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Cumulative Preferred Stock (Authorized) (Details) - CAD ($) $ / shares in Units, $ in Millions |  | 12 Months Ended |
--- | --- | --- | ---
Aug. 15, 2020 | Dec. 31, 2020 | Dec. 31, 2019 | Aug. 17, 2020
Class of Stock [Line Items] |  |  |  |
Issued and Outstanding |  | 41,000,000 | 41,000,000 |
Net Proceeds |  | $ 1,004 | $ 1,004 |
Series A Preferred Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Annual Dividend Per Share | $ 0.5456 | $ 0.6388 |  |
Preferred Stock, Redemption Price Per Share |  | $ 25 |  |
Issued and Outstanding |  | 4,866,814 | 3,864,636 | 3,864,636
Net Proceeds |  | $ 95 | $ 95 |
Series B Preferred Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Preferred Stock, Redemption Price Per Share |  | $ 25 |  |
Issued and Outstanding |  | 1,133,186 | 2,135,364 | 2,135,364
Net Proceeds |  | $ 52 | $ 52 |
Preferred Stock Dividend Payment Rate Variable |  | Floating |  |
Series C Preferred Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Annual Dividend Per Share | 1.1802 | $ 1.1802 | $ 1.0505 |
Preferred Stock, Redemption Price Per Share |  | $ 25 |  |
Issued and Outstanding |  | 10,000,000 | 10,000,000 |
Net Proceeds |  | $ 245 | $ 245 |
Series E Preferred Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Annual Dividend Per Share |  | $ 1.125 |  |
Preferred Stock, Redemption Price Per Share |  | $ 25.75 |  |
Issued and Outstanding |  | 5,000,000 | 5,000,000 |
Net Proceeds |  | $ 122 | $ 122 |
Series F Preferred Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Annual Dividend Per Share | $ 1.0505 | $ 1.0625 |  |
Preferred Stock, Redemption Price Per Share |  | $ 25 |  |
Issued and Outstanding |  | 8,000,000 | 8,000,000 |
Net Proceeds |  | $ 195 | $ 195 |
Series H Preferred Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Annual Dividend Per Share |  | $ 1.225 |  |
Preferred Stock, Redemption Price Per Share |  | $ 25 |  |
Issued and Outstanding |  | 12,000,000 | 12,000,000 |
Net Proceeds |  | $ 295 | $ 295 |

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