<?xml version='1.0' encoding='UTF-8'?>
<FilingSummary>
  <Version>3.8.0.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>260</ContextCount>
  <ElementCount>227</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>73</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>11</UnitCount>
  <MyReports>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001000 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/DocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1001000 - Statement - Condensed Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/CondensedConsolidatedBalanceSheets</Role>
      <ShortName>Condensed Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1001501 - Statement - Condensed Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/CondensedConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Condensed Consolidated Balance Sheets (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1002000 - Statement - Condensed Consolidated Statements of Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/CondensedConsolidatedStatementsOfIncome</Role>
      <ShortName>Condensed Consolidated Statements of Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1003000 - Statement - Condensed Consolidated Statements of Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/CondensedConsolidatedStatementsOfComprehensiveIncome</Role>
      <ShortName>Condensed Consolidated Statements of Comprehensive Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1003501 - Statement - Condensed Consolidated Statements of Comprehensive Income (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/CondensedConsolidatedStatementsOfComprehensiveIncomeParenthetical</Role>
      <ShortName>Condensed Consolidated Statements of Comprehensive Income (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1004000 - Statement - Condensed Consolidated Statements of Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/CondensedConsolidatedStatementsOfStockholdersEquity</Role>
      <ShortName>Condensed Consolidated Statements of Stockholders' Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>1004501 - Statement - Condensed Consolidated Statements of Stockholders' Equity (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/CondensedConsolidatedStatementsOfStockholdersEquityParenthetical</Role>
      <ShortName>Condensed Consolidated Statements of Stockholders' Equity (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>1005000 - Statement - Condensed Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/CondensedConsolidatedStatementsOfCashFlows</Role>
      <ShortName>Condensed Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2101100 - Disclosure - Basis of Presentation and Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/BasisOfPresentationAndSignificantAccountingPolicies</Role>
      <ShortName>Basis of Presentation and Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2102100 - Disclosure - Revenue Recognition</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/RevenueRecognition</Role>
      <ShortName>Revenue Recognition</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2106100 - Disclosure - Pension Plans and Postretirement Benefits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/PensionPlansAndPostretirementBenefits</Role>
      <ShortName>Pension Plans and Postretirement Benefits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2107100 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/CommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2108100 - Disclosure - Derivative Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/DerivativeFinancialInstruments</Role>
      <ShortName>Derivative Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2109100 - Disclosure - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/FairValueMeasurements</Role>
      <ShortName>Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2111100 - Disclosure - Stock-Based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/StockBasedCompensation</Role>
      <ShortName>Stock-Based Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2112100 - Disclosure - Segment Reporting</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/SegmentReporting</Role>
      <ShortName>Segment Reporting</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2113100 - Disclosure - Earnings Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/EarningsPerShare</Role>
      <ShortName>Earnings Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2201201 - Disclosure - Basis of Presentation and Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/BasisOfPresentationAndSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Basis of Presentation and Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.bwxt.com/role/BasisOfPresentationAndSignificantAccountingPolicies</ParentRole>
      <Position>19</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2301302 - Disclosure - Basis of Presentation and Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/BasisOfPresentationAndSignificantAccountingPoliciesTables</Role>
      <ShortName>Basis of Presentation and Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bwxt.com/role/BasisOfPresentationAndSignificantAccountingPolicies</ParentRole>
      <Position>20</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2302301 - Disclosure - Revenue Recognition (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/RevenueRecognitionTables</Role>
      <ShortName>Revenue Recognition (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bwxt.com/role/RevenueRecognition</ParentRole>
      <Position>21</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2306301 - Disclosure - Pension Plans and Postretirement Benefits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/PensionPlansAndPostretirementBenefitsTables</Role>
      <ShortName>Pension Plans and Postretirement Benefits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bwxt.com/role/PensionPlansAndPostretirementBenefits</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2308301 - Disclosure - Derivative Financial Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/DerivativeFinancialInstrumentsTables</Role>
      <ShortName>Derivative Financial Instruments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bwxt.com/role/DerivativeFinancialInstruments</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2309301 - Disclosure - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/FairValueMeasurementsTables</Role>
      <ShortName>Fair Value Measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bwxt.com/role/FairValueMeasurements</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2312301 - Disclosure - Segment Reporting (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/SegmentReportingTables</Role>
      <ShortName>Segment Reporting (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bwxt.com/role/SegmentReporting</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2313301 - Disclosure - Earnings Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/EarningsPerShareTables</Role>
      <ShortName>Earnings Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bwxt.com/role/EarningsPerShare</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2401403 - Disclosure - Basis of Presentation and Significant Accounting Policies - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/BasisOfPresentationAndSignificantAccountingPoliciesAdditionalInformationDetails</Role>
      <ShortName>Basis of Presentation and Significant Accounting Policies - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2401404 - Disclosure - Basis of Presentation and Significant Accounting Policies - Accumulated Other Comprehensive Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/BasisOfPresentationAndSignificantAccountingPoliciesAccumulatedOtherComprehensiveIncomeDetails</Role>
