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Common Stock (Narrative) (Details) - CAD ($) $ / shares in Units, $ in Millions |  | 12 Months Ended |
--- | --- | --- | ---
Nov. 17, 2020 | Dec. 31, 2020 | Dec. 31, 2019 | Jul. 11, 2019
Debt Instrument [Line Items] |  |  |  |
Issuance of common stock |  | 4,540,000 | 1,770,000 |
Gross proceeds from Issuance of Common Stock |  | $ 285 | $ 203 |
Maximum daily trading limit of shares percent | 25.00% |  |  |
ATM Program |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Stock Repurchase Program, Authorized Amount |  |  |  | $ 600
Stock Repurchase Program, Remaining Authorized Repurchase Amount |  | $ 245 |  |
ATM Program | Common Stock |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Issuance of common stock |  | 4,544,025 |  |
Shares, Issued |  |  | 1,768,120 |
Gross proceeds from Issuance of Common Stock |  | $ 255 | $ 100 |
Net proceeds from issuance of common stock |  | $ 251 | $ 99 |
Average price per share, issued |  | $ 56.04 |  |
ATM Program | Average | Common Stock |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Shares Issued, Price Per Share |  |  | $ 56.56 |
Employee Stock Option Plan |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Common Stock, Capital Shares Reserved for Future Issuance |  | 3,500,000 | 3,900,000 |
Share Unit Plans |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Common Stock, Capital Shares Reserved for Future Issuance |  | 3,500,000 | 900,000 |
Dividend Reinvestment |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Common Stock, Capital Shares Reserved for Future Issuance |  | 5,100,000 | 8,800,000 |

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