Revenue - Additional Information (Details) $ in Thousands	9 Months Ended
Jun. 30, 2019 USD ($)
Disaggregation Of Revenue [Line Items]
Revenue, remaining performance obligations, amount	$ 339,352
Contract with customer billed revenue	113,181
Contract with customer unbilled revenue additions	$ 115,653
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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Namespace Prefix:	bv_
Data Type:	xbrli:durationItemType
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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130569-203045

+ Details

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Data Type:	xbrli:booleanItemType
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=109196051&loc=SL49130556-203045

+ Details

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

- Details

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Balance Type:
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- Details

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Namespace Prefix:
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Balance Type:
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- Details

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