Revenue - Additional Information (Details) $ in Millions	12 Months Ended
Sep. 30, 2020 USD ($)
Disaggregation Of Revenue [Line Items]
Revenue, remaining performance obligations, amount	$ 295.3
Contract with customer billed revenue	142.5
Contract with customer unbilled revenue additions	$ 129.5
Revenue, practical expedient, financing component	true
Minimum \| Maintenance Services
Disaggregation Of Revenue [Line Items]
Recurring annual contract period	1 year
Maximum \| Maintenance Services
Disaggregation Of Revenue [Line Items]
Recurring annual contract period	3 years

Contract with customer billed revenue.

No definition available.

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Namespace Prefix:	bv_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Contract with customer unbilled revenue additions.

+ References

No definition available.

+ Details

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Namespace Prefix:	bv_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Recurring annual contract period.

+ References

No definition available.

+ Details

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- 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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- Definition

Indicates (true false) whether practical expedient was elected not to adjust consideration for effect of financing component when transfer and customer payment for product or service occurs within one year or less.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130569-203045

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130556-203045

+ Details

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Period Type:	instant

- Details

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- Details

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- Details

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