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Financial Instruments (Details)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
CAD
Dec. 31, 2013
CAD
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Dec. 31, 2014
Carrying amount [Member]
CAD
Dec. 31, 2013
Carrying amount [Member]
CAD
Dec. 31, 2014
Fair value [Member]
CAD
Dec. 31, 2013
Fair value [Member]
CAD
Risk Management [Abstract]                
Unamortized gain included in accumulated other comprehensive loss 7us-gaap_UnrealizedGainLossOnInterestRateCashFlowHedgesPretaxAccumulatedOtherComprehensiveIncomeLoss 8us-gaap_UnrealizedGainLossOnInterestRateCashFlowHedgesPretaxAccumulatedOtherComprehensiveIncomeLoss            
Unamortized gain included in accumulated other comprehensive loss, after-tax 5us-gaap_AccumulatedOtherComprehensiveIncomeLossCumulativeChangesInNetGainLossFromCashFlowHedgesEffectNetOfTax 6us-gaap_AccumulatedOtherComprehensiveIncomeLossCumulativeChangesInNetGainLossFromCashFlowHedgesEffectNetOfTax            
Financial assets [Abstract]                
Investments         58us-gaap_CostMethodInvestmentsFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
57us-gaap_CostMethodInvestmentsFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
183us-gaap_CostMethodInvestmentsFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
164us-gaap_CostMethodInvestmentsFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Financial liabilities [Abstract]                
Total debt         8,409us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
7,840us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
9,767us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
8,683us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Gain related to the fair value of foreign exchange forward contracts 9cni_GainRelatedToFairValueOfForeignExchangeForwardContracts 6cni_GainRelatedToFairValueOfForeignExchangeForwardContracts            
Value Of Foreign Exchange Forward Contracts     $ 350cni_ValueOfForeignExchangeForwardContracts $ 325cni_ValueOfForeignExchangeForwardContracts