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Retirement Plans - Narrative (Details) $ in Millions | 9 Months Ended
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Sep. 30, 2025 USD ($)
Retirement Benefits [Abstract] |
Expected return on plan assets, changes in fair value, period (in years) | 5 years
Cash contributions for pension plans | $ 0

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- Definition The expected long-term rate of return on plan assets that is applied to a calculated value of plan assets that recognizes the changes in fair value over this period. + References No definition available. + Details
Name: | unp_DefinedBenefitPlanExpectedReturnOnPlanAssetsChangesInFairValuePeriod
Namespace Prefix: | unp_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_CompensationAndRetirementDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of cash outflow for pension benefit. Includes, but is not limited to, employer contribution to fund plan asset and payment to retiree. Excludes other postretirement benefit. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 45 -Paragraph 25 -Subparagraph (g) -SubTopic 10 -Topic 230 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482740/230-10-45-25 + Details
Name: | us-gaap_PensionContributions
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration