<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.20.4</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>347</ContextCount>
  <ElementCount>614</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>90</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>8</UnitCount>
  <MyReports>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>00090 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>00100 - Statement - Consolidated Statements Of Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/StatementConsolidatedStatementsOfIncome</Role>
      <ShortName>Consolidated Statements Of Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>00200 - Statement - Consolidated Statements Of Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/StatementConsolidatedStatementsOfComprehensiveIncome</Role>
      <ShortName>Consolidated Statements Of Comprehensive Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>00205 - Statement - Consolidated Statements Of Comprehensive Income (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/StatementConsolidatedStatementsOfComprehensiveIncomeParentheticals</Role>
      <ShortName>Consolidated Statements Of Comprehensive Income (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>00300 - Statement - Consolidated Statements Of Financial Position</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/StatementConsolidatedStatementsOfFinancialPosition</Role>
      <ShortName>Consolidated Statements Of Financial Position</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>00305 - Statement - Consolidated Statements Of Financial Position (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/StatementConsolidatedStatementsOfFinancialPositionParentheticals</Role>
      <ShortName>Consolidated Statements Of Financial Position (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>00400 - Statement - Consolidated Statements Of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/StatementConsolidatedStatementsOfCashFlows</Role>
      <ShortName>Consolidated Statements Of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>00500 - Statement - Consolidated Statements Of Changes In Common Shareholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/StatementConsolidatedStatementsOfChangesInCommonShareholdersEquity</Role>
      <ShortName>Consolidated Statements Of Changes In Common Shareholders' Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>00505 - Statement - Consolidated Statements of Changes in Common Shareholders' Equity (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/StatementConsolidatedStatementsOfChangesInCommonShareholdersEquityParentheticals</Role>
      <ShortName>Consolidated Statements of Changes in Common Shareholders' Equity (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>10101 - Disclosure - Nature Of Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureNatureOfOperations</Role>
      <ShortName>Nature Of Operations</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>10201 - Disclosure - Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureSignificantAccountingPolicies</Role>
      <ShortName>Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>10301 - Disclosure - Accounting Pronouncements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureAccountingPronouncements</Role>
      <ShortName>Accounting Pronouncements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>10501 - Disclosure - Stock Options And Other Stock Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureStockOptionsAndOtherStockPlans</Role>
      <ShortName>Stock Options And Other Stock Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>10601 - Disclosure - Retirement Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureRetirementPlans</Role>
      <ShortName>Retirement Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>10701 - Disclosure - Other Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureOtherIncome</Role>
      <ShortName>Other Income</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>10801 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureIncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>10901 - Disclosure - Earnings Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureEarningsPerShare</Role>
      <ShortName>Earnings Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>11001 - Disclosure - Accumulated Other Comprehensive Income/(Loss)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureAccumulatedOtherComprehensiveIncomeLoss</Role>
      <ShortName>Accumulated Other Comprehensive Income/(Loss)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>11101 - Disclosure - Accounts Receivable</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureAccountsReceivable</Role>
      <ShortName>Accounts Receivable</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>11201 - Disclosure - Properties</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureProperties</Role>
      <ShortName>Properties</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>11301 - Disclosure - Accounts Payable And Other Current Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureAccountsPayableAndOtherCurrentLiabilities</Role>
      <ShortName>Accounts Payable And Other Current Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>11401 - Disclosure - Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureFinancialInstruments</Role>
      <ShortName>Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>11501 - Disclosure - Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureDebt</Role>
      <ShortName>Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>11601 - Disclosure - Variable Interest Entities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureVariableInterestEntities</Role>
      <ShortName>Variable Interest Entities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>11701 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureLeases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>11801 - Disclosure - Commitments And Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureCommitmentsAndContingencies</Role>
      <ShortName>Commitments And Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>11901 - Disclosure - Share Repurchase Program</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureShareRepurchaseProgram</Role>
