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Debt (Shelf Registration And Significant New Borrowings) (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Nov. 14, 2019
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