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Retirement Plans (Cash Contributions For Qualified Pension Plan Benefit Payments For Non-Qualified OPEB Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash contribution to non-qualified OPEB plan	$ 19	$ 22
Pension Non-qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to non-qualified pension plan	$ 31	$ 30

X

- Definition

The amount of contributions made by the employer that were non-qualified.

+ References

No definition available.

+ Details

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Namespace Prefix:	unp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- 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

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

X

- Definition

Amount of cash outflow for other postretirement benefit. Includes, but is not limited to, employer contribution to fund plan asset and payment to retiree. Excludes pension benefit.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3536-108585

+ Details

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Balance Type:	credit
Period Type:	duration

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: