Leases - Additional Information (Details)	12 Months Ended
Sep. 30, 2021
Leased Assets [Line Items]
Lease renewal option term	one or more exercisable renewal periods and specified increases in lease payments upon exercise of the renewal options.
Maximum
Leased Assets [Line Items]
Remaining lease terms	10 years 6 months

Lessee operating and finance leases remaining lease term.

No definition available.

Name:	bv_LesseeOperatingAndFinanceLeasesRemainingLeaseTerm
Namespace Prefix:	bv_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration