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Capital stock (Detail) (CAD)
In Millions, except Per Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Share Repurchase Programs [Abstract]      
Weighted-average price per share (in dollars per share) 67.38cni_ShareRepurchaseProgramWeightedAveragePricePerShare [1] 50.65cni_ShareRepurchaseProgramWeightedAveragePricePerShare [1] 41.36cni_ShareRepurchaseProgramWeightedAveragePricePerShare [1]
Amount of repurchase 1,505cni_TotalValueSharesRepurchaseDuringPeriod 1,400cni_TotalValueSharesRepurchaseDuringPeriod 1,400cni_TotalValueSharesRepurchaseDuringPeriod
Shares approved for repurchase in share repurchase program (in shares) 28.0cni_ShareRepurchaseProgramSharesApproved    
Share repurchase program expiration date 2015-10-23    
Share repurchase program initiation date 2014-10-24    
Total shares repurchased in repurchase program (in shares) 5.6cni_ShareRepurchaseProgramTotalSharesRepurchased    
Share Trust Purchase [Abstract]      
Number Of Shares Held By Share Trust 0cni_NumberOfSharesHeldByShareTrust    
Number Of Shares Share Trust Authorized Purchase Next Period 2.0cni_NumberOfSharesShareTrustAuthorizedPurchaseNextPeriod    
Share Based Compensation Program Modification [Abstract]      
Share Based Compensation Modification DSU 132cni_ShareBasedCompensationModificationDSU    
Share Based Compensation Modification PSU 60cni_ShareBasedCompensationModificationPSU    
Share Based Compensation Modification Other 17cni_ShareBasedCompensationModificationOther    
Common shares [Member]      
Issued and outstanding common shares [Abstract]      
Shareholders', common share, beginning balance (in shares) 830.6us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 856.8us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 884.2us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
Number of shares repurchased through buyback programs (in shares) (22.4)us-gaap_StockRepurchasedAndRetiredDuringPeriodShares
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 [2] (27.6)us-gaap_StockRepurchasedAndRetiredDuringPeriodShares
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 [2] (33.8)us-gaap_StockRepurchasedAndRetiredDuringPeriodShares
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 [2]
Stock options exercised (in shares) 1.2us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 1.4us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 6.4us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
Shareholders', common share, ending balance (in shares) 809.4us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 830.6us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 856.8us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
Share Repurchase Programs [Abstract]      
Number of common shares (in shares) 22.4us-gaap_StockRepurchasedAndRetiredDuringPeriodShares
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 [2] 27.6us-gaap_StockRepurchasedAndRetiredDuringPeriodShares
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 [2] 33.8us-gaap_StockRepurchasedAndRetiredDuringPeriodShares
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 [2]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock Issued During Period Shares Stock Options Exercised 1.2us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 1.4us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 6.4us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
Common shares and additional paid in capital [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock Issued During Period Value Stock Options Exercised 25cni_StockIssuedDuringPeriodValueStockOptionsExercised1
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember		 28cni_StockIssuedDuringPeriodValueStockOptionsExercised1
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember		 102cni_StockIssuedDuringPeriodValueStockOptionsExercised1
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember
Equity settled SBC expense value 11us-gaap_StockOptionPlanExpense
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember		 9us-gaap_StockOptionPlanExpense
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember		 10us-gaap_StockOptionPlanExpense
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember
Excess Tax Benefit From Share Based Compensation Financing Activities 5us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember		 3us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember		 16us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember
Modification of SBC award agreements 209cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsValueModification
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember		      
Share Based Compensation Other Total 250cni_ShareBasedCompensationOtherTotal
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember		 40cni_ShareBasedCompensationOtherTotal
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember		 128cni_ShareBasedCompensationOtherTotal
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember
[1] Includes brokerage fees.
[2] Includes common shares purchased in the first and fourth quarters of 2014, 2013 and 2012 pursuant to private agreements between the Company and arm’s-length third-party sellers.