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Debt (Shelf Registration And Significant New Borrowings) (Narrative) (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Nov. 14, 2019
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Debt [Abstract] |  |
Board of Directors authorized debt issuance |  | $ 6,000
Board of Directors remaining debt issuance | $ 2,250 |

X
- Definition Amount of debt issuance authorized by Board of Directors. + References No definition available. + Details
Name: | unp_BoardOfDirectorsAuthorizedDebtIssuance
Namespace Prefix: | unp_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of debt issuance remaining under current authority by Board of Directors. + References No definition available. + Details
Name: | unp_BoardOfDirectorsRemainingDebtIssuance
Namespace Prefix: | unp_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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