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Derivatives (Realized and Unrealized Gains (Losses) with Respect to HFT Derivatives) (Details) - Designated as Hedging Instrument [Member] - Held for Trading Hedge [Member] - CAD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items] |  |
Gain (Loss) on Derivative Instruments Held for Trading Purposes, Net | $ 420 | $ 59
Power swaps and physical contracts [Member] | Non-regulated fuel for generation and purchased power [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Gain (Loss) on Derivative Instruments Held for Trading Purposes, Net | 2 | 0
Power swaps and physical contracts [Member] | Other Operating Income (Expense) [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Gain (Loss) on Derivative Instruments Held for Trading Purposes, Net | 7 | (1)
Natural gas swaps, futures, forwards, physical contracts [Member] | Non-regulated fuel for generation and purchased power [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Gain (Loss) on Derivative Instruments Held for Trading Purposes, Net | 10 | (7)
Natural gas swaps, futures, forwards, physical contracts [Member] | Other Operating Income (Expense) [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Gain (Loss) on Derivative Instruments Held for Trading Purposes, Net | 401 | 69
Foreign Exchange Option [Member] | Other Operating Income (Expense) [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Gain (Loss) on Derivative Instruments Held for Trading Purposes, Net | $ 0 | $ (2)

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- Definition Aggregate net gain and losses (whether realized or unrealized) on all derivative instruments held for trading purposes whether or not settled physically. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41322-113958 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4F -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624186-113959 + Details
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