Income Taxes - Aggregate Changes in Balance of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2015
Income Tax Disclosure [Abstract]
Beginning unrecognized tax benefits	$ 4,193	$ 3,619	$ 2,157
Tax positions taken in prior periods:
Gross increases	0	88	883
Gross decreases	(199)	(42)	0
Tax positions taken in current period:
Gross increases	595	528	579
Lapse of statute of limitations	(68)	0	0
Ending unrecognized tax benefits	$ 4,521	$ 4,193	$ 3,619

Tax Positions Taken In Current Period [Abstract]

No definition available.

Name:	bv_TaxPositionsTakenInCurrentPeriodAbstract
Namespace Prefix:	bv_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Tax Positions Taken In Prior Periods [Abstract]

+ References

No definition available.

+ Details

Name:	bv_TaxPositionsTakenInPriorPeriodsAbstract
Namespace Prefix:	bv_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

Amount of unrecognized tax benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=65882285&loc=SL6600010-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Unrecognized Tax Benefit
-URI http://asc.fasb.org/extlink&oid=6527854

+ Details

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

- Definition

Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

+ References

+ Details

Name:	us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

+ References

+ Details

Name:	us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

+ References

+ Details

Name:	us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations.

+ References

+ Details

Name:	us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration