Debt instrument issue price percentage.

No definition available.

Name:	becn_DebtInstrumentIssuePricePercentage
Namespace Prefix:	becn_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

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+ Details

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

- Definition

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

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Description of debt redemption features under terms of the debt agreement.

+ References

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

- Definition

Percentage of principal amount of debt redeemed.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentRedemptionPricePercentageOfPrincipalAmountRedeemed
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

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

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+ Details

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

- Definition

The cash outflow for a long-term debt where the holder has highest claim on the entity's asset in case of bankruptcy or liquidation during the period.

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+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_DebtInstrumentAxis=becn_SeniorNotesDueTwoThousandTwentyFiveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Financing Arrangements - 2025 Senior Notes - Additional Information (Detail) - Senior Notes, Matures 2025 [Member] - USD ($)	May 19, 2021	Jun. 30, 2021	Sep. 30, 2020	Jun. 30, 2020	Oct. 25, 2017
Debt Instrument [Line Items]
Debt instrument, aggregate principal amount					$ 1,300,000,000
Debt instrument interest rate		4.125%	4.88%	4.88%	4.875%
Debt instrument, issue price percentage					100.00%
Debt instrument, redemption description	On May 19, 2021, in conjunction with the 2021 Debt Refinancing, the Company redeemed all $1.30 billion aggregate principal amount outstanding of the 2025 Senior Notes at a redemption price of 102.438% plus accrued interest
Redemption of senior debt	$ 1,300,000,000
Debt redemption price percentage of principal amount	102.438%
Unamortized debt issuance costs	$ 12,500,000