      <ShortName>Share Repurchase Program</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>12001 - Disclosure - Related Parties</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureRelatedParties</Role>
      <ShortName>Related Parties</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>12101 - Disclosure - Selected Quarterly Data (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureSelectedQuarterlyDataUnaudited</Role>
      <ShortName>Selected Quarterly Data (Unaudited)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>12201 - Disclosure - Schedule Of Valuation And Qualifying Accounts</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureScheduleOfValuationAndQualifyingAccounts</Role>
      <ShortName>Schedule Of Valuation And Qualifying Accounts</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>20202 - Disclosure - Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureSignificantAccountingPolicies</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>30103 - Disclosure - Nature Of Operations (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureNatureOfOperationsTables</Role>
      <ShortName>Nature Of Operations (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureNatureOfOperations</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>30203 - Disclosure - Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureSignificantAccountingPoliciesTables</Role>
      <ShortName>Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureSignificantAccountingPolicies</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>30503 - Disclosure - Stock Options And Other Stock Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureStockOptionsAndOtherStockPlansTables</Role>
      <ShortName>Stock Options And Other Stock Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureStockOptionsAndOtherStockPlans</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>30603 - Disclosure - Retirement Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureRetirementPlansTables</Role>
      <ShortName>Retirement Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureRetirementPlans</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>30703 - Disclosure - Other Income (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureOtherIncomeTables</Role>
      <ShortName>Other Income (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureOtherIncome</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>30803 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureIncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureIncomeTaxes</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>30903 - Disclosure - Earnings Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureEarningsPerShareTables</Role>
      <ShortName>Earnings Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureEarningsPerShare</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>31003 - Disclosure - Accumulated Other Comprehensive Income/(Loss) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureAccumulatedOtherComprehensiveIncomeLossTables</Role>
      <ShortName>Accumulated Other Comprehensive Income/(Loss) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureAccumulatedOtherComprehensiveIncomeLoss</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>31203 - Disclosure - Properties (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosurePropertiesTables</Role>
      <ShortName>Properties (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureProperties</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>31303 - Disclosure - Accounts Payable And Other Current Liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureAccountsPayableAndOtherCurrentLiabilitiesTables</Role>
      <ShortName>Accounts Payable And Other Current Liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureAccountsPayableAndOtherCurrentLiabilities</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>31503 - Disclosure - Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureDebtTables</Role>
      <ShortName>Debt (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureDebt</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>31703 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureLeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureLeases</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>31803 - Disclosure - Commitments And Contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureCommitmentsAndContingenciesTables</Role>
      <ShortName>Commitments And Contingencies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureCommitmentsAndContingencies</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>31903 - Disclosure - Share Repurchase Program (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureShareRepurchaseProgramTables</Role>
      <ShortName>Share Repurchase Program (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureShareRepurchaseProgram</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>32103 - Disclosure - Selected Quarterly Data (Unaudited) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureSelectedQuarterlyDataUnauditedTables</Role>
      <ShortName>Selected Quarterly Data (Unaudited) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureSelectedQuarterlyDataUnaudited</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>40101 - Disclosure - Nature Of Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureNatureOfOperationsDetails</Role>
      <ShortName>Nature Of Operations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureNatureOfOperationsTables</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>40201 - Disclosure - Significant Accounting Policies (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureSignificantAccountingPoliciesDetails</Role>
      <ShortName>Significant Accounting Policies (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureSignificantAccountingPoliciesTables</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>40202 - Disclosure - Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restriced Cash (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureSignificantAccountingPoliciesReconciliationOfCashCashEquivalentsAndRestricedCashDetails</Role>
