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Derivatives (Derivative Assets and Liabilities) (Details) - Designated as Hedging Instrument [Member] - CAD CAD in Millions | Dec. 31, 2016 | Dec. 31, 2015
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Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Collateral, Obligation to Return Cash, Offset | CAD (131.0) | CAD (240.0)
Derivative Asset, Total | 276.0 | 418.0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Collateral, Right to Reclaim Cash, Offset | (131.0) | (240.0)
Derivative Liability, Total | 475.0 | 445.0
Current Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 250.0 | 439.0
Derivative Asset, Collateral, Obligation to Return Cash, Offset | (105.0) | (189.0)
Derivative Asset, Total | 145.0 | 250.0
Noncurrent Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 157.0 | 219.0
Derivative Asset, Collateral, Obligation to Return Cash, Offset | (26.0) | (51.0)
Derivative Asset, Total | 131.0 | 168.0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 176.0 | 147.3
Derivative Liability, Collateral, Right to Reclaim Cash, Offset | (26.0) | (51.3)
Derivative Liability, Total | 150.0 | 96.0
Current Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 430.0 | 538.0
Derivative Liability, Collateral, Right to Reclaim Cash, Offset | (105.0) | (189.0)
Derivative Liability, Total | 325.0 | 349.0
Cash Flow Hedging [Member] | Current Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 5.0 | 8.0
Cash Flow Hedging [Member] | Noncurrent Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 5.0 | 12.0
Cash Flow Hedging [Member] | Current Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 14.0 | 15.0
Cash Flow Hedging [Member] | Noncurrent Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 13.0 | 31.3
Cash Flow Hedging [Member] | Power swaps [Member] | Current Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 5.0 | 8.0
Cash Flow Hedging [Member] | Power swaps [Member] | Current Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 2.0 | 1.0
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] | Current Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 0.0 | 0.0
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] | Noncurrent Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 0.0 | 0.0
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] | Current Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 12.0 | 14.0
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] | Noncurrent Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 10.0 | 27.2
Cash Flow Hedging [Member] | Power swaps and physical contracts [Member] | Noncurrent Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 5.0 | 12.0
Cash Flow Hedging [Member] | Power swaps and physical contracts [Member] | Noncurrent Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 3.0 | 4.1
Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Noncurrent Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 0.0 | 0.0
Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Noncurrent Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 0.0 | 0.0
Regulatory Deferral Hedge [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Collateral, Obligation to Return Cash, Offset | (10.0) | 0.0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Collateral, Right to Reclaim Cash, Offset | (10.0) | 0.0
Regulatory Deferral Hedge [Member] | Current Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 119.0 | 89.0
Regulatory Deferral Hedge [Member] | Noncurrent Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 120.0 | 121.0
Regulatory Deferral Hedge [Member] | Current Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 14.0 | 43.0
Regulatory Deferral Hedge [Member] | Noncurrent Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 8.0 | 21.0
Regulatory Deferral Hedge [Member] | Foreign Exchange Forward [Member] | Current Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 56.0 | 85.0
Regulatory Deferral Hedge [Member] | Foreign Exchange Forward [Member] | Noncurrent Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 50.0 | 121.0
Regulatory Deferral Hedge [Member] | Foreign Exchange Forward [Member] | Current Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 0.0 | 10.0
Regulatory Deferral Hedge [Member] | Foreign Exchange Forward [Member] | Noncurrent Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 0.0 | 0.0
Regulatory Deferral Hedge [Member] | Derivative instrument whose primary underlying risk is tied to commodity prices for coal purchases. | Current Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 26.0 | 0.0
Regulatory Deferral Hedge [Member] | Derivative instrument whose primary underlying risk is tied to commodity prices for coal purchases. | Noncurrent Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 57.0 | 0.0
Regulatory Deferral Hedge [Member] | Derivative instrument whose primary underlying risk is tied to commodity prices for coal purchases. | Current Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 9.0 | 12.0
Regulatory Deferral Hedge [Member] | Derivative instrument whose primary underlying risk is tied to commodity prices for coal purchases. | Noncurrent Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 0.0 | 4.4
Regulatory Deferral Hedge [Member] | Natural gas purchases and sales [Member] | Current Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 28.0 | 2.0
