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Earnings Per Share (Reconciliation of Basic and Diluted Earnings Per Share) (Details) - CAD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share Reconciliation [Abstract]
Net Income (Loss) Available to Common Stockholders, Basic	$ 709.6	$ 266.1
Net Income (Loss) Available to Common Stockholders, Diluted, Total	$ 709.6	$ 266.1
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Weighted Average Number of Shares Issued, Basic	231.7	212.3
Weighted Average Number of Shares deferred share units outstanding	1.3	1.1
Weighted Average Number of Shares Outstanding, Basic, Total	233.0	213.4
Incremental Weighted Average Shares Attributable to Dilutive Effect [Abstract]
IncrementalCommonSharesAttributableToShareBasedPaymentArrangements	0.4	0.6
IncrementalCommonSharesAttributableToConversionOfDebtSecurities	0.1	0.1
Weighted Average Number of Shares Outstanding, Diluted, Total	233.5	214.1
Basic	$ 3.05	$ 1.25
Diluted	$ 3.04	$ 1.24

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of convertible debt securities using the if-converted method.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

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Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities, and addition from assumption of issuance of common shares for dilutive potential common shares; of income (loss) available to common shareholders.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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