      <ShortName>Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restriced Cash (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>40301 - Disclosure - Accounting Pronouncements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureAccountingPronouncementsDetails</Role>
      <ShortName>Accounting Pronouncements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureAccountingPronouncements</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>40501 - Disclosure - Stock Options And Other Stock Plans (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureStockOptionsAndOtherStockPlansNarrativeDetails</Role>
      <ShortName>Stock Options And Other Stock Plans (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureStockOptionsAndOtherStockPlansTables</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>40502 - Disclosure - Stock Options And Other Stock Plans (Stock Options, Retention Awards And Performance Retention Awards) (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureStockOptionsAndOtherStockPlansStockOptionsRetentionAwardsAndPerformanceRetentionAwardsNarrativeDetails</Role>
      <ShortName>Stock Options And Other Stock Plans (Stock Options, Retention Awards And Performance Retention Awards) (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureStockOptionsAndOtherStockPlansTables</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>40503 - Disclosure - Stock-based Compensation (Schedule Of Stock-Based Compensation) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureStockBasedCompensationScheduleOfStockBasedCompensationDetails</Role>
      <ShortName>Stock-based Compensation (Schedule Of Stock-Based Compensation) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>40504 - Disclosure - Stock-based Compensation (Schedule Of Stock Options Weighted Average Assumptions) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureStockBasedCompensationScheduleOfStockOptionsWeightedAverageAssumptionsDetails</Role>
      <ShortName>Stock-based Compensation (Schedule Of Stock Options Weighted Average Assumptions) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>40505 - Disclosure - Stock-based Compensation (Summary Of Stock Option Activities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureStockBasedCompensationSummaryOfStockOptionActivitiesDetails</Role>
      <ShortName>Stock-based Compensation (Summary Of Stock Option Activities) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>40506 - Disclosure - Stock-based Compensation (Schedule Of Stock Option Exercises) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureStockBasedCompensationScheduleOfStockOptionExercisesDetails</Role>
      <ShortName>Stock-based Compensation (Schedule Of Stock Option Exercises) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>40507 - Disclosure - Stock-based Compensation (Schedule Of Retention Awards) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureStockBasedCompensationScheduleOfRetentionAwardsDetails</Role>
      <ShortName>Stock-based Compensation (Schedule Of Retention Awards) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>40509 - Disclosure - Stock-based Compensation (Schedule Of Performance Retention Awards) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureStockBasedCompensationScheduleOfPerformanceRetentionAwardsDetails</Role>
      <ShortName>Stock-based Compensation (Schedule Of Performance Retention Awards) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>40601 - Disclosure - Retirement Plans (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureRetirementPlansNarrativeDetails</Role>
      <ShortName>Retirement Plans (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureRetirementPlansTables</ParentRole>
      <Position>59</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>40602 - Disclosure - Retirement Plans (Other Retirement Programs) (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureRetirementPlansOtherRetirementProgramsNarrativeDetails</Role>
      <ShortName>Retirement Plans (Other Retirement Programs) (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureRetirementPlansTables</ParentRole>
      <Position>60</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>40603 - Disclosure - Retirement Plans (Schedule Of Changes In Projected Benefit Obligation And Plan Assets) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureRetirementPlansScheduleOfChangesInProjectedBenefitObligationAndPlanAssetsDetails</Role>
      <ShortName>Retirement Plans (Schedule Of Changes In Projected Benefit Obligation And Plan Assets) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureRetirementPlansTables</ParentRole>
      <Position>61</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>40604 - Disclosure - Retirement Plans (Schedule Of Amounts Recognized In Statement Of Financial Position) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureRetirementPlansScheduleOfAmountsRecognizedInStatementOfFinancialPositionDetails</Role>
      <ShortName>Retirement Plans (Schedule Of Amounts Recognized In Statement Of Financial Position) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureRetirementPlansTables</ParentRole>
      <Position>62</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>40605 - Disclosure - Retirement Plans (Schedule Of Pre-Tax Amounts Recognized In Accumulated Other Comprehensive Income/(Loss)) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureRetirementPlansScheduleOfPreTaxAmountsRecognizedInAccumulatedOtherComprehensiveIncomeLossDetails</Role>
      <ShortName>Retirement Plans (Schedule Of Pre-Tax Amounts Recognized In Accumulated Other Comprehensive Income/(Loss)) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureRetirementPlansTables</ParentRole>
      <Position>63</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>40606 - Disclosure - Retirement Plans (Schedule Of Other Pre-Tax Changes Recognized In Other Comprehensive Income) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureRetirementPlansScheduleOfOtherPreTaxChangesRecognizedInOtherComprehensiveIncomeDetails</Role>
      <ShortName>Retirement Plans (Schedule Of Other Pre-Tax Changes Recognized In Other Comprehensive Income) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureRetirementPlansTables</ParentRole>
