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Retirement Plans (Weighted-Average Actuarial Assumptions Used To Determine Benefit Obligations) (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Discount rate - benefit obligations	2.42%	3.26%
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate - benefit obligations	2.42%	3.26%
Compensation increase - benefit obligations	4.40%	4.10%
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate - benefit obligations	2.22%	3.13%
Health care cost trend rate (employees under 65) - benefit obligations	5.42%	5.64%
Ultimate health care cost trend rate - benefit obligations	4.50%	4.50%
Year ultimate trend reached - benefit obligations	2038	2038

X

- Definition

The ultimate trend rate for health care costs.

+ References

No definition available.

+ Details

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Namespace Prefix:	unp_
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Balance Type:	na
Period Type:	instant

X

- Definition

The assumed health care cost trend rate used to measure the expected cost of benefits covered by the plan (gross eligible charges) for employees under age 65. This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status.

+ References

No definition available.

+ Details

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Namespace Prefix:	unp_
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Balance Type:	na
Period Type:	instant

X

- Definition

Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(1)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

+ Details

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X

- Definition

Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(2)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

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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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X

- Definition

Year ultimate health care cost trend rate is expected to be reached, in CCYY format.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (l)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

+ Details

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- Details

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X

- Details

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