      <ShortName>Basis of Presentation and Significant Accounting Policies - Accumulated Other Comprehensive Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2401405 - Disclosure - Basis of Presentation and Significant Accounting Policies - Schedule of Amounts Reclassified from Accumulated Other Comprehensive Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/BasisOfPresentationAndSignificantAccountingPoliciesScheduleOfAmountsReclassifiedFromAccumulatedOtherComprehensiveIncomeDetails</Role>
      <ShortName>Basis of Presentation and Significant Accounting Policies - Schedule of Amounts Reclassified from Accumulated Other Comprehensive Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2401406 - Disclosure - Basis of Presentation and Significant Accounting Policies - Reconciliation of Cash and Cash Equivalents and Restricted Cash and Cash Equivalents (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/BasisOfPresentationAndSignificantAccountingPoliciesReconciliationOfCashAndCashEquivalentsAndRestrictedCashAndCashEquivalentsDetails</Role>
      <ShortName>Basis of Presentation and Significant Accounting Policies - Reconciliation of Cash and Cash Equivalents and Restricted Cash and Cash Equivalents (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2401407 - Disclosure - Basis of Presentation and Significant Accounting Policies - Summary of Changes in Carrying Amount of Accrued Warranty Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/BasisOfPresentationAndSignificantAccountingPoliciesSummaryOfChangesInCarryingAmountOfAccruedWarrantyExpenseDetails</Role>
      <ShortName>Basis of Presentation and Significant Accounting Policies - Summary of Changes in Carrying Amount of Accrued Warranty Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2402402 - Disclosure - Revenue Recognition - Initial Impact of New Revenue Recognition Standard (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/RevenueRecognitionInitialImpactOfNewRevenueRecognitionStandardDetails</Role>
      <ShortName>Revenue Recognition - Initial Impact of New Revenue Recognition Standard (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2402403 - Disclosure - Revenue Recognition - Disaggregated Revenues (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/RevenueRecognitionDisaggregatedRevenuesDetails</Role>
      <ShortName>Revenue Recognition - Disaggregated Revenues (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2402404 - Disclosure - Revenue Recognition (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/RevenueRecognitionDetails</Role>
      <ShortName>Revenue Recognition (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bwxt.com/role/RevenueRecognitionTables</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2402405 - Disclosure - Revenue Recognition - Contract Assets and Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/RevenueRecognitionContractAssetsAndLiabilitiesDetails</Role>
      <ShortName>Revenue Recognition - Contract Assets and Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2402406 - Disclosure - Revenue Recognition - Remaining Performance Obligations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/RevenueRecognitionRemainingPerformanceObligationsDetails</Role>
      <ShortName>Revenue Recognition - Remaining Performance Obligations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2402407 - Disclosure - Revenue Recognition - Historical Method (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/RevenueRecognitionHistoricalMethodDetails</Role>
      <ShortName>Revenue Recognition - Historical Method (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2406402 - Disclosure - Pension Plans and Postretirement Benefits - (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/PensionPlansAndPostretirementBenefitsDetails</Role>
      <ShortName>Pension Plans and Postretirement Benefits - (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bwxt.com/role/PensionPlansAndPostretirementBenefitsTables</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2406403 - Disclosure - Pension Plans and Postretirement Benefits - Components of Net Periodic Benefit Cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/PensionPlansAndPostretirementBenefitsComponentsOfNetPeriodicBenefitCostDetails</Role>
      <ShortName>Pension Plans and Postretirement Benefits - Components of Net Periodic Benefit Cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2407401 - Disclosure - Commitments and Contingencies - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/CommitmentsAndContingenciesAdditionalInformationDetails</Role>
      <ShortName>Commitments and Contingencies - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2408402 - Disclosure - Derivative Financial Instruments - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/DerivativeFinancialInstrumentsAdditionalInformationDetails</Role>
      <ShortName>Derivative Financial Instruments - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>2408403 - Disclosure - Derivative Financial Instruments - Summary of Derivative Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/DerivativeFinancialInstrumentsSummaryOfDerivativeFinancialInstrumentsDetails</Role>
      <ShortName>Derivative Financial Instruments - Summary of Derivative Financial Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>2408404 - Disclosure - Derivative Financial Instruments - Schedule of Effect of Derivative Instruments on Statements of Financial Performance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/DerivativeFinancialInstrumentsScheduleOfEffectOfDerivativeInstrumentsOnStatementsOfFinancialPerformanceDetails</Role>
      <ShortName>Derivative Financial Instruments - Schedule of Effect of Derivative Instruments on Statements of Financial Performance (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>2409402 - Disclosure - Fair Value Measurements - Summary of Investments and Available-for-Sale Securities Measured at Fair Value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/FairValueMeasurementsSummaryOfInvestmentsAndAvailableForSaleSecuritiesMeasuredAtFairValueDetails</Role>
      <ShortName>Fair Value Measurements - Summary of Investments and Available-for-Sale Securities Measured at Fair Value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>2409403 - Disclosure - Fair Value Measurements - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/FairValueMeasurementsAdditionalInformationDetails</Role>
      <ShortName>Fair Value Measurements - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>2411401 - Disclosure - Stock-Based Compensation - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/StockBasedCompensationAdditionalInformationDetails</Role>
      <ShortName>Stock-Based Compensation - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>2412402 - Disclosure - Segment Reporting - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/SegmentReportingAdditionalInformationDetails</Role>
      <ShortName>Segment Reporting - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>2412403 - Disclosure - Segment Reporting - Schedule of Operating Results by Segment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/SegmentReportingScheduleOfOperatingResultsBySegmentDetails</Role>
      <ShortName>Segment Reporting - Schedule of Operating Results by Segment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="bwxt-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>2413402 - Disclosure - Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bwxt.com/role/EarningsPerShareComputationOfBasicAndDilutedEarningsPerShareDetails</Role>
      <ShortName>Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>bwxt-20180331.xml</File>
    <File>bwxt-20180331.xsd</File>
    <File>bwxt-20180331_cal.xml</File>
    <File>bwxt-20180331_def.xml</File>
    <File>bwxt-20180331_lab.xml</File>
    <File>bwxt-20180331_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://xbrl.sec.gov/invest/2013-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/country/2017-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2017-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2014-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