      <Position>64</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>40608 - Disclosure - Retirement Plans (Underfunded Accumulated Benefit Obligation) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureRetirementPlansUnderfundedAccumulatedBenefitObligationDetails</Role>
      <ShortName>Retirement Plans (Underfunded Accumulated Benefit Obligation) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureRetirementPlansTables</ParentRole>
      <Position>65</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>40609 - Disclosure - Retirement Plans (Weighted-Average Actuarial Assumptions Used To Determine Benefit Obligations) (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureRetirementPlansWeightedAverageActuarialAssumptionsUsedToDetermineBenefitObligationsDetail</Role>
      <ShortName>Retirement Plans (Weighted-Average Actuarial Assumptions Used To Determine Benefit Obligations) (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureRetirementPlansTables</ParentRole>
      <Position>66</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>40610 - Disclosure - Retirement Plans (Net Periodic Pension And OPEB Cost) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureRetirementPlansNetPeriodicPensionAndOpebCostDetails</Role>
      <ShortName>Retirement Plans (Net Periodic Pension And OPEB Cost) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureRetirementPlansTables</ParentRole>
      <Position>67</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>40611 - Disclosure - Retirement Plans (Weighted-Average Actuarial Assumptions Used To Determine Benefit Expense) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureRetirementPlansWeightedAverageActuarialAssumptionsUsedToDetermineBenefitExpenseDetails</Role>
      <ShortName>Retirement Plans (Weighted-Average Actuarial Assumptions Used To Determine Benefit Expense) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureRetirementPlansTables</ParentRole>
      <Position>68</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>40613 - Disclosure - Retirement Plans (Cash Contributions For Qualified Pension Plan Benefit Payments For Non-Qualified OPEB Plans) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureRetirementPlansCashContributionsForQualifiedPensionPlanBenefitPaymentsForNonQualifiedOpebPlansDetails</Role>
      <ShortName>Retirement Plans (Cash Contributions For Qualified Pension Plan Benefit Payments For Non-Qualified OPEB Plans) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureRetirementPlansTables</ParentRole>
      <Position>69</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>40614 - Disclosure - Retirement Plans (Schedule Of Expected Benefit Payments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureRetirementPlansScheduleOfExpectedBenefitPaymentsDetails</Role>
      <ShortName>Retirement Plans (Schedule Of Expected Benefit Payments) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureRetirementPlansTables</ParentRole>
      <Position>70</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>40615 - Disclosure - Retirement Plans (Schedule Of Pension Plan Asset Allocation) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureRetirementPlansScheduleOfPensionPlanAssetAllocationDetails</Role>
      <ShortName>Retirement Plans (Schedule Of Pension Plan Asset Allocation) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureRetirementPlansTables</ParentRole>
      <Position>71</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>40616 - Disclosure - Retirement Plans (Schedule Of Assets Measured At Fair Value On A Recurring Basis) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureRetirementPlansScheduleOfAssetsMeasuredAtFairValueOnRecurringBasisDetails</Role>
      <ShortName>Retirement Plans (Schedule Of Assets Measured At Fair Value On A Recurring Basis) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureRetirementPlansTables</ParentRole>
      <Position>72</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>40701 - Disclosure - Other Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureOtherIncomeDetails</Role>
      <ShortName>Other Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureOtherIncomeTables</ParentRole>
      <Position>73</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>40801 - Disclosure - Income Taxes (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureIncomeTaxesNarrativeDetails</Role>
      <ShortName>Income Taxes (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureIncomeTaxesTables</ParentRole>
      <Position>74</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>40802 - Disclosure - Income Taxes (Schedule Of Components Of Income Tax Expense) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureIncomeTaxesScheduleOfComponentsOfIncomeTaxExpenseDetails</Role>
      <ShortName>Income Taxes (Schedule Of Components Of Income Tax Expense) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureIncomeTaxesTables</ParentRole>
      <Position>75</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>40803 - Disclosure - Income Taxes (Reconciliations Between Statutory And Effective Tax Rates) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureIncomeTaxesReconciliationsBetweenStatutoryAndEffectiveTaxRatesDetails</Role>
      <ShortName>Income Taxes (Reconciliations Between Statutory And Effective Tax Rates) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureIncomeTaxesTables</ParentRole>
      <Position>76</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>40804 - Disclosure - Income Taxes (Schedule Of Deferred Income Tax Liabilities And Assets) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureIncomeTaxesScheduleOfDeferredIncomeTaxLiabilitiesAndAssetsDetails</Role>
      <ShortName>Income Taxes (Schedule Of Deferred Income Tax Liabilities And Assets) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureIncomeTaxesTables</ParentRole>
      <Position>77</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>40805 - Disclosure - Income Taxes (Reconciliation Of Changes In Unrecognized Tax Benefits Liabilities/(Assets)) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureIncomeTaxesReconciliationOfChangesInUnrecognizedTaxBenefitsLiabilitiesAssetsDetails</Role>
