Note 4 - Stock-based Compensation (Details Textual) shares in Thousands, $ in Millions	6 Months Ended
Jun. 30, 2022 USD ($) shares
Share-Based Payment Arrangement, Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year)	10 years
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)	3 years
Share-based Compensation Arrangement by Share-based Payment Award, Conditions (in shares) \| shares	0
Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total	$ 23
Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year)	1 year 6 months
Retention Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)	4 years
Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total	$ 113
Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year)	2 years
Performance Retention Awards [Member]
Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total	$ 37
Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year)	1 year 8 months 12 days
Share-based Compensation Arrangement by Share-based Payment Award, Award Requisite Service Period (Month)	37 months
Share-based Compensation Arrangement by Share-based Payment Award, Performance Period (Year)	3 years

Represents the number of awards subject to performance or market-based vesting conditions.

No definition available.

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Namespace Prefix:	unp_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the performance period of share-based payment awards.

+ References

No definition available.

+ Details

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Namespace Prefix:	unp_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

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- Definition

Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

+ Details

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- Definition

Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

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Period Type:	duration

- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

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Namespace Prefix:
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- Details

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- Details

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