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Stock Plans - Options Year of Grant (Details) (CAD)
In Millions, except Per Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Share Based Compensation Arrangement By Share Based Payment Award Valuation And Expense Information Stock Option Awards By Year Of Grant [Line Items]      
Stock-based compensation expense recognized over requisite service period 9us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount [1] 9us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount [1] 10us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount [1]
Fair value of awards vested during the year 9us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1 11us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1 11us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1
Unrecognized compensation cost on nonvested awards at end of period 7us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized    
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]      
Valuation method used in calculating compensation cost Black-Scholes option-pricing model    
Stock Option Plans [Member] | Granted Year 2013 [Member]      
Share Based Compensation Arrangement By Share Based Payment Award Valuation And Expense Information Stock Option Awards By Year Of Grant [Line Items]      
Stock-based compensation expense recognized over requisite service period 1us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2013Member
[1] 5us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2013Member
[1]  
Fair value per unit at grant date (in dollars per share) 8.52us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2013Member
   
Fair value of awards vested during the year 2us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2013Member
0us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2013Member
 
Unrecognized compensation cost on nonvested awards at end of period 2us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2013Member
   
Remaining recognition period for unrecognized compensation cost on nonvested awards at end of period (years) 2.0cni_EmployeeServiceShareBasedCompensationUnrecognizedCompensationCostsOnNonvestedAwardsPeriodOfRecognition
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2013Member
   
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]      
Grant price 47.47cni_StockOptionGrantPrice
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2013Member
   
Expected stock price volatility 23.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2013Member
[2]    
Expected term (years) 5 years 5 months [3]    
Risk-free interest rate 1.41%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2013Member
[4]    
Dividend rate per share (in dollars per share) 0.86cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRatePerShare
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2013Member
[5]    
Stock Option Plans [Member] | Granted Year 2012 [Member]      
Share Based Compensation Arrangement By Share Based Payment Award Valuation And Expense Information Stock Option Awards By Year Of Grant [Line Items]      
Stock-based compensation expense recognized over requisite service period 1us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2012Member
[1] 2us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2012Member
[1] 4us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2012Member
[1]
Fair value per unit at grant date (in dollars per share) 7.74us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2012Member
   
Fair value of awards vested during the year 2us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2012Member
2us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2012Member
0us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2012Member
Unrecognized compensation cost on nonvested awards at end of period 1us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2012Member
   
Remaining recognition period for unrecognized compensation cost on nonvested awards at end of period (years) 1.0cni_EmployeeServiceShareBasedCompensationUnrecognizedCompensationCostsOnNonvestedAwardsPeriodOfRecognition
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2012Member
   
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]      
Grant price 38.35cni_StockOptionGrantPrice
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2012Member
   
Expected stock price volatility 26.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2012Member
[2]    
Expected term (years) 5 years 5 months [3]    
Risk-free interest rate 1.33%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2012Member
[4]    
Dividend rate per share (in dollars per share) 0.75cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRatePerShare
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2012Member
[5]    
Stock Option Plans [Member] | Granted Year 2011 [Member]      
Share Based Compensation Arrangement By Share Based Payment Award Valuation And Expense Information Stock Option Awards By Year Of Grant [Line Items]      
Stock-based compensation expense recognized over requisite service period 1us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2011Member
[1] 1us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2011Member
[1] 2us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2011Member
[1]
Fair value per unit at grant date (in dollars per share) 7.83us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2011Member
   
Fair value of awards vested during the year 3us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2011Member
3us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2011Member
2us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2011Member
Unrecognized compensation cost on nonvested awards at end of period 0us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2011Member
   
Remaining recognition period for unrecognized compensation cost on nonvested awards at end of period (years) 0cni_EmployeeServiceShareBasedCompensationUnrecognizedCompensationCostsOnNonvestedAwardsPeriodOfRecognition
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2011Member
   
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]      
Grant price 34.47cni_StockOptionGrantPrice
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2011Member
   
Expected stock price volatility 26.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2011Member
[2]    
Expected term (years) 5 years 4 months [3]    
Risk-free interest rate 2.53%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2011Member
[4]    
Dividend rate per share (in dollars per share) 0.65cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRatePerShare
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2011Member
[5]    
Stock Option Plans [Member] | Granted Year 2010 [Member]      
Share Based Compensation Arrangement By Share Based Payment Award Valuation And Expense Information Stock Option Awards By Year Of Grant [Line Items]      
Stock-based compensation expense recognized over requisite service period 0us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2010Member
[1] 1us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2010Member
[1] 2us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2010Member
[1]
Fair value per unit at grant date (in dollars per share) 6.55us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2010Member
   
