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Employee Benefits - Defined Contribution Plans - Narrative (Details) $ in Millions	12 Months Ended
	Jan. 01, 2022 USD ($) defined_contribution_plan	Jan. 02, 2021 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Number of defined contribution plans | defined_contribution_plan	2
Hourly Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 0.7	$ 0.7
Salaried Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 2.0	$ 1.8

X

- Definition

Defined Contribution Plan, Number of Plans

+ References

No definition available.

+ Details

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Namespace Prefix:	bxc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

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

X

- Definition

Amount of cost for defined contribution plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

+ Details

Name:	us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

Name:	us-gaap_RetirementPlanNameAxis=bxc_HourlySavingsPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RetirementPlanNameAxis=bxc_SalariedSavingsPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: