Name:	cni_PensionPlanAmortizationThresholdGainLoss
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Other postretirement benefits amortization of cumulative net actuarial gains and losses percentage in excess of the projected benefit obligation at the beginning of the year, over the expected average remaining service life of the employee group covered by the plan.

No definition available.

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Describes the minimum level of activity of costs an entity capitalizes as part of property, plant and equipment, including the costs incurred for repairs and maintenance activities.

No definition available.

Name:	cni_PropertyPlantAndEquipmentCostCapitalizationMinimumLevelOfActivityBallast
Namespace Prefix:	cni_
Data Type:	xbrli:integerItemType
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Describes the minimum level of activity of costs an entity capitalizes as part of property, plant and equipment, including the costs incurred for repairs and maintenance activities.

No definition available.

Name:	cni_PropertyPlantAndEquipmentCostCapitalizationMinimumLevelOfActivityTiesPerMinimumLevelOfActivityOfRailAndRelatedTrackMaterial
Namespace Prefix:	cni_
Data Type:	xbrli:integerItemType
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Period Type:	instant

Number of depreciable asset classes.

No definition available.

Name:	cni_PropertyPlantAndEquipmentNumberOfDepreciableAssetClasses
Namespace Prefix:	cni_
Data Type:	xbrli:integerItemType
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Describes the minimum level of activity of costs an entity capitalizes as part of property, plant and equipment, including the costs incurred for repairs and maintenance activities.

No definition available.

Name:	cni_Propertyplantandequipmentcostcapitalizationminimumlevelofacivityrailandtrackmaterial
Namespace Prefix:	cni_
Data Type:	num:lengthItemType
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Period Type:	duration

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

The amortization method of a major finite-lived intangible asset class. A major class is composed of intangible assets that can be grouped together because they are similar either by their nature or by their use in the operations of a company. The straight-line method is the preferred amortization method, unless another method better reflects the pattern in which the asset is consumed.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 35
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=66005732&loc=d3e15372-109273

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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.

Name:	us-gaap_FiniteLivedIntangibleAssetsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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- References

No definition available.

Name:	us-gaap_GeneralDiscussionOfPensionAndOtherPostretirementBenefitsAbstract
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Data Type:	xbrli:stringItemType
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- References

No definition available.