Long-Term Debt - Rates and Ratios (Details)	9 Months Ended
Jun. 30, 2020
Debt Instrument
Debt covenant, leverage ratio	6.00
Term Loan
Debt Instrument
Discount percentage on debt instrument	98.00%
Debt Instrument, Interest Rate, Stated Percentage	6.00%
Term Loan \| Federal Funds
Debt Instrument
Basis spread on variable interest rate	0.50%
Term Loan \| One-Month Eurodollar
Debt Instrument
Basis spread on variable interest rate	1.00%
Term Loan \| Base Rate
Debt Instrument
Basis spread on variable interest rate	4.00%
Term Loan \| Eurodollar
Debt Instrument
Basis spread on variable interest rate	5.00%
Revolving Credit Facility
Debt Instrument
Unused capacity on line of credit commitment fee percentage	0.50%
Debt Instrument, Interest Rate, Stated Percentage	5.25%
Revolving Credit Facility \| Minimum
Debt Instrument
Unused capacity on line of credit commitment fee percentage	0.25%
Revolving Credit Facility \| Maximum
Debt Instrument
Unused capacity on line of credit commitment fee percentage	0.50%
Revolving Credit Facility \| Base Rate
Debt Instrument
Basis spread on variable interest rate	3.25%
Revolving Credit Facility \| Base Rate \| Minimum
Debt Instrument
Basis spread on variable interest rate	2.75%
Revolving Credit Facility \| Base Rate \| Median
Debt Instrument
Basis spread on variable interest rate	3.00%
Revolving Credit Facility \| Base Rate \| Maximum
Debt Instrument
Basis spread on variable interest rate	3.25%
Revolving Credit Facility \| Eurodollar
Debt Instrument
Basis spread on variable interest rate	4.25%
Revolving Credit Facility \| Eurodollar \| Minimum
Debt Instrument
Basis spread on variable interest rate	3.75%
Revolving Credit Facility \| Eurodollar \| Median
Debt Instrument
Basis spread on variable interest rate	4.00%
Revolving Credit Facility \| Eurodollar \| Maximum
Debt Instrument
Basis spread on variable interest rate	4.25%
Revolving Credit Facility \| Excess Cash Flow Ratio
Debt Instrument
Debt covenant, leverage ratio	3.35
Revolving Credit Facility \| High-End Ratio
Debt Instrument
Debt covenant, leverage ratio	3.50
Revolving Credit Facility \| Low-End Ratio
Debt Instrument
Debt covenant, leverage ratio	2.50

Debt Covenant, Leverage Ratio

No definition available.

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Namespace Prefix:	brbr_
Data Type:	xbrli:pureItemType
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

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Namespace Prefix:	us-gaap_
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=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num: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

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Data Type:	xbrli:stringItemType
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Period Type:	duration

- Definition

Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

+ 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 e
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

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

- 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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Period Type:	duration

- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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