Financial Instruments	3 Months Ended
Financial Instruments	Mar. 31, 2021
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Financial Instruments	Financial instruments A. Fair values of financial instruments The Company categorizes its financial assets and liabilities measured at fair value into a three-level hierarchy established by GAAP that prioritizes those inputs to valuation techniques used to measure fair value based on the degree to which they are observable. The three levels of the fair value hierarchy are as follows: Level 1 inputs are quoted prices in active markets for identical assets and liabilities; Level 2 inputs, other than quoted prices included within Level 1, are observable for the asset or liability either directly or indirectly; and Level 3 inputs are not observable in the market. For non-exchange traded derivatives classified as Level 2, the Company uses standard valuation techniques to calculate fair value. Primary inputs to these techniques include observable market prices (interest, FX, and commodity) and volatility, depending on the type of derivative and nature of the underlying risk. The Company uses inputs and data used by willing market participants when valuing derivatives and considers its own credit default swap spread as well as those of its counterparties in its determination of fair value. All derivatives are classified as Level 2. The Company’s short-term financial instruments include cash and cash equivalents, accounts receivable, accounts payable and accrued liabilities, and short-term borrowings including commercial paper. The carrying value of short-term financial instruments all approximate their fair values. The carrying value of the Company’s debt and finance lease liabilities do not approximate their fair value. Their estimated fair value has been determined based on market information where available, or by discounting future payments of principal and interest at estimated interest rates expected to be available to the Company at period end. All measurements are classified as Level 2. The Company’s debt and finance lease liabilities, including current maturities, with a carrying value of $8,841 million at March 31, 2021 (December 31, 2020 - $8,951 million), had a fair value of $10,609 million (December 31, 2020 - $11,597 million). B. Financial risk management FX management Net investment hedge The effect of the Company's net investment hedge for the three months ended March 31, 2021 was an unrealized FX gain of $76 million (three months ended March 31, 2020 - an unrealized FX loss of $555 million) recognized in “Other comprehensive income”. FX forward contracts During the first quarter of 2021, the Company entered into various FX forward contracts totalling a notional U.S. $800 million to fix the FX rate and lock-in a portion of the amount of Canadian dollars it may borrow to finance the U.S. dollar-denominated cash purchase consideration of the pending KCS transaction. The changes in fair value on the FX forward contracts were recorded in "Other (income) expense" on the Company's Interim Consolidated Statements of Income, with the offsetting unrealized gains and losses included in "Other current assets" and "Accounts payable and accrued liabilities" on the Company's Interim Consolidated Balance Sheets. For the three months ended March 31, 2021, the change in fair value on the FX forward contracts was negligible. During April 2021, the Company entered into additional FX forward contracts totalling a notional U.S. $200 million to fix the FX rate and lock-in a portion of the amount of Canadian dollars it may use to finance the pending U.S. dollar-denominated KCS transaction. Interest rate management Forward starting swaps During the first quarter of 2021, the Company entered into forward starting floating-to-fixed interest rate swap agreements ("forward starting swaps") with terms of up to 30 years, totalling a notional U.S. $1.8 billion to fix the benchmark rate on cash flows associated with highly probable forecasted issuances of long-term notes. The changes in fair value on the forward starting swaps is recorded in “Accumulated other comprehensive loss”, net of tax, as cash flow hedges until the notes are issued. Subsequent to the notes issuance, amounts in “Accumulated other comprehensive loss” will be reclassified to “Net interest expense”. As at March 31, 2021, the unrealized fair value gain derived from the forward starting swaps of $20 million was included in "Other current assets" on the Company’s Interim Consolidated Balance Sheets, with the offset reflected in "Other comprehensive income" on the Company’s Interim Consolidated Statements of Comprehensive Income. During April 2021, the Company entered into additional forward starting swaps with terms of up to 30 years, totalling a notional U.S. $600 million to fix the benchmark rate on cash flows associated with highly probable issuances of long-term notes. Bond locks During the first quarter of 2021, the Company entered into 7-year interest rate bond locks totalling a notional $600 million to fix the benchmark rate on cash flows associated with highly probable forecasted issuance of long-term notes. The changes in fair value on the bond locks is recorded in “Accumulated other comprehensive loss”, net of tax, as cash flow hedges until the notes are issued. Subsequent to the notes issuance, amounts in “Accumulated other comprehensive loss” will be reclassified to “Net interest expense”. As at March 31, 2021, the unrealized fair value gain derived from the bond locks of $2 million was included in "Other current assets" on the Company’s Interim Consolidated Balance Sheets, with the offset reflected in "Other comprehensive income" on the Company’s Interim Consolidated Statements of Comprehensive Income.