Summary of significant accounting policies (Policies)	12 Months Ended
Summary of significant accounting policies (Policies)	Dec. 31, 2016
Summary of significant accounting policies
Basis of presentation	Basis of presentation These consolidated financial statements are expressed in Canadian dollars, except where otherwise indicated, and have been prepared in accordance with United States generally accepted accounting principles (GAAP) as codified in the Financial Accounting Standards Board (FASB) Accounting Standards Codification (ASC).
Principles of consolidation	Principles of consolidation These consolidated financial statements include the accounts of all subsidiaries and variable interest entities for which the Company is the primary beneficiary. The Company is the primary beneficiary of the Employee Benefit Plan Trusts (“Share Trusts”) as the Company funds the Share Trusts. The Company’s investments in which it has significant influence are accounted for using the equity method and all other investments are accounted for using the cost method.
Use of estimates	Use of estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of revenues, expenses, assets and liabilities, and the disclosure of contingent assets and liabilities at the date of the financial statements. On an ongoing basis, management reviews its estimates, including those related to income taxes, depreciation, pensions and other postretirement benefits, personal injury and other claims, and environmental matters, based upon available information. Actual results could differ from these estimates.
Revenues	Revenues Freight revenues are recognized using the percentage of completed service method based on the transit time of freight as it moves from origin to destination. The allocation of revenues between reporting periods is based on the relative transit time in each period with expenses being recorded as incurred. Revenues related to non-rail transportation services are recognized as service is performed or as contractual obligations are met. Revenues are presented net of taxes collected from customers and remitted to governmental authorities.
Income taxes	Income taxes The Company follows the asset and liability method of accounting for income taxes. Under the asset and liability method, the change in the net deferred income tax asset or liability is included in the computation of Net income or Other comprehensive income (loss). Deferred income tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which temporary differences are expected to be recovered or settled.
Earnings per share	Earnings per share Basic earnings per share is calculated based on the weighted-average number of common shares outstanding over each period. The weighted-average number of basic shares outstanding excludes shares held in the Share Trusts and includes fully vested equity settled stock-based compensation awards excluding stock options. Diluted earnings per share is calculated based on the weighted-average number of diluted shares outstanding using the treasury stock method. Included in the diluted earnings per share calculation are dilutive effects of common shares issuable upon exercise of outstanding stock options and nonvested equity settled awards.
Foreign currency	Foreign currency All of the Company’s United States (U.S.) subsidiaries use the US dollar as their functional currency. Accordingly, the U.S. subsidiaries’ assets and liabilities are translated into Canadian dollars at the rate in effect at the balance sheet date and the revenues and expenses are translated at the average exchange rates during the year. All adjustments resulting from the translation of the foreign operations are recorded in Other comprehensive income (loss). The Company designates the US dollar-denominated long-term debt of the parent company as a foreign currency hedge of its net investment in U.S. subsidiaries. Accordingly, foreign exchange gains and losses, from the dates of designation, on the translation of the US dollar-denominated long-term debt are also included in Other comprehensive income (loss).
Cash and cash equivalents	Cash and cash equivalents Cash and cash equivalents include highly liquid investments purchased three months or less from maturity and are stated at cost, which approximates market value.
Restricted cash and cash equivalents	Restricted cash and cash equivalents The Company has the option, under its bilateral letter of credit facility agreements with various banks, to pledge collateral in the form of cash and cash equivalents for a minimum term of one month, equal to at least the face value of the letters of credit issued. Restricted cash and cash equivalents are shown separately on the balance sheet and include highly liquid investments purchased three months or less from maturity and are stated at cost, which approximates market value.
Accounts receivable	Accounts receivable Accounts receivable are recorded at cost net of billing adjustments and an allowance for doubtful accounts. The allowance for doubtful accounts is based on expected collectability and considers historical experience as well as known trends or uncertainties related to account collectability. When a receivable is deemed uncollectible, it is written off against the allowance for doubtful accounts. Subsequent recoveries of amounts previously written off are credited to bad debt expense in Casualty and other in the Consolidated Statement of Income.
Material and supplies	Material and supplies Material and supplies, which consist mainly of rail, ties, and other items for construction and maintenance of property and equipment, as well as diesel fuel, are valued at weighted-average cost.
