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Common Stock (Narrative) (Details) - CAD ($) $ / shares in Units, shares in Thousands, $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Jul. 11, 2019
Debt Instrument [Line Items] |  |  |
Issuance of common stock | 1,770 | 450 |
Gross proceeds from Issuance of Common Stock | $ 203.0 | $ 10.0 |
ATM Program |  |  |
Debt Instrument [Line Items] |  |  |
Stock Repurchase Program, Authorized Amount |  |  | $ 600.0
Stock Repurchase Program, Remaining Authorized Repurchase Amount | $ 500.0 |  |
ATM Program | Common Stock |  |  |
Debt Instrument [Line Items] |  |  |
Shares, Issued | 1,770 |  |
Gross proceeds from Issuance of Common Stock | $ 100.0 |  |
Net proceeds from issuance of common stock | $ 98.7 |  |
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 | 3,900 | 6,500 |
Share Unit Plans |  |  |
Debt Instrument [Line Items] |  |  |
Common Stock, Capital Shares Reserved for Future Issuance | 900 | 1,000 |
Dividend Reinvestment |  |  |
Debt Instrument [Line Items] |  |  |
Common Stock, Capital Shares Reserved for Future Issuance | 8,800 | 12,600 |

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