Consolidated Statement of Income (CAD) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Revenues	12,134us-gaap_Revenues	10,575us-gaap_Revenues	9,920us-gaap_Revenues
Operating expenses
Labor and fringe benefits	2,319us-gaap_LaborAndRelatedExpense	2,182us-gaap_LaborAndRelatedExpense	1,952us-gaap_LaborAndRelatedExpense
Purchased services and material	1,598cni_PurchasedServicesAndMaterial	1,351cni_PurchasedServicesAndMaterial	1,248cni_PurchasedServicesAndMaterial
Fuel	1,846us-gaap_FuelCosts	1,619us-gaap_FuelCosts	1,524us-gaap_FuelCosts
Depreciation and amortization	1,050us-gaap_DepreciationAndAmortization	980us-gaap_DepreciationAndAmortization	924us-gaap_DepreciationAndAmortization
Equipment rents	329cni_EquipmentRents	275cni_EquipmentRents	249cni_EquipmentRents
Casualty and other	368cni_CasualtyAndOther	295cni_CasualtyAndOther	338cni_CasualtyAndOther
Total operating expenses	7,510us-gaap_OperatingExpenses	6,702us-gaap_OperatingExpenses	6,235us-gaap_OperatingExpenses
Operating income	4,624us-gaap_OperatingIncomeLoss	3,873us-gaap_OperatingIncomeLoss	3,685us-gaap_OperatingIncomeLoss
Interest expense	(371)us-gaap_InterestExpense	(357)us-gaap_InterestExpense	(342)us-gaap_InterestExpense
Other income	107us-gaap_OtherNonoperatingIncomeExpense	73us-gaap_OtherNonoperatingIncomeExpense	315us-gaap_OtherNonoperatingIncomeExpense
Income before income taxes	4,360	3,589	3,658
Income tax expense	(1,193)us-gaap_IncomeTaxExpenseBenefit	(977)us-gaap_IncomeTaxExpenseBenefit	(978)us-gaap_IncomeTaxExpenseBenefit
Net income	3,167us-gaap_NetIncomeLoss	2,612us-gaap_NetIncomeLoss	2,680us-gaap_NetIncomeLoss
Earnings per share abstract
Basic (in dollars per share)	3.86us-gaap_EarningsPerShareBasic	3.10us-gaap_EarningsPerShareBasic	3.08us-gaap_EarningsPerShareBasic
Diluted (in dollars per share)	3.85us-gaap_EarningsPerShareDiluted	3.09us-gaap_EarningsPerShareDiluted	3.06us-gaap_EarningsPerShareDiluted
Weighted-average number of shares
Basic (in shares)	819.9	843.1	871.1
Diluted (in shares)	823.5	846.1	875.4

Other expenses including costs associated with casualty, environmental, insurance and operating taxes.

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Rent paid for freight cars owned by other railroads or private companies, net of rents received, including lease expenses primarily for locomotives, railcars, containers and trailers, office and other rentals.

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Materials and contracted services to maintain infrastructure and equipment and terminal services at intermodal and automotive facilities.

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The current period expense charged against earnings on long-lived, physical assets not used in production, and which are not intended for resale, to allocate or recognize the cost of such assets over their useful lives; or to record the reduction in book value of an intangible asset over the benefit period of such asset; or to reflect consumption during the period of an asset that is not used in production.

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Fuel costs incurred that are directly related to goods produced and sold and services rendered during the reporting period.

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This element represents the income or loss from continuing operations attributable to the economic entity which may also be defined as revenue less expenses from ongoing operations, after income or loss from equity method investments, but before income taxes, extraordinary items, and noncontrolling interest.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of the cost of borrowed funds accounted for as interest expense.

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The aggregate amount of expenditures for salaries, wages, profit sharing and incentive compensation, and other employee benefits, including equity-based compensation, and pension and other postretirement benefit expense.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Generally recurring costs associated with normal operations except for the portion of these expenses which can be clearly related to production and included in cost of sales or services. Includes selling, general and administrative expense.

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The net result for the period of deducting operating expenses from operating revenues.

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The net amount of other income and expense amounts, the components of which are not separately disclosed on the income statement, resulting from ancillary business-related activities (that is, excluding major activities considered part of the normal operations of the business) also known as other nonoperating income (expense) recognized for the period. Such amounts may include: (a) dividends, (b) interest on securities, (c) net gains or losses on securities, (d) unusual costs, (e) gains or losses on foreign exchange transactions, and (f) miscellaneous other income and expense items.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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