Deferred compensation arrangement percentage of participants compensation deferred.

No definition available.

Name:	bv_DeferredCompensationArrangementPercentageOfParticipantsCompensationDeferred
Namespace Prefix:	bv_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Net deferred losses net of taxes expected to be recognized in interest expense.

+ References

No definition available.

+ Details

Name:	bv_NetDeferredLossesNetOfTaxesExpectedToBeRecognizedInInterestExpense
Namespace Prefix:	bv_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Number of outstanding fuel contracts.

+ References

No definition available.

+ Details

Name:	bv_NumberOfOutstandingFuelContracts
Namespace Prefix:	bv_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
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

Name:	us-gaap_DerivativeLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Nominal volume used to calculate payments on a derivative instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeNonmonetaryNotionalAmountVolume
Namespace Prefix:	us-gaap_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=bv_FuelSwapContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Fair Value Measurements and Derivatives Instruments - Additional Information (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2018 USD ($) Fuel	Dec. 31, 2018 USD ($) Fuel gal	Sep. 30, 2018 USD ($)
Interest Rate Contracts
Derivative [Line Items]
Notional amount	$ 1,560,000,000	$ 1,560,000,000	$ 1,560,000,000
Interest Rate Swaps
Derivative [Line Items]
Net deferred losses on interest rate swap net of taxes expected to be recognized over the next 12 months	$ 6,310,000
Fuel Swap Contracts
Derivative [Line Items]
Number of outstanding fuel contracts \| Fuel	1	1
Notional amount \| gal		5,000
Maximum
Derivative [Line Items]
Percentage of participants compensation deferred	70.00%