      <ShortName>Income Taxes (Reconciliation Of Changes In Unrecognized Tax Benefits Liabilities/(Assets)) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureIncomeTaxesTables</ParentRole>
      <Position>78</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>40806 - Disclosure - Income Taxes (Unrecognized Tax Benefits That Would/Not Reduce The Effective Tax Rate) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureIncomeTaxesUnrecognizedTaxBenefitsThatWouldNotReduceEffectiveTaxRateDetails</Role>
      <ShortName>Income Taxes (Unrecognized Tax Benefits That Would/Not Reduce The Effective Tax Rate) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureIncomeTaxesTables</ParentRole>
      <Position>79</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>40901 - Disclosure - Earnings Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureEarningsPerShareDetails</Role>
      <ShortName>Earnings Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureEarningsPerShareTables</ParentRole>
      <Position>80</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>41001 - Disclosure - Accumulated Other Comprehensive Income/(Loss) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureAccumulatedOtherComprehensiveIncomeLossDetails</Role>
      <ShortName>Accumulated Other Comprehensive Income/(Loss) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureAccumulatedOtherComprehensiveIncomeLossTables</ParentRole>
      <Position>81</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>41101 - Disclosure - Accounts Receivable (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureAccountsReceivableDetails</Role>
      <ShortName>Accounts Receivable (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureAccountsReceivable</ParentRole>
      <Position>82</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>41201 - Disclosure - Properties (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosurePropertiesDetails</Role>
      <ShortName>Properties (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosurePropertiesTables</ParentRole>
      <Position>83</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>41301 - Disclosure - Accounts Payable And Other Current Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureAccountsPayableAndOtherCurrentLiabilitiesDetails</Role>
      <ShortName>Accounts Payable And Other Current Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureAccountsPayableAndOtherCurrentLiabilitiesTables</ParentRole>
      <Position>84</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>41401 - Disclosure - Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureFinancialInstrumentsDetails</Role>
      <ShortName>Financial Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureFinancialInstruments</ParentRole>
      <Position>85</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>41501 - Disclosure - Debt (Equipment Encumbrances) (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureDebtEquipmentEncumbrancesNarrativeDetails</Role>
      <ShortName>Debt (Equipment Encumbrances) (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureDebtTables</ParentRole>
      <Position>86</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>41502 - Disclosure - Debt (Credit Facilities) (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureDebtCreditFacilitiesNarrativeDetails</Role>
      <ShortName>Debt (Credit Facilities) (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureDebtTables</ParentRole>
      <Position>87</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>41504 - Disclosure - Debt (Shelf Registration And Significant New Borrowings) (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureDebtShelfRegistrationAndSignificantNewBorrowingsNarrativeDetails</Role>
      <ShortName>Debt (Shelf Registration And Significant New Borrowings) (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureDebtTables</ParentRole>
      <Position>88</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>41506 - Disclosure - Debt (Debt Exchange) (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureDebtDebtExchangeNarrativeDetails</Role>
      <ShortName>Debt (Debt Exchange) (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureDebtTables</ParentRole>
      <Position>89</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>41507 - Disclosure - Debt (Debt Redemption) (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureDebtDebtRedemptionNarrativeDetails</Role>
      <ShortName>Debt (Debt Redemption) (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureDebtTables</ParentRole>
      <Position>90</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>41508 - Disclosure - Debt (Receivables Securitization Facility) (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureDebtReceivablesSecuritizationFacilityNarrativeDetails</Role>
      <ShortName>Debt (Receivables Securitization Facility) (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureDebtTables</ParentRole>
      <Position>91</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>41509 - Disclosure - Debt (Schedule Of Debt) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureDebtScheduleOfDebtDetails</Role>
      <ShortName>Debt (Schedule Of Debt) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureDebtTables</ParentRole>
      <Position>92</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>41510 - Disclosure - Debt (Aggregate Debt Maturities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureDebtAggregateDebtMaturitiesDetails</Role>
      <ShortName>Debt (Aggregate Debt Maturities) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureDebtTables</ParentRole>
      <Position>93</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>41511 - Disclosure - Debt (Unsecured Fixed Rate Debt Securities Issued Under Current Shelf Registration) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureDebtUnsecuredFixedRateDebtSecuritiesIssuedUnderCurrentShelfRegistrationDetails</Role>
      <ShortName>Debt (Unsecured Fixed Rate Debt Securities Issued Under Current Shelf Registration) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureDebtTables</ParentRole>
      <Position>94</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>41601 - Disclosure - Variable Interest Entities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureVariableInterestEntitiesDetails</Role>
