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Document And Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2021
Jan. 28, 2022
Jun. 30, 2021
Document Information [Line Items]      
Entity Central Index Key 0000100885    
Entity Registrant Name UNION PACIFIC CORP    
Amendment Flag false    
Current Fiscal Year End Date --12-31    
Document Fiscal Period Focus FY    
Document Fiscal Year Focus 2021    
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2021    
Document Transition Report false    
Entity File Number 1-6075    
Entity Incorporation, State or Country Code UT    
Entity Tax Identification Number 13-2626465    
Entity Address, Address Line One 1400 Douglas Street    
Entity Address, City or Town Omaha    
Entity Address, State or Province NE    
Entity Address, Postal Zip Code 68179    
City Area Code 402    
Local Phone Number 544-5000    
Title of 12(b) Security Common Stock (Par Value $2.50 per share)    
Trading Symbol UNP    
Security Exchange Name NYSE    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Public Float     $ 142,000,000,000.0
Entity Common Stock, Shares Outstanding   636,898,957  
Auditor Name Deloitte & Touche LLP    
Auditor Location Omaha, Nebraska    
Auditor Firm ID 34    
ICFR Auditor Attestation Flag true