLeaseObligation
Namespace Prefix:	bxc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Present value of lessee's discounted obligation for lease payments from finance lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=121609121&loc=SL77918701-209980

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121603541&loc=SL77918627-209977

+ Details

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

- Definition

Term of lessee's finance lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=121609121&loc=SL77918673-209980

+ Details

Name:	us-gaap_LesseeFinanceLeaseRenewalTerm1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

Term of lessee's finance lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_LesseeFinanceLeaseTermOfContract1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
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_LesseeLeaseDescriptionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_BuildingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=bxc_UniversityParkILMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=bxc_YuleeFLMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_TransportationEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Leases - Narrative (Details) $ in Thousands						3 Months Ended		12 Months Ended
Leases - Narrative (Details) $ in Thousands	Feb. 28, 2020 USD ($)	Jan. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) warehouse	Jun. 20, 2019 USD ($)	May 19, 2019 USD ($)	Apr. 03, 2021 USD ($) option	Mar. 28, 2020 USD ($)	Jan. 02, 2021 USD ($)	Dec. 28, 2019 USD ($)	Dec. 29, 2018
Lessee, Lease, Description [Line Items]
Number of Options \| option						1
Number of financing transactions \| warehouse			2
Lease renewal term						5 years
Finance lease, renewal term										5 years
Finance lease obligations						$ 281,274		$ 273,118
Proceeds from real estate financing transactions						0	$ 78,329
Warehouse faciility
Lessee, Lease, Description [Line Items]
Number of financing transactions \| warehouse			2
Finance lease, term of contract									15 years
Finance lease, renewal term								5 years	5 years
Finance lease obligations						$ 81,100		$ 82,000
Proceeds from real estate financing transactions	$ 7,500	$ 34,100	$ 27,200					$ 78,300	$ 45,000
Warehouse faciility \| University Park, IL
Lessee, Lease, Description [Line Items]
Proceeds from real estate financing transactions					$ 21,800
Warehouse faciility \| Yulee, FL
Lessee, Lease, Description [Line Items]
Proceeds from real estate financing transactions				$ 13,300
Transportation Equipment
Lessee, Lease, Description [Line Items]
Finance lease, term of contract						4 years
Finance lease obligations						$ 10,200
Minimum
Lessee, Lease, Description [Line Items]
Lease term						1 year
Finance lease, renewal term										10 years
Minimum \| Warehouse faciility
Lessee, Lease, Description [Line Items]
Finance lease, term of contract								15 years
Maximum
Lessee, Lease, Description [Line Items]
Lease term						15 years
Finance lease, term of contract										15 years
Maximum \| Warehouse faciility
Lessee, Lease, Description [Line Items]
Finance lease, term of contract								18 years