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Note 4 - Retirement Plans (Details Textual) $ in Thousands | 6 Months Ended
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Jun. 30, 2023 USD ($)
Defined Benefit Plan, Expected Return on Plan Assets, Changes In Fair Value, Period (Year) | 5 years
Payment for Pension Benefits | $ 0

X
- 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

X
- 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