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Debt (Unsecured Fixed Rate Debt Securities Issued Under Current Shelf Registration) (Details) (Unsecured Debt [Member], USD $)
 3 Months Ended
Mar. 31, 2015
Jan. 29, 2015
1.80% Notes Due 2020 [Member]
    
Debt Instrument [Line Item]    
Issuance date Jan. 29, 2015  
Note amount   $ 250,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= unp_NotesDue2020180PercentMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_UnsecuredDebtMember
Interest rate on note   1.80%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= unp_NotesDue2020180PercentMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_UnsecuredDebtMember
Maturity date on new borrowing Feb. 01, 2020  
3.375% Notes Due 2035 [Member]
    
Debt Instrument [Line Item]    
Issuance date Jan. 29, 2015  
Note amount   450,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= unp_NotesDue20353375PercentMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_UnsecuredDebtMember
Interest rate on note   3.375%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= unp_NotesDue20353375PercentMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_UnsecuredDebtMember
Maturity date on new borrowing Feb. 01, 2035  
3.875% Notes Due 2055 [Member]
    
Debt Instrument [Line Item]    
Issuance date Jan. 29, 2015  
Note amount   $ 450,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= unp_NotesDue20553875PercentMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_UnsecuredDebtMember
Interest rate on note   3.875%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= unp_NotesDue20553875PercentMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_UnsecuredDebtMember
Maturity date on new borrowing Feb. 01, 2055