Stock-Based Compensation - Stock Option Plan (Details) - Stock Option Plan - CAD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Number of options, Options outstanding (in shares)
Outstanding, January 1, Options	1,416,346
Granted, Options	217,240
Exercised, Options	(232,034)
Forfeited, Options	(13,839)
Expired, Options	(347)
Outstanding, December 31, Options	1,387,366	1,416,346
Vested or expected to vest at December 31, Options	1,366,649
Exercisable, December 31, Options	610,289
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Outstanding, January 1	$ 199.12
Granted	344.04
Exercised	162.87
Forfeited	271.75
Expired	168.84
Outstanding at December 31	225.20	$ 199.12
Vested or expected to vest as at December 31	223.98
Exercisable at December 31	$ 177.65
Number of options, Nonvested options (in shares)
Non-vested, Outstanding January 1, Options	761,784
Granted, Options	217,240
Vested, Options	(188,108)
Non-vested, Forfeited, Options	(13,839)
Non-vested, Outstanding December 31, Options	777,077	761,784
Weighted average grant date fair value, Nonvested options
Non-vested, Outstanding, January 1	$ 53.54
Non-vested, Granted	69.00	$ 63.69	$ 55.63
Vested	50.91
Non-vested, Forfeited	58.29
Non-vested, Outstanding, December 31	$ 58.40	$ 53.54
Weighted average remaining term of vested or expected to vest options	4 years 6 months
Aggregate intrinsic value of vested or expected to vest options	$ 297

Share based Compensation Arrangement by Share based Payment Award, Weighted Average Grant Date Fair Value [Abstract]

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest exercisable options that may be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options of the plan that expired.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Number of non-vested options outstanding.

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Weighted average grant-date fair value of non-vested options outstanding.

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Weighted average remaining contractual term for fully vested and expected to vest exercisable or convertible options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of options vested.

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Weighted average grant-date fair value of options vested.

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Number of share options (or share units) exercised during the current period.

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