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Earnings Per Share (Reconciliation of Basic and Diluted Earnings Per Share) (Details) - CAD CAD / shares in Units, shares in Thousands, CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share Reconciliation [Abstract]
Net Income (Loss) Available to Common Stockholders, Basic	CAD 227.2	CAD 397.2
Interest on Convertible Debt, Net of Tax	0.2	0.0
Net Income (Loss) Available to Common Stockholders, Diluted, Total	CAD 227.4	CAD 397.2
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Weighted Average Number of Shares Issued, Basic	170,400	144,900
Weighted Average Number of Shares deferred share units outstanding	1,000	900
Weighted Average Number of Shares Outstanding, Basic, Total	171,400	145,800
Incremental Weighted Average Shares Attributable to Dilutive Effect [Abstract]
IncrementalCommonSharesAttributableToShareBasedPaymentArrangements	600	600
Incremental Common Shares Attributable to Dilutive Effect of Dividend Reinvesatment Program	0	0
IncrementalCommonSharesAttributableToConversionOfDebtSecurities	200	0
Weighted Average Number of Shares Outstanding, Diluted, Total	172,200	146,400
Basic	CAD 1.33	CAD 2.72
Diluted	CAD 1.32	CAD 2.71
Earnings Per Share, Additional Infomration [Abstract]
Stock Issued During Period, Shares, Dividend Reinvestment Plan	0	0

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of dividend reinvestment program arrangements using the treasury stock method.

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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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The after-tax amount of interest recognized in the period associated with any convertible debt.

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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Net Income or Loss Available to Common Stockholders plus adjustments resulting from the assumption that dilutive convertible securities were converted, options or warrants were exercised, or that other shares were issued upon the satisfaction of certain conditions.

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Number of shares issued during the period from a dividend reinvestment plan (DRIP). A dividend reinvestment plan allows the shareholders to reinvest dividends paid to them by the entity on new issues of stock by the entity.

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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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