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Revenue - Schedule of Changes in Deferred Revenue (Details) $ in Millions | 9 Months Ended
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Jun. 30, 2021 USD ($)
Revenue From Contract With Customer [Abstract] |
Balance, September 30, 2020 | $ 57.1
Recognition of revenue | (770.8)
Deferral of revenue | 766.8
Balance, June 30, 2021 | $ 53.1

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- Definition Contract with customer liability deferral of revenue. + References No definition available. + Details
Name: | bv_ContractWithCustomerLiabilityDeferralOfRevenue
Namespace Prefix: | bv_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Contract with customer liability revenue recognized including additions. + References No definition available. + Details
Name: | bv_ContractWithCustomerLiabilityRevenueRecognizedIncludingAdditions
Namespace Prefix: | bv_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable, classified as current. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121556615&loc=SL49130531-203044 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=121556615&loc=SL49130532-203044 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130549-203045 + Details
Name: | us-gaap_ContractWithCustomerLiabilityCurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_RevenueFromContractWithCustomerAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration