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Fair Values (Fair Value - Derivatives) (Details) - Fair Value, Measurements, Recurring [Member] - Designated as Hedging Instrument [Member] - CAD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | $ 253 | $ 276
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 310 | 475
Fair Value, Net Asset (Liability), Total | (57) | (199)
Cash Flow Hedging [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 7 | 10
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 7 | 27
Cash Flow Hedging [Member] | Power swaps [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 5 | 10
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 2 | 4
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 2 |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 5 | 23
Regulatory Deferral Hedge [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 181 | 229
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 13 | 12
Regulatory Deferral Hedge [Member] | Foreign Exchange Forward [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 32 | 106
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 4 |
Regulatory Deferral Hedge [Member] | Derivative instrument whose primary underlying risk is tied to commodity prices for coal purchases. |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 127 | 74
Regulatory Deferral Hedge [Member] | Natural gas purchases and sales [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 5 | 33
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 6 | 2
Regulatory Deferral Hedge [Member] | Heavy fuel oil purchases [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 12 | 9
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 6
Regulatory Deferral Hedge [Member] | Power purchases [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 5 | 7
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 3 | 4
Held for Trading Hedge [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 63 | 37
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 290 | 434
Held for Trading Hedge [Member] | Power swaps [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 12 | (6)
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 17
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 50 |
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 51 | 43
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 240 | 417
Other Derivatives Hedge [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 2 |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 2
Other Derivatives Hedge [Member] | Foreign Exchange Forward [Member] |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 1
Other Derivatives Hedge [Member] | Interest Rate Swap [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 2 |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 1
Fair Value, Inputs, Level 1 [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 14 | 21
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 65 | 25
Fair Value, Net Asset (Liability), Total | (51) | (4)
Fair Value, Inputs, Level 1 [Member] | Foreign Exchange Forward [Member] |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 0
Fair Value, Inputs, Level 1 [Member] | Cash Flow Hedging [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 5 | 10
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 2 | 4
Fair Value, Inputs, Level 1 [Member] | Cash Flow Hedging [Member] | Power swaps [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 5 | 10
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 2 | 4
Fair Value, Inputs, Level 1 [Member] | Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0 | 0
Fair Value, Inputs, Level 1 [Member] | Regulatory Deferral Hedge [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 9 | 18
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 8 | 5
Fair Value, Inputs, Level 1 [Member] | Regulatory Deferral Hedge [Member] | Foreign Exchange Forward [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 | 0
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0 |
Fair Value, Inputs, Level 1 [Member] | Regulatory Deferral Hedge [Member] | Derivative instrument whose primary underlying risk is tied to commodity prices for coal purchases. |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 | 0
Fair Value, Inputs, Level 1 [Member] | Regulatory Deferral Hedge [Member] | Natural gas purchases and sales [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 | 8
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 5 |
Fair Value, Inputs, Level 1 [Member] | Regulatory Deferral Hedge [Member] | Heavy fuel oil purchases [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 4 | 3
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 0
Fair Value, Inputs, Level 1 [Member] | Regulatory Deferral Hedge [Member] | Power purchases [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 5 | 7
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 3 | 4
Fair Value, Inputs, Level 1 [Member] | Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member] |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 1
Fair Value, Inputs, Level 1 [Member] | Held for Trading Hedge [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 | (7)
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 55 | 16
Fair Value, Inputs, Level 1 [Member] | Held for Trading Hedge [Member] | Power swaps [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 | (7)
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 12
Fair Value, Inputs, Level 1 [Member] | Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 49 |
Fair Value, Inputs, Level 1 [Member] | Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 | 0
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 6 | 4
Fair Value, Inputs, Level 1 [Member] | Other Derivatives Hedge [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 0
Fair Value, Inputs, Level 1 [Member] | Other Derivatives Hedge [Member] | Foreign Exchange Forward [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 |
Fair Value, Inputs, Level 1 [Member] | Other Derivatives Hedge [Member] | Interest Rate Swap [Member] |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 0
Fair Value, Inputs, Level 2 [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 205 | 215
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 62 | 61
Fair Value, Net Asset (Liability), Total | 143 | 154
Fair Value, Inputs, Level 2 [Member] | Foreign Exchange Forward [Member] |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 1
Fair Value, Inputs, Level 2 [Member] | Cash Flow Hedging [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 2 | 0
