Stock-based compensation (Stock price volatility) (Details 9) CAD / shares in Units, CAD in Millions	12 Months Ended
Dec. 31, 2016 CAD CAD / shares
Stock price volatility
Amount of change in Company's stock price used for sensitivity analysis which would impact stock-based compensation expense for cash settlement awards \| CAD / shares	CAD 1
Amount of change in Company's stock-based compensation expense if stock price changed by $1 \| CAD	CAD 1

The hypothetical change in entity's stock-based compensation expense, on awards granted to employees, based on change in entity's stock price.

No definition available.

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