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

No definition available.

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

No definition available.

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Namespace Prefix:	cni_
Data Type:	num:percentItemType
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Period Type:	duration

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

No definition available.

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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
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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.

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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.

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Data Type:	xbrli:stringItemType
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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=108410482&loc=d3e1928-114920

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Percentage of investment to total investment within defined benefit plan asset category.

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=112275985&loc=d3e4179-114921

Reference 2: 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=108410482&loc=d3e1928-114920

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Data Type:	num:percentItemType
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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=112275985&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=108410482&loc=d3e1928-114920

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