Stock-Based Compensation - Narrative (Details) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2019 CAD ($) $ / shares shares	Dec. 31, 2018 CAD ($) shares	Dec. 31, 2017 CAD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 133.0	$ 75.0	$ 35.0
Share-based liabilities paid	58.0	37.0	39.0
Performance Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 89.0	$ 54.0	30.0
Forfeited awards other than options \| shares	12,976	38,635
Vesting period	3 years
Unrecognized compensation related to stock options	$ 42.0
Weighted-average period of recognition for unrecognized compensation	1 year 6 months
Units issued \| shares	133,681	125,280
Grant date fair value	$ 36.0
Payout percentage	193.00%	177.00%
PSUs vested in period \| shares	121,098
Fair value of PSUs vested in period	$ 75.0
Number of trading days	30 days	30 days
Share-based liabilities paid	$ 54.0	$ 30.0	31.0
Units paid in period \| shares	117,228	66,243
Performance Share Units \| Regular and Retention Grant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Units issued \| shares	134,260	162,255
Performance Share Units \| Retention Grant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Units issued \| shares	579	36,975
Deferred Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 20.0	$ 4.0	(3.0)
Forfeited awards other than options \| shares	951	1,747
Vesting period	48 months
Unrecognized compensation related to stock options	$ 0.7
Weighted-average period of recognition for unrecognized compensation	1 year 2 months 12 days
Units issued \| shares	19,912	16,481
Grant date fair value	$ 5.0
Number of trading days	10 days
Share-based liabilities paid	$ 4.0	$ 6.0	6.0
Units paid in period \| shares	12,110	20,072
Matching % of DSU's granted to senior managers	25.00%
Ownership target period	5 years
Regular and Performance Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 14.0	$ 10.0	3.0
Forfeited options \| shares	7,855
Company's closing stock price (CAD) \| $ / shares	$ 331.03
Expiration period	7 years
Fair value of options at grant date	$ 14.0	16.0	17.0
Unrecognized compensation related to stock options	$ 14.0
Weighted-average period of recognition for unrecognized compensation	1 year 3 months 18 days
Fair value of shares vested for stock option plan	$ 8.0	11.0	14.0
Regular and Performance Stock Options \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	12 months
Regular and Performance Stock Options \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	48 months
Employee Share Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 6.0	$ 5.0	$ 4.0
Vesting period	1 year
Maximum percentage of annual salary that an employee can contribute	6.00%
Number of shares purchased on behalf of participants \| shares	137,942	118,865	130,041
Employer contributions	$ 8.0	$ 6.0	$ 5.0
Employer matching ratio per dollar contributed	0.3333
Chief Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation and benefits recovery			(51.0)
Pension and other post-retirement benefits recovery			$ (27.0)
Chief Executive Officer \| Performance Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Forfeited awards other than options \| shares			22,514
Chief Executive Officer \| Deferred Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Forfeited awards other than options \| shares			68,612
Chief Executive Officer \| Regular and Performance Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Forfeited options \| shares			752,145

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Share-based Compensation Arrangement by Share-based Payment Award, Ratio Employer Match Per Dollar Contributed

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The highest percentage of annual salary that an employee is permitted to utilize with respect to the plan.

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Number of non-option equity instruments exercised by participants.

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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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Number of shares purchased for issuance under share-based payment arrangement.

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Price of a single share of a number of saleable stocks of a company.

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Percentage of vesting of award under share-based payment arrangement.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

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Value, before forfeiture, of shares granted under share-based payment arrangement. Excludes employee stock ownership plan (ESOP).

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