Pensions and Other Postretirement Benefits (Adoption of Spot Rate Approach) (Details) CAD in Millions	12 Months Ended
Dec. 31, 2017 CAD
Change in Assumptions for Pension Plans [Member]
Adoption of the spot rate approach
Net periodic benefit cost (income) due to change in accounting estimate	CAD (130)

The amount of increase or decrease in net periodic benefit cost related to the effects of a change in the accounting estimate of an existing defined benefit plan.

No definition available.

Name:	cni_DefinedBenefitPlanEffectOfChangeInAccountingEstimateOnNetPeriodicBenefitCost
Namespace Prefix:	cni_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration