Stock-Based Compensation - Narrative (Detail) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2018 CAD ($) $ / shares shares	Dec. 31, 2017 CAD ($) shares	Dec. 31, 2016 CAD ($) shares
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense (recovery)	$ 75.0	$ 35.0	$ 51.0
Share based liabilities paid	37.0	39.0	48.0
Performance Share Units [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense (recovery)	$ 54.0	$ 30.0	29.0
Forfeited awards other than options \| shares	38,635	44,399
Vesting period	3 years
Unrecognized compensation related to stock options	$ 29.0
Expected to be recognized over a weighted-average period	1 year 7 months 6 days
Units issued \| shares	125,280	134,991
Grant date fair value	$ 40.0
Payout Percentage	177.00%	160.00%
Number of trading days	30 days	30 days
PSUs vested in period \| shares	117,095	82,800
PSU payout amount	$ 54.0
Share based liabilities paid	$ 30.0	$ 31.0	31.0
Performance Share Units [Member] \| Regular and Retention Grant [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Units issued \| shares	162,255
Performance Share Units [Member] \| Retention Grant [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Units issued \| shares	36,975
Deferred Share Units [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense (recovery)	$ 4.0	$ (3.0)	2.0
Forfeited awards other than options \| shares	1,747	69,708
Vesting period	48 months
Unrecognized compensation related to stock options	$ 0.6
Expected to be recognized over a weighted-average period	1 year
Units issued \| shares	16,481	23,932
Grant date fair value	$ 4.0
Share based liabilities paid	$ 6.0	$ 6.0	17.0
Matching % of DSU's granted to senior managers	25.00%
Ownership target threshold	5 years
Regular and Performance Stock Options [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense (recovery)	$ 10.0	3.0	14.0
Vested and nonvested options forfeited, Number of shares \| shares	26,650
Company's closing stock price (in CAD per share) \| $ / shares	$ 242.24
Stock-based compensation, expiration period	7 years
Fair value of options at grant date	$ 16.0	17.0	16.0
Unrecognized compensation related to stock options	$ 15.0
Expected to be recognized over a weighted-average period	1 year 7 months 6 days
Fair value of shares vested for stock option plan	$ 11.0	14.0	15.0
Regular and Performance Stock Options [Member] \| Minimum [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Vesting period	12 months
Regular and Performance Stock Options [Member] \| Maximum [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Vesting period	48 months
Employee Share Purchase Plan [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense (recovery)	$ 5.0	$ 4.0	$ 5.0
Vesting period	1 year
Employer contribution to employee share purchase plan	$1 for every $3 contributed by employees
Maximum percentage of annual salary that an employee can contribute	6.00%
Number of shares purchased on behalf of participants \| shares	118,865	130,041	140,560
Employer contributions	$ 6.0	$ 5.0	$ 5.0
Employer matching ratio per dollar contributed	0.3333
Chief Executive Officer [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Employee benefits and stock-based compensation recovery		(51.0)
Pension and other post-retirement benefits reversal of cost		$ (27.0)
Chief Executive Officer [Member] \| Performance Share Units [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Forfeited awards other than options \| shares		22,514
Chief Executive Officer [Member] \| Deferred Share Units [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Forfeited awards other than options \| shares		68,612
Chief Executive Officer [Member] \| Regular and Performance Stock Options [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Vested and nonvested options forfeited, Number of shares \| shares		752,145

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Number of Deferred Share Units That Can Be Granted as Percentage of Shares Acquired

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Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Ownership Target, Period

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

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Amount of expense for employee benefit and equity-based compensation.

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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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This item represents the amount of cash contributions during the period made by the entity to the Employee Stock Ownership Plan (ESOP).

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Amount of cost (reversal of cost) for pension and other postretirement benefits.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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Amount of cash paid related to share-based compensation liabilities.

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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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The highest percentage of annual salary that an employee is permitted to utilize with respect to the 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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The number of shares purchased on the open market during the period for issuance to employees under the plan.

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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 share-based compensation awards.

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Amount of capitalized and expensed compensation cost from equity-based compensation arrangements.

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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 forfeitures, of stock or other type of equity granted of any equity-based compensation plan other than an employee stock ownership plan (ESOP).

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