Properties	Properties Accounting policy for capitalization of costs The Company’s railroad operations are highly capital intensive. The Company’s properties mainly consist of homogeneous or network-type assets such as rail, ties, ballast and other structures, which form the Company’s Track and roadway properties, and Rolling stock. The Company’s capital expenditures are for the replacement of existing assets and for the purchase or construction of new assets to enhance operations or provide new service offerings to customers. A large portion of the Company’s capital expenditures are for self-constructed properties including the replacement of existing track and roadway assets and track line expansion, as well as major overhauls and large refurbishments of rolling stock. Expenditures are capitalized if they extend the life of the asset or provide future benefits such as increased revenue-generating capacity, functionality, or physical or service capacity. The Company has a process in place to determine whether its capital programs qualify for capitalization. For Track and roadway properties, the Company establishes basic capital programs to replace or upgrade the track infrastructure assets which are capitalized if they meet the capitalization criteria. In addition, for Track and roadway properties, expenditures that meet the minimum level of activity as defined by the Company are also capitalized as follows: grading: installation of road bed, retaining walls, and drainage structures; rail and related track material: installation of 39 or more continuous feet of rail; ties: installation of 5 or more ties per 39 feet; and ballast: installation of 171 cubic yards of ballast per mile. For purchased assets, the Company capitalizes all costs necessary to make the asset ready for its intended use. Expenditures that are capitalized as part of self-constructed properties include direct material, labor, and contracted services, as well as other allocated costs which are not charged directly to capital projects. These allocated costs include, but are not limited to, fringe benefits, small tools and supplies, maintenance on equipment used on projects and project supervision. The Company reviews and adjusts its allocations, as required, to reflect the actual costs incurred each year. For the rail asset, the Company capitalizes the costs of rail grinding which consists of restoring and improving the rail profile and removing irregularities from worn rail to extend the service life. The service life of the rail asset is increased incrementally as rail grinding is performed thereon, and as such, the costs incurred are capitalized given that the activity extends the service life of the rail asset beyond its original or current condition as additional gross tons can be carried over the rail for its remaining service life. For the ballast asset, the Company engages in shoulder ballast undercutting that consists of removing some or all of the ballast, which has deteriorated over its service life, and replacing it with new ballast. When ballast is installed as part of a shoulder ballast undercutting project, it represents the addition of a new asset and not the repair or maintenance of an existing asset. As such, the Company capitalizes expenditures related to shoulder ballast undercutting given that an existing asset is retired and replaced with a new asset. Under the group method of accounting for properties, the deteriorated ballast is retired at its average cost measured using the quantities of new ballast added. Costs of deconstruction and removal of replaced assets, referred to herein as dismantling costs, are distinguished from installation costs for self-constructed properties based on the nature of the related activity. For Track and roadway properties, employees concurrently perform dismantling and installation of new track and roadway assets and, as such, the Company estimates the amount of labor and other costs that are related to dismantling. The Company determines dismantling costs based on an analysis of the track and roadway installation process. Expenditures relating to the Company’s properties that do not meet the Company’s capitalization criteria are considered normal repairs and maintenance and are expensed as incurred. For Track and roadway properties, such expenditures include but are not limited to spot tie replacement, spot or broken rail replacement, physical track inspection for detection of rail defects and minor track corrections, and other general maintenance of track infrastructure. Accounting policy for depreciation Railroad properties are carried at cost less accumulated depreciation including asset impairment write-downs. The cost of properties, including those under capital leases, net of asset impairment write-downs, is depreciated on a straight-line basis over their estimated service lives, measured in years, except for rail and ballast which are measured in millions of gross tons. The Company follows the group method of depreciation whereby a single composite depreciation rate is applied to the gross investment in a class of similar assets, despite small differences in the service life or salvage value of individual property units within the same asset class. The Company uses approximately 40 different depreciable asset classes. For all depreciable assets, the depreciation rate is based on the estimated service lives of the assets. Assessing the reasonableness of the estimated service lives of properties requires judgment and is based on currently available information, including periodic depreciation studies conducted by the Company. The Company’s U.S. properties are subject to comprehensive depreciation studies as required by the Surface Transportation Board (STB) and are conducted by external experts. Depreciation studies for Canadian properties are not required by regulation and are conducted internally. Studies are performed on specific asset groups on a periodic basis. Changes in the estimated service lives of the assets and their related composite depreciation rates are implemented prospectively. The service life of the rail asset is based on expected future usage of the rail in its existing condition, determined using railroad industry research and testing (based on rail characteristics such as weight, curvature and metallurgy), less the rail asset’s usage to date. The annual composite depreciation rate for rail assets is determined by dividing the estimated annual number of gross tons carried over the rail by the estimated service life of the rail measured in millions of gross tons. The Company amortizes the cost of rail grinding over the remaining life of the rail asset, which includes the incremental life extension generated by rail grinding.
