Commitments And Contingencies (Narrative) (Details) $ in Millions	Mar. 31, 2021 USD ($) item
Commitments and Contingencies [Line Items]
Sites identified which we are or may be liable for remediation costs	368
Sites subject of actions taken by the U.S. government	26
Sites on the Superfund National Priorities List	16
Personal Injury [Member]
Commitments and Contingencies [Line Items]
Percent of liability recorded related to asserted claims	94.00%
Percent of liability recorded related to unasserted claims	6.00%
Personal Injury [Member] \| Minimum [Member]
Commitments and Contingencies [Line Items]
Reasonably possible outcome of related claims \| $	$ 293
Personal Injury [Member] \| Maximum [Member]
Commitments and Contingencies [Line Items]
Reasonably possible outcome of related claims \| $	$ 320

Percentage of the recorded liability related to asserted claims.

No definition available.

Name:	unp_RecordedLiabilityRelatedToAssertedClaims
Namespace Prefix:	unp_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of the recorded liability related to unasserted claims.

+ References

No definition available.

+ Details

Name:	unp_RecordedLiabilityRelatedToUnassertedClaims
Namespace Prefix:	unp_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Sites identified at which Railroad is or may be liable for remediation costs associated with alleged contamination or for violations of environmental requirements.

+ References

No definition available.

+ Details

Name:	unp_SitesIdentified
Namespace Prefix:	unp_
Data Type:	xbrli:positiveIntegerItemType
Balance Type:	na
Period Type:	instant

- Definition

Sites on the Superfund National Priorities List.

+ References

No definition available.

+ Details

Name:	unp_SitesOnSuperfundNationalPrioritiesList
Namespace Prefix:	unp_
Data Type:	xbrli:positiveIntegerItemType
Balance Type:	na
Period Type:	instant

- Definition

Sites subject of actions taken by the U.S. government.

+ References

No definition available.

+ Details

Name:	unp_SitesSubjectOfActionsTakenByUsGovernment
Namespace Prefix:	unp_
Data Type:	xbrli:positiveIntegerItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_LossContingenciesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Reflects the estimated amount of loss from the specified contingency as of the balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14435-108349

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 9
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14557-108349

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=121555522&loc=d3e12021-110248

+ Details

Name:	us-gaap_LossContingencyEstimateOfPossibleLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=unp_PersonalInjuryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: