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Derivatives Instruments (Realized and Unrealized Gains (Losses) with Respect to HFT Derivatives) (Details) - HFT derivatives - CAD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items] |  |
Realized and unrealized gains (losses) with respect to HFT derivatives | $ 276 | $ 195
Operating revenues | Power | Non-Regulated |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Realized and unrealized gains (losses) with respect to HFT derivatives | 1 | (12)
Operating revenues | Natural gas | Non-Regulated |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Realized and unrealized gains (losses) with respect to HFT derivatives | 281 | 205
Fuel for generation and purchased power | Power | Non-Regulated |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Realized and unrealized gains (losses) with respect to HFT derivatives | (6) | 2
Fuel for generation and purchased power | Natural gas | Non-Regulated |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Realized and unrealized gains (losses) with respect to HFT derivatives | $ 0 | $ 0

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