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 5 | 23
Fair Value, Inputs, Level 2 [Member] | Cash Flow Hedging [Member] | Power swaps [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 | 0
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0 | 0
Fair Value, Inputs, Level 2 [Member] | Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 2 |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 5 | 23
Fair Value, Inputs, Level 2 [Member] | Regulatory Deferral Hedge [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 172 | 210
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 5 | 7
Fair Value, Inputs, Level 2 [Member] | Regulatory Deferral Hedge [Member] | Foreign Exchange Forward [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 32 | 106
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 4 |
Fair Value, Inputs, Level 2 [Member] | Regulatory Deferral Hedge [Member] | Derivative instrument whose primary underlying risk is tied to commodity prices for coal purchases. |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 127 | 74
Fair Value, Inputs, Level 2 [Member] | Regulatory Deferral Hedge [Member] | Natural gas purchases and sales [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 5 | 25
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 1 | 1
Fair Value, Inputs, Level 2 [Member] | Regulatory Deferral Hedge [Member] | Heavy fuel oil purchases [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 8 | 5
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 6
Fair Value, Inputs, Level 2 [Member] | Regulatory Deferral Hedge [Member] | Power purchases [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 | 0
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0 | 0
Fair Value, Inputs, Level 2 [Member] | Held for Trading Hedge [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 29 | 5
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 52 | 29
Fair Value, Inputs, Level 2 [Member] | Held for Trading Hedge [Member] | Power swaps [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 3 | 1
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 5
Fair Value, Inputs, Level 2 [Member] | Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 5 |
Fair Value, Inputs, Level 2 [Member] | Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 26 | 4
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 47 | 24
Fair Value, Inputs, Level 2 [Member] | Other Derivatives Hedge [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 2 |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 2
Fair Value, Inputs, Level 2 [Member] | Other Derivatives Hedge [Member] | Foreign Exchange Forward [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 2 |
Fair Value, Inputs, Level 2 [Member] | Other Derivatives Hedge [Member] | Interest Rate Swap [Member] |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 1
Fair Value, Inputs, Level 3 [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 34 | 40
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 183 | 389
Fair Value, Net Asset (Liability), Total | (149) | (349)
Fair Value, Inputs, Level 3 [Member] | Foreign Exchange Forward [Member] |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 0
Fair Value, Inputs, Level 3 [Member] | Cash Flow Hedging [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 | 0
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0 | 0
Fair Value, Inputs, Level 3 [Member] | Cash Flow Hedging [Member] | Power swaps [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 | 0
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0 | 0
Fair Value, Inputs, Level 3 [Member] | Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0 | 0
Fair Value, Inputs, Level 3 [Member] | Regulatory Deferral Hedge [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 | 1
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0 | 0
Fair Value, Inputs, Level 3 [Member] | Regulatory Deferral Hedge [Member] | Foreign Exchange Forward [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 | 0
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0 |
Fair Value, Inputs, Level 3 [Member] | Regulatory Deferral Hedge [Member] | Derivative instrument whose primary underlying risk is tied to commodity prices for coal purchases. |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 | 0
Fair Value, Inputs, Level 3 [Member] | Regulatory Deferral Hedge [Member] | Natural gas purchases and sales [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 | 0
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0 |
Fair Value, Inputs, Level 3 [Member] | Regulatory Deferral Hedge [Member] | Heavy fuel oil purchases [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 | 1
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 0
Fair Value, Inputs, Level 3 [Member] | Regulatory Deferral Hedge [Member] | Power purchases [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 | 0
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0 | 0
Fair Value, Inputs, Level 3 [Member] | Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member] |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 0
Fair Value, Inputs, Level 3 [Member] | Held for Trading Hedge [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 34 | 39
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 183 | 389
Fair Value, Inputs, Level 3 [Member] | Held for Trading Hedge [Member] | Power swaps [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 9 | 0
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 0
Fair Value, Inputs, Level 3 [Member] | Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | (4) |
Fair Value, Inputs, Level 3 [Member] | Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 25 | 39
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 187 | 389
Fair Value, Inputs, Level 3 [Member] | Other Derivatives Hedge [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0 |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 0
Fair Value, Inputs, Level 3 [Member] | Other Derivatives Hedge [Member] | Foreign Exchange Forward [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | $ 0 |
Fair Value, Inputs, Level 3 [Member] | Other Derivatives Hedge [Member] | Interest Rate Swap [Member] |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | $ 0

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- Definition Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13495-108611 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226008-175313 + Details
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