Represents carrying amount of capital leases related to trucks and trailers.

No definition available.

Name:	bxc_CarryingAmountOfCapitalLeases
Namespace Prefix:	bxc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents net book value of capital leases related to trucks and trailers.

+ References

No definition available.

+ Details

Name:	bxc_NetBookValueOfCapitalLeases
Namespace Prefix:	bxc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Number of Properties Subject to Sale-Leaseback Arrangement

+ References

No definition available.

+ Details

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

- Definition

Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84164817&loc=d3e45280-112737

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_CapitalLeaseObligations
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_CapitalLeasedAssetsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line.

+ References

No definition available.

+ Details

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

- Definition

The amount of the gain recorded in connection with the transaction involving the sale of property to another party and the lease of the property back to the seller to be recognized in future periods.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 40
-Section 25
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=77913853&loc=d3e49567-112750

+ Details

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

- Details

Name:	us-gaap_StatementGeographicalAxis=bxc_TampaFloridaAndFt.WorthTexasMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Lease Commitments - Narrative (Details) $ in Millions	12 Months Ended
Lease Commitments - Narrative (Details) $ in Millions	Dec. 30, 2017 USD ($) property	Dec. 31, 2016 USD ($)	Jan. 02, 2016 USD ($)	Apr. 01, 2017 USD ($) property
Capital Leased Assets [Line Items]
Total operating lease rent expense	$ 6.1	$ 4.2	$ 4.8
Basis of assets under capital leases	30.3	20.8
Net book value of assets under capital leases	$ 16.4	$ 9.0
Capital lease asset and obligations				$ 8.0
Number of properties subject to sale-leaseback arrangement \| property	3			3
Deferred gain on sale-leaseback transactions	$ 13.7			$ 13.7
Tampa, Florida and Ft. Worth, Texas
Capital Leased Assets [Line Items]
Capital lease asset and obligations	$ 8.0
Number of properties subject to sale-leaseback arrangement \| property	2