v3.5.0.1

Commitments and Contingencies - Additional Information (Details) $ in Thousands |  |  |  |  |  |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 14, 2015 | Nov. 21, 2014 USD ($) | Nov. 13, 2014 USD ($) ft² Term | Jun. 04, 2014 USD ($) | Mar. 12, 2013 USD ($) | Jun. 12, 2012 USD ($) | Apr. 30, 2016 USD ($) | Jul. 31, 2013 USD ($) | Apr. 30, 2016 USD ($) | Apr. 30, 2015 USD ($) | Apr. 30, 2014 USD ($)
Operating Leased Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Office space | ft² |  |  | 137,615 |  |  |  |  |  |  |  |
Lease commencement date | Dec. 14, 2015 |  |  |  |  |  |  |  |  |  |
Number of successive periods of five years terms | Term |  |  | 2 |  |  |  |  |  |  |  |
Lease extension period |  |  | 5 years |  |  |  |  |  |  |  |
Operating leases, rent expense |  |  |  |  |  |  |  |  | $ 5,000 | $ 4,100 | $ 4,400
Operating leases, future minimum payments due, future minimum sublease rentals |  |  |  |  |  |  | $ 800 |  | 800 |  |
Revolving line of credit |  |  |  |  |  |  | 42,000 |  | 42,000 | 57,000 |
Business combination, consideration transferred |  |  |  |  |  | $ 150,800 |  |  |  |  |
Cash consideration |  |  |  | $ 30,000 |  |  |  |  |  | 30,000 |
Amount that placed into escrow as partial security for the Company's indemnification obligations |  |  |  | $ 4,500 |  |  |  |  |  |  |
Discontinued operation, loss on disposal of discontinued operation, net of tax previously recognized |  |  |  |  |  |  |  | $ (9,192) |  | (10,729) |
Loss on disposal of discontinued operations, net of tax |  |  |  |  |  |  |  |  | 0 | $ (1,537) | $ (9,192)
Loss contingency, damages sought, value |  |  |  |  | $ 91,400 |  |  |  |  |  |
Office space lease contract |  |  |  |  |  |  |  |  |  |  |
Operating Leased Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Security deposit for the lease |  |  | $ 8,000 |  |  |  |  |  |  |  |
Revolving credit facility |  |  |  |  |  |  |  |  |  |  |
Operating Leased Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Repayments of debt |  |  |  |  |  |  | 15,000 |  |  |  |
Revolving line of credit |  |  |  |  |  |  | $ 42,000 |  | $ 42,000 |  |
Credit facility | Revolving credit facility |  |  |  |  |  |  |  |  |  |  |
Operating Leased Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility, maximum borrowing capacity |  | $ 70,000 |  |  |  |  |  |  |  |  |
Revolving letter of credit with a borrowing capacity |  | 15,000 |  |  |  |  |  |  |  |  |
Line of Credit Facility, expiration date |  |  |  |  |  |  |  |  | Nov. 21, 2017 |  |
Swingline Loan | Credit facility | Revolving credit facility |  |  |  |  |  |  |  |  |  |  |
Operating Leased Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility, maximum borrowing capacity |  | $ 3,000 |  |  |  |  |  |  |  |  |
Daily adjusting LIBOR rate | Credit facility | Revolving credit facility |  |  |  |  |  |  |  |  |  |  |
Operating Leased Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest above daily adjusting LIBOR rate |  | 3.50% |  |  |  |  |  |  |  |  |

X
- Definition Amount of gain (loss), after tax expense or benefit, resulting from the sale of a business component. + References No definition available. + Details
Name: | bv_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTaxIncludingPreviouslyRecognized
Namespace Prefix: | bv_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Lease Commencement Date + References No definition available. + Details
Name: | bv_LeaseCommencementDate
Namespace Prefix: | bv_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition Lease Extension Period + References No definition available. + Details
Name: | bv_LeaseExtensionPeriod
Namespace Prefix: | bv_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Letter of credit maximum borrowing capacity. + References No definition available. + Details
Name: | bv_LetterOfCreditMaximumBorrowingCapacity
Namespace Prefix: | bv_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Number Of Successive Five Year Terms + References No definition available. + Details
Name: | bv_NumberOfSuccessiveFiveYearTerms
Namespace Prefix: | bv_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Office Space Under Lease + References No definition available. + Details
Name: | bv_OfficeSpaceUnderLease
Namespace Prefix: | bv_
Data Type: | num:areaItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6578-128477 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6613-128477 + Details
Name: | us-gaap_BusinessCombinationConsiderationTransferred1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount after tax of gain (loss) not previously recognized resulting from the disposal of a discontinued operation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=64827811&loc=d3e1361-107760 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 3B -URI http://asc.fasb.org/extlink&oid=65878427&loc=SL51721525-107759 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 5C -Subparagraph (b)(2) -URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721675-107760 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=65878427&loc=d3e957-107759 + Details
Name: | us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The designation of funds furnished by a borrower to a lender to assure future payments of the borrower's real estate taxes and insurance obligations with respect to a mortgaged property. Escrow deposits may be made for a variety of other purposes such as earnest money and contingent payments. This element excludes replacement reserves which are an escrow separately provided for within the US GAAP taxonomy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.10) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
Name: | us-gaap_EscrowDeposit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line. + References No definition available. + Details
Name: | us-gaap_LeaseAndRentalExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Date the credit facility terminates, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LineOfCreditFacilityExpirationDate1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The carrying value as of the balance sheet date of the noncurrent portion of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 45 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=28361426&loc=d3e1314-112600 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=28361426&loc=d3e1336-112600 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Line-of-Credit Arrangement -URI http://asc.fasb.org/extlink&oid=6517033 + Details
Name: | us-gaap_LongTermLineOfCredit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The value (monetary amount) of the award the plaintiff seeks in the legal matter. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349 + Details
Name: | us-gaap_LossContingencyDamagesSoughtValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- 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_OperatingLeasedAssetsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Contractually required future rental payments receivable on noncancelable subleasing arrangements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717 + Details
Name: | us-gaap_OperatingLeasesFutureMinimumPaymentsDueFutureMinimumSubleaseRentals
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The cash inflow associated with the amount received from the sale of a portion of the company's business, for example a segment, division, branch or other business, during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3179-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 + Details
Name: | us-gaap_ProceedsFromDivestitureOfBusinesses
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations. + References No definition available. + Details
Name: | us-gaap_RepaymentsOfDebt
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition This element represents money paid in advance to protect the provider of a product or service, such as a lessor, against damage or nonpayment by the buyer or tenant (lessee) during the term of the agreement. Such damages may include physical damage to the property, theft of property, and other contractual breaches. Security deposits held may be interest or noninterest bearing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.15(a)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
Name: | us-gaap_SecurityDepositLiability
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_LeaseArrangementTypeAxis=bv_OfficeSpaceLeaseContractMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | bv_LoanAgreementAmendmentAxis=bv_CreditAgreementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ShortTermDebtTypeAxis=bv_SwingLineLoanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |