Name:	cni_DefinedBenefitPlanActualEmployerContributionsInNextFiscalYear
Namespace Prefix:	cni_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The excess (deficit) funded status is measured as the difference between the fair value of plan assets and the benefit obligation based on an actuarial valuation, for funding purposes. The excess (deficit) is based on current projected results of actuarial funding valuations on a solvency basis. Actuarial valuations for funding purposes are subject to revaluation on an annual basis.

No definition available.

Name:	cni_DefinedBenefitPlanFundedSolvencyBasisStatusOfPlan
Namespace Prefix:	cni_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Maximum percentage of equity investments allocated to an individual issuer during the period.

No definition available.

Name:	cni_DefinedBenefitPlanMaximumPercentageOfEquityInvestmentsAllocatedToIndividualIssuer
Namespace Prefix:	cni_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Maximum percentage of equity investments allocated to an industry sector during the period.

No definition available.

Name:	cni_DefinedBenefitPlanMaximumPercentageOfEquityInvestmentsAllocatedToIndustrySector
Namespace Prefix:	cni_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Percentage of bonds issued or guaranteed by Canadian, U.S. or other governments and were of investment grade.

No definition available.

Name:	cni_DefinedBenefitPlanPercentageOfBondsHeldInShortTermInvestmentsThatAreInvestmentGradeIssuedByGovernments
Namespace Prefix:	cni_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Defined Benefit Plan, Plan Liabilities, Actual Allocation, Percentage

No definition available.

Name:	cni_DefinedBenefitPlanPlanLiabilitiesActualAllocationPercentage
Namespace Prefix:	cni_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Defined Benefit Plan, Plan Liabilities, Target Allocation, Percentage

No definition available.

Name:	cni_DefinedBenefitPlanPlanLiabilitiesTargetAllocationPercentage
Namespace Prefix:	cni_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

The expected excess (deficit) funded status is measured as the difference between the fair value of plan assets and the benefit obligation based on an upcoming or future actuarial valuation, for funding purposes. The estimated excess (deficit) is based on current projected results of actuarial funding valuations on a solvency basis. Actuarial valuations for funding purposes are subject to revaluation on an annual basis.

No definition available.

Name:	cni_ExpectedDefinedBenefitPlanFundedStatusOfPlan
Namespace Prefix:	cni_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The expected going concern excess (deficit) funded status is measured as the difference between the fair value of plan assets and the benefit obligation based on an upcoming or future actuarial valuation, for funding purposes. The estimated excess (deficit) is based on current projected results of actuarial funding valuations on a solvency basis. Actuarial valuations for funding purposes are subject to revaluation on an annual basis.

No definition available.

Name:	cni_ExpectedGoingConcernDefinedBenefitPlanFundedStatusOfPlan
Namespace Prefix:	cni_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The going concern excess (deficit) funded status is measured as the difference between the fair value of plan assets and the benefit obligation based on an actuarial valuation, for funding purposes. The excess (deficit) is based on current projected results of actuarial funding valuations on a going concern basis. Actuarial valuations for funding purposes are subject to revaluation on an annual basis.

No definition available.

Name:	cni_GoingConcernDefinedBenefitPlanFundedStatusOfPlan
Namespace Prefix:	cni_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Amount of contributions expected to be received by defined benefit plan from employer in next fiscal year following latest fiscal year.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Name:	us-gaap_DefinedBenefitPlanExpectedFutureEmployerContributionsNextFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Name:	us-gaap_DefinedBenefitPlanPlanAssetsTargetAllocationPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Percentage of actual investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Name:	us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_DefinedBenefitPlanCashAndCashEquivalentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=cni_BondsMortgagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=cni_EmergingMarketDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=cni_PrivateDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_DefinedBenefitPlanEquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_DefinedBenefitPlanRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_OilAndGasMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=cni_InfrastructureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_HedgeFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=cni_RiskFactorAllocationMemberMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=cni_InvestmentRelatedLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=cni_BondsAndDerivativeFinancialInstrumentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: