Debt instrument covenant required borrowings availability.

No definition available.

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Namespace Prefix:	becn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Debt instrument covenant required borrowings base percentage.

+ References

No definition available.

+ Details

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Namespace Prefix:	becn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Minimum fixed charge ratio per covenant of senior secured credit facility.

+ References

No definition available.

+ Details

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

- Definition

Number Of Financial Covenants

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

States whether the entity was in compliance with the debt covenants throughout the reporting period, and describes facts and circumstances of any compliance failure.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

Reference 2: 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))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- 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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
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

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of unamortized debt discount (premium) and debt issuance costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28555-108399

+ Details

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

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The carrying value as of the balance sheet date of the noncurrent portion of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the 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
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section 45
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-Topic 470
-SubTopic 10
-Section 45
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=99376301&loc=d3e1314-112600

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of cash inflow from contractual arrangement with the lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3255-108585

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-Name Accounting Standards Codification
-Topic 235
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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- Details

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Financing Arrangements - 2023 ABL - Additional Information (Detail)		1 Months Ended	12 Months Ended
	Jan. 02, 2018 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2020 USD ($) Covenant	Sep. 30, 2019 USD ($)
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 3,570,000,000
Debt instrument maturity date			Nov. 15, 2026
Borrowings under revolving lines of credit/term loans, net			$ 251,100,000	$ 81,000,000.0
Unamortized debt issuance costs			47,700,000
2023 ABL [Member]
Debt Instrument [Line Items]
Unamortized debt issuance costs			5,900,000
Revolving Lines of Credit [Member]
Debt Instrument [Line Items]
Borrowings under revolving lines of credit/term loans, net			$ 251,100,000	$ 81,000,000.0
Revolving Lines of Credit [Member] \| 2023 ABL [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,300,000,000
Debt instrument maturity date	Jan. 02, 2023
Line of credit facility, unused fees			0.25%
Number of financial covenants \| Covenant			1
Debt instrument covenant compliance			Per the covenant, the Company’s FCCR must be a minimum of 1.00 at the end of each fiscal quarter, calculated on a trailing four quarter basis (or under certain circumstances, at the end of each fiscal month, calculated on a trailing twelve-month basis). Compliance is only required at such times as borrowing availability (subject to certain adjustments) is less than the greater of (i) 10% of the lesser of the borrowing base or the aggregate commitments or (ii) $90.0 million, and for a period of thirty days thereafter. The Company was in compliance with this covenant as of September 30, 2020.
Debt Instrument Covenant Required Borrowings Base Percentage			10.00%
Debt Instrument Covenant Required Borrowings Availability			$ 90,000,000.0
Fixed charge coverage ratio			100.00%
Proceeds from Lines of Credit	$ 525,000,000.0	$ 725,000,000
Borrowings under revolving lines of credit/term loans, net			$ 251,100,000
Unamortized debt issuance costs			5,900,000
Revolving Lines of Credit [Member] \| 2023 ABL [Member] \| Base Rate [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Base rate borrowings	0.25%
Revolving Lines of Credit [Member] \| 2023 ABL [Member] \| Base Rate [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Base rate borrowings	0.75%
Revolving Lines of Credit [Member] \| 2023 ABL [Member] \| LIBOR [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Base rate borrowings	1.25%
Revolving Lines of Credit [Member] \| 2023 ABL [Member] \| LIBOR [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Base rate borrowings	1.75%
Revolving Lines of Credit [Member] \| U.S. [Member] \| 2023 ABL [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,250,000,000
Standby letters of credit outstanding			$ 13,000,000.0
Revolving Lines of Credit [Member] \| Canada [Member] \| 2023 ABL [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 50,000,000.0