Intangible assets	Intangible assets Intangible assets consist mainly of customer contracts and relationships assumed through past acquisitions and are being amortized on a straight-line basis over 40 to 50 years. The Company reviews the carrying amounts of intangible assets held and used whenever events or changes in circumstances indicate that such carrying amounts may not be recoverable based on future undiscounted cash flows. Assets that are deemed impaired as a result of such review are recorded at the lower of carrying amount or fair value.
Accounts receivable securitization	Accounts receivable securitization Based on the structure of its accounts receivable securitization program, the Company accounts for the proceeds received as a secured borrowing.
Pensions and postretirement benefits other than pensions	Pensions Pension costs are determined using actuarial methods. Net periodic benefit cost (income) is recorded in Labor and fringe benefits expense and includes: the cost of pension benefits provided in exchange for employees’ services rendered during the year; the interest cost of pension obligations; the expected long-term return on pension fund assets; the amortization of prior service costs and amendments over the expected average remaining service life of the employee group covered by the plans; and the amortization of cumulative net actuarial gains and losses in excess of 10% of the greater of the beginning of year balances of the projected benefit obligation or market-related value of plan assets, over the expected average remaining service life of the employee group covered by the plans. The pension plans are funded through contributions determined in accordance with the projected unit credit actuarial cost method. Postretirement benefits other than pensions The Company accrues the cost of postretirement benefits other than pensions using actuarial methods. These benefits, which are funded as they become due, include life insurance programs, medical benefits and, for a closed group of employees, free rail travel benefits. The Company amortizes the cumulative net actuarial gains and losses in excess of 10% 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.
Stock-based compensation	Stock-based compensation For equity settled awards, stock-based compensation costs are accrued over the requisite service period based on the fair value of the awards at the grant date. The fair value of performance share unit (PSU) awards is dependent on the type of PSU award. The fair value of PSU-ROIC awards is determined using a lattice-based model incorporating a minimum share price condition and the fair value of PSU-TSR awards is determined using a Monte Carlo simulation model. The fair value of deferred share unit (DSU) awards is determined using the stock price at the grant date. The fair value of stock option awards is determined using the Black-Scholes option-pricing model. For cash settled awards, stock-based compensation costs are accrued over the requisite service period based on the fair value determined at each period-end.
Personal injury and other claims	Personal injury and other claims In Canada, the Company accounts for costs related to employee work-related injuries based on actuarially developed estimates on a discounted basis of the ultimate cost associated with such injuries, including compensation, health care and third-party administration costs. In the U.S., the Company accrues the expected cost for personal injury, property damage and occupational disease claims, based on actuarial estimates of their ultimate cost on an undiscounted basis. For all other legal actions in Canada and the U.S., the Company maintains, and regularly updates on a case-by-case basis, provisions for such items when the expected loss is both probable and can be reasonably estimated based on currently available information.
Environmental expenditures	Environmental expenditures Environmental expenditures that relate to current operations, or to an existing condition caused by past operations, are expensed as incurred unless they can contribute to current or future operations. Environmental liabilities are recorded when environmental assessments occur, remedial efforts are probable, and when the costs, based on a specific plan of action in terms of the technology to be used and the extent of the corrective action required, can be reasonably estimated. The Company accrues its allocable share of liability taking into account the Company’s alleged responsibility, the number of potentially responsible parties and their ability to pay their respective shares of the liability. Recoveries of environmental remediation costs from other parties are recorded as assets when their receipt is deemed probable and collectability is reasonably assured.
Derivative financial instruments	Derivative financial instruments The Company uses derivative financial instruments from time to time in the management of its interest rate and foreign currency exposures. Derivative instruments are recorded on the balance sheet at fair value and the changes in fair value are recorded in Net income or Other comprehensive income (loss) depending on the nature and effectiveness of the hedge transaction. Income and expense related to hedged derivative financial instruments are recorded in the same category as that generated by the underlying asset or liability.