Percentage of fixed interest rate of equipment financing facilities.

No definition available.

Name:	becn_PercentageOfFixedInterestRateOfEquipmentFinancingFacilities
Namespace Prefix:	becn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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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

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Date the credit facility terminates, in CCYY-MM-DD format.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

The effective interest rate at the end of the reporting period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_LongtermDebtTypeAxis=becn_EquipmentFinancialFacilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
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Data Type:	na
Balance Type:
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- Details

Name:	srt_RangeAxis=srt_MaximumMember
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Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_DebtInstrumentAxis=becn_SeniorNotesDueInOctoberTwoThousandTwentyThreeMember
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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=becn_SeniorNotesDueInNovemberTwoThousandTwentyFiveMember
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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
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Data Type:	na
Balance Type:
Period Type:

- Details

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Data Type:	na
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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_CreditFacilityAxis=becn_TermLoanMember
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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=becn_TermLoanDueInJanuaryTwoTwoThousandAndTwentyFiveMember
Namespace Prefix:
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Financing Arrangements - Long-term Debt Instruments (Parenthetical) (Detail)		12 Months Ended
	Jan. 02, 2018	Sep. 30, 2020	Sep. 30, 2019	Oct. 25, 2017	Oct. 01, 2015
Debt Instrument [Line Items]
Debt interest rate at period end		4.50%
Debt instrument maturity date		Nov. 15, 2026
Equipment Financing Facilities [Member]
Debt Instrument [Line Items]
Debt instrument maturity date		Sep. 30, 2021	Sep. 30, 2021
Equipment Financing Facilities [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Equipment financing facilities, fixed interest rate percentage		2.33%	2.33%
Equipment Financing Facilities [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Equipment financing facilities, fixed interest rate percentage		2.89%	2.89%
2023 Senior Notes [Member]
Debt Instrument [Line Items]
Debt interest rate at period end			6.38%		6.38%
Debt instrument maturity date			Oct. 01, 2023
2025 Senior Notes [Member]
Debt Instrument [Line Items]
Debt interest rate at period end		4.88%	4.88%	4.875%
Debt instrument maturity date		Nov. 01, 2025	Nov. 01, 2025
Revolving Lines of Credit [Member] \| 2023 ABL [Member]
Debt Instrument [Line Items]
Debt instrument maturity date	Jan. 02, 2023
Revolving Lines of Credit [Member] \| 2023 ABL [Member] \| U.S. [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate at Period End		1.89%	5.41%
Line of Credit Facility, Expiration Date		Jan. 02, 2023	Jan. 02, 2023
Term Loan [Member] \| Term Loan, Matures January 2025 [Member]
Debt Instrument [Line Items]
Debt interest rate at period end		2.41%	4.36%
Debt instrument maturity date	Jan. 02, 2025	Jan. 02, 2025	Jan. 02, 2025