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Derivatives Instruments (Changes in Realized and Unrealized Gains (Losses) with Respect to Derivatives Receiving Regulatory Deferral) (Details) - CAD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items] |  |
Unrealized gain (loss) on derivatives receiving regulatory deferral | $ 39 | $ (55)
Realized (gain) loss on derivatives receiving regulatory deferral | 28 | (1)
Regulatory deferral | Commodity swaps and forwards |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total change derivative instruments on derivatives receiving regulatory deferral | (115) | (70)
Regulatory deferral | Commodity swaps and forwards | Regulatory Assets |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Unrealized gain (loss) on derivatives receiving regulatory deferral | (89) | (34)
Regulatory deferral | Commodity swaps and forwards | Regulatory Liabilities |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Unrealized gain (loss) on derivatives receiving regulatory deferral | 9 | 29
Realized (gain) loss on derivatives receiving regulatory deferral | (2) | (8)
Regulatory deferral | Commodity swaps and forwards | Inventory |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Realized (gain) loss on derivatives receiving regulatory deferral | (36) | (55)
Regulatory deferral | Commodity swaps and forwards | Regulated | Fuel for generation and purchased power |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Realized (gain) loss on derivatives receiving regulatory deferral | 3 | (2)
Regulatory deferral | Foreign exchange forwards |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total change derivative instruments on derivatives receiving regulatory deferral | (33) | 1
Regulatory deferral | Foreign exchange forwards | Regulatory Assets |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Unrealized gain (loss) on derivatives receiving regulatory deferral | (6) | 4
Regulatory deferral | Foreign exchange forwards | Regulatory Liabilities |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Unrealized gain (loss) on derivatives receiving regulatory deferral | (8) | 24
Realized (gain) loss on derivatives receiving regulatory deferral | 0 | 0
Regulatory deferral | Foreign exchange forwards | Inventory |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Realized (gain) loss on derivatives receiving regulatory deferral | (11) | (18)
Regulatory deferral | Foreign exchange forwards | Regulated | Fuel for generation and purchased power |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Realized (gain) loss on derivatives receiving regulatory deferral | $ (8) | $ (9)

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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Gain (loss) incurred in hedging activities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959 + Details
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- Definition The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585 + Details
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