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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, 2017	Dec. 31, 2016
Earnings Per Share Reconciliation [Abstract]
Net Income (Loss) Available to Common Stockholders, Basic	$ 266.1	$ 227.2
Interest on Convertible Debt, Net of Tax	0.0	0.2
Net Income (Loss) Available to Common Stockholders, Diluted, Total	$ 266.1	$ 227.4
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Weighted Average Number of Shares Issued, Basic	212.3	170.4
Weighted Average Number of Shares deferred share units outstanding	1.1	1.0
Weighted Average Number of Shares Outstanding, Basic, Total	213.4	171.4
Incremental Weighted Average Shares Attributable to Dilutive Effect [Abstract]
IncrementalCommonSharesAttributableToShareBasedPaymentArrangements	0.6	0.6
IncrementalCommonSharesAttributableToConversionOfDebtSecurities	0.1	0.2
Weighted Average Number of Shares Outstanding, Diluted, Total	214.1	172.2
Basic	$ 1.25	$ 1.33
Diluted	$ 1.24	$ 1.32

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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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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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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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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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