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Common Stock (Common Stock Rollfoward) (Details) - CAD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Increase (Decrease) In Common Stock Shares [Roll Forward] |  |
Balance, January 1 | 210,020,000 | 147,210,000
Conversion of Convertible Debentures | 150,000 | 51,990,000
Issuance of common stock | 14,610,000 | 7,690,000
Issued for cash under Purchase Plans at market rate | 3,890,000 | 2,510,000
Discount on shares purchased under Dividend Reinvestment Plan | 0 | 0
Options exercised under senior management share option plan | 100,000 | 620,000
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period | 0 | 0
Balance, December 31 | 228,770,000 | 210,020,000
Increase (Decrease) In Common Stock Value [Roll Forward] |  |
Common Stock, Value, Issued, Beginning Balance | $ 4,738 | $ 2,157
Conversion of Convertible Debentures | 6 | 2,115
Issuance of common shares | 680 | 338
Issued for cash under Purchase Plans at market rate | 182 | 115
Discount on shares purchased under Dividend Reinvestment Plan Value | (9) | (5)
Options exercised under senior management share option plan | 3 | 17
Stock-based compensation | 1 | 1
Common Stock, Value, Issued, Ending Balance | $ 5,601 | $ 4,738
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Outstanding Stock Maximum | 10.00% |
Employee Stock Option [Member] |  |
Increase (Decrease) In Common Stock Shares [Roll Forward] |  |
Options exercised under senior management share option plan | (103,825) |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Common Stock, Capital Shares Reserved for Future Issuance | 1,300,000 | 1,500,000
Stock Compensation Plan [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Common Stock, Capital Shares Reserved for Future Issuance | 6,500,000 | 6,600,000
Dividend Reinvestment [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Common Stock, Capital Shares Reserved for Future Issuance | 4,200,000 | 7,900,000

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