      <ShortName>Variable Interest Entities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureVariableInterestEntities</ParentRole>
      <Position>95</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>41701 - Disclosure - Leases (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureLeasesNarrativeDetails</Role>
      <ShortName>Leases (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureLeasesTables</ParentRole>
      <Position>96</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>41702 - Disclosure - Leases (Lease Assets and Liabilities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureLeasesLeaseAssetsAndLiabilitiesDetails</Role>
      <ShortName>Leases (Lease Assets and Liabilities) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureLeasesTables</ParentRole>
      <Position>97</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>41703 - Disclosure - Leases (Lease Cost) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureLeasesLeaseCostDetails</Role>
      <ShortName>Leases (Lease Cost) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureLeasesTables</ParentRole>
      <Position>98</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>41704 - Disclosure - Leases (Schedule of Aggregate Lease Maturities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureLeasesScheduleOfAggregateLeaseMaturitiesDetails</Role>
      <ShortName>Leases (Schedule of Aggregate Lease Maturities) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureLeasesTables</ParentRole>
      <Position>99</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>41705 - Disclosure - Leases (Lease Term and Discount Rate) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureLeasesLeaseTermAndDiscountRateDetails</Role>
      <ShortName>Leases (Lease Term and Discount Rate) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureLeasesTables</ParentRole>
      <Position>100</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>41706 - Disclosure - Leases (Other Information) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureLeasesOtherInformationDetails</Role>
      <ShortName>Leases (Other Information) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureLeasesTables</ParentRole>
      <Position>101</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>41801 - Disclosure - Commitments And Contingencies (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureCommitmentsAndContingenciesNarrativeDetails</Role>
      <ShortName>Commitments And Contingencies (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureCommitmentsAndContingenciesTables</ParentRole>
      <Position>102</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>41802 - Disclosure - Commitments And Contingencies (Commitments And Contingencies Activity) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureCommitmentsAndContingenciesCommitmentsAndContingenciesActivityDetails</Role>
      <ShortName>Commitments And Contingencies (Commitments And Contingencies Activity) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureCommitmentsAndContingenciesTables</ParentRole>
      <Position>103</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R104.htm</HtmlFileName>
      <LongName>41901 - Disclosure - Share Repurchase Program (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureShareRepurchaseProgramDetails</Role>
      <ShortName>Share Repurchase Program (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureShareRepurchaseProgramTables</ParentRole>
      <Position>104</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R105.htm</HtmlFileName>
      <LongName>42001 - Disclosure - Related Parties (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureRelatedPartiesDetails</Role>
      <ShortName>Related Parties (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureRelatedParties</ParentRole>
      <Position>105</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R106.htm</HtmlFileName>
      <LongName>42101 - Disclosure - Selected Quarterly Data (Unaudited) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureSelectedQuarterlyDataUnauditedDetails</Role>
      <ShortName>Selected Quarterly Data (Unaudited) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureSelectedQuarterlyDataUnauditedTables</ParentRole>
      <Position>106</Position>
    </Report>
    <Report instance="unp-20201231x10k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R107.htm</HtmlFileName>
      <LongName>42201 - Disclosure - Schedule Of Valuation And Qualifying Accounts (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.up.com/role/DisclosureScheduleOfValuationAndQualifyingAccountsDetails</Role>
      <ShortName>Schedule Of Valuation And Qualifying Accounts (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.up.com/role/DisclosureScheduleOfValuationAndQualifyingAccounts</ParentRole>
      <Position>107</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="10-K" original="unp-20201231x10k.htm">unp-20201231x10k.htm</File>
    <File>unp-20201231.xsd</File>
    <File>unp-20201231_cal.xml</File>
    <File>unp-20201231_def.xml</File>
    <File>unp-20201231_lab.xml</File>
    <File>unp-20201231_pre.xml</File>
    <File>unp-20201231xex10_a.htm</File>
    <File>unp-20201231xex10_b.htm</File>
    <File>unp-20201231xex10_c.htm</File>
    <File>unp-20201231xex10_d.htm</File>
    <File>unp-20201231xex21.htm</File>
    <File>unp-20201231xex23.htm</File>
    <File>unp-20201231xex24.htm</File>
    <File>unp-20201231xex31_a.htm</File>
    <File>unp-20201231xex31_b.htm</File>
    <File>unp-20201231xex32.htm</File>
  </InputFiles>
  <SupplementalFiles>
    <File>unp-20201231x10kg001.jpg</File>
    <File>unp-20201231x10kg002.jpg</File>
    <File>unp-20201231x10kg003.jpg</File>
    <File>unp-20201231x10kg004.jpg</File>
    <File>unp-20201231x10kg005.jpg</File>
    <File>unp-20201231x10kg006.jpg</File>
    <File>unp-20201231x10kg007.jpg</File>
    <File>unp-20201231x10kg008.jpg</File>
    <File>unp-20201231x10kg009.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy>http://xbrl.sec.gov/country/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/srt/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2020-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