Fair value of awards vested during the year 2us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2010Member
2us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2010Member
2us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2010Member
Unrecognized compensation cost on nonvested awards at end of period 0us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2010Member
   
Remaining recognition period for unrecognized compensation cost on nonvested awards at end of period (years) 0cni_EmployeeServiceShareBasedCompensationUnrecognizedCompensationCostsOnNonvestedAwardsPeriodOfRecognition
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2010Member
   
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]      
Grant price 27.38cni_StockOptionGrantPrice
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2010Member
   
Expected stock price volatility 28.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2010Member
[2]    
Expected term (years) 5 years 5 months [3]    
Risk-free interest rate 2.44%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2010Member
[4]    
Dividend rate per share (in dollars per share) 0.54cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRatePerShare
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2010Member
[5]    
Stock Option Plans [Member] | Granted Year 2009 [Member]      
Share Based Compensation Arrangement By Share Based Payment Award Valuation And Expense Information Stock Option Awards By Year Of Grant [Line Items]      
Stock-based compensation expense recognized over requisite service period   0us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2009Member
[1] 2us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2009Member
[1]
Fair value per unit at grant date (in dollars per share) 6.30us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2009Member
   
Fair value of awards vested during the year   4us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2009Member
4us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2009Member
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]      
Grant price 21.07cni_StockOptionGrantPrice
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2009Member
   
Expected stock price volatility 39.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2009Member
[2]    
Expected term (years) 5 years 4 months [3]    
Risk-free interest rate 1.97%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2009Member
[4]    
Dividend rate per share (in dollars per share) 0.51cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRatePerShare
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2009Member
[5]    
Stock Option Plans [Member] | Granted Year 2008 [Member]      
Share Based Compensation Arrangement By Share Based Payment Award Valuation And Expense Information Stock Option Awards By Year Of Grant [Line Items]      
Stock-based compensation expense recognized over requisite service period     0us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2008Member
[1]
Fair value per unit at grant date (in dollars per share) 6.22us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2008Member
   
Fair value of awards vested during the year     3us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2008Member
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]      
Grant price 24.25cni_StockOptionGrantPrice
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2008Member
   
Expected stock price volatility 27.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2008Member
[2]    
Expected term (years) 5 years 4 months [3]    
Risk-free interest rate 3.58%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2008Member
[4]    
Dividend rate per share (in dollars per share) 0.46cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRatePerShare
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2008Member
[5]    
Stock Option Plans [Member] | Granted Year 2014 [Member]      
Share Based Compensation Arrangement By Share Based Payment Award Valuation And Expense Information Stock Option Awards By Year Of Grant [Line Items]      
Stock-based compensation expense recognized over requisite service period 6us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2014Member
[1]    
Fair value per unit at grant date (in dollars per share) 11.09us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2014Member
   
Fair value of awards vested during the year 0us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2014Member
   
Unrecognized compensation cost on nonvested awards at end of period 4us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2014Member
   
Remaining recognition period for unrecognized compensation cost on nonvested awards at end of period (years) 3.0cni_EmployeeServiceShareBasedCompensationUnrecognizedCompensationCostsOnNonvestedAwardsPeriodOfRecognition
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2014Member
   
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]      
Grant price 58.74cni_StockOptionGrantPrice
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2014Member
   
Expected stock price volatility 23.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2014Member
[2]    
Expected term (years) 5 years 5 months [3]    
Risk-free interest rate 1.51%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2014Member
[4]    
Dividend rate per share (in dollars per share) 1.00cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRatePerShare
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByPlanAxis
= cni_StockOptionPlansMember
/ cni_ShareBasedCompensationArrangementByShareBasedPaymentAwardValuationAndExpenseInformationStockOptionAwardsByYearOfGrantAxis
= cni_GrantedYear2014Member
[5]    
[1] Compensation cost is based on the grant date fair value using the Black-Scholes option-pricing model that uses the assumptions at the grant date.
[2] Based on the average of the historical volatility of the Company's stock over a period commensurate with the expected term of the award and the implied volatility from traded options on the Company's stock.
[3] Represents the period of time that awards are expected to be outstanding. The Company uses historical data to estimate option exercise and employee termination, and groups of employees that have similar historical exercise behavior are considered separately.
[4] Based on the implied yield available on zero-coupon government issues with an equivalent term commensurate with the expected term of the awards.
[5] Based on the annualized dividend rate.