Regulatory Deferral Hedge [Member] | Natural gas purchases and sales [Member] | Noncurrent Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 5.0 | 0.0
Regulatory Deferral Hedge [Member] | Natural gas purchases and sales [Member] | Current Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 0.0 | 1.0
Regulatory Deferral Hedge [Member] | Natural gas purchases and sales [Member] | Noncurrent Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 2.0 | 0.0
Regulatory Deferral Hedge [Member] | Heavy fuel oil purchases [Member] | Current Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 6.0 | 0.0
Regulatory Deferral Hedge [Member] | Heavy fuel oil purchases [Member] | Noncurrent Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 4.0 | 0.0
Regulatory Deferral Hedge [Member] | Heavy fuel oil purchases [Member] | Current Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 4.0 | 20.0
Regulatory Deferral Hedge [Member] | Heavy fuel oil purchases [Member] | Noncurrent Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 3.0 | 16.6
Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member] | Current Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 0.0 | 2.0
Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member] | Noncurrent Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 0.0 | 0.0
Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member] | Current Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 0.0 | 0.0
Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member] | Noncurrent Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 0.0 | 0.0
Held for Trading Hedge [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Collateral, Obligation to Return Cash, Offset | (121.0) | (240.0)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Collateral, Right to Reclaim Cash, Offset | (121.0) | (240.0)
Held for Trading Hedge [Member] | Current Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 126.0 | 250.0
Held for Trading Hedge [Member] | Noncurrent Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 32.0 | 86.0
Held for Trading Hedge [Member] | Current Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 401.0 | 480.0
Held for Trading Hedge [Member] | Noncurrent Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 154.0 | 92.0
Held for Trading Hedge [Member] | Foreign Exchange Forward [Member] | Current Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 0.0 | 0.0
Held for Trading Hedge [Member] | Foreign Exchange Forward [Member] | Noncurrent Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 0.0 | 1.0
Held for Trading Hedge [Member] | Foreign Exchange Forward [Member] | Current Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 0.0 | 2.0
Held for Trading Hedge [Member] | Foreign Exchange Forward [Member] | Noncurrent Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 0.0 | 1.0
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Current Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 33.0 | 151.0
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Noncurrent Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 14.0 | 13.0
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Current Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 44.0 | 119.0
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Noncurrent Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 27.0 | 28.0
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Current Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 93.0 | 99.0
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Noncurrent Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 18.0 | 72.0
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Current Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 357.0 | 359.0
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Noncurrent Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 127.0 | 63.0
Other Derivatives Hedge [Member] | Current Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 0.0 | 92.0
Other Derivatives Hedge [Member] | Noncurrent Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 0.0 | 0.0
Other Derivatives Hedge [Member] | Current Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 1.0 | 0.0
Other Derivatives Hedge [Member] | Noncurrent Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 1.0 | 3.0
Other Derivatives Hedge [Member] | Foreign Exchange Forward [Member] | Current Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 0.0 | 92.0
Other Derivatives Hedge [Member] | Foreign Exchange Forward [Member] | Current Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 1.0 | 0.0
Other Derivatives Hedge [Member] | Interest Rate Swap [Member] | Noncurrent Assets |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 0.0 | 0.0
Other Derivatives Hedge [Member] | Interest Rate Swap [Member] | Noncurrent Liabilities |  |
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | CAD 1.0 | CAD 3.0

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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=73719730&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=73719730&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=51825145&loc=SL20226008-175313 + Details
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- Definition Fair value, after the effects of master netting arrangements, of a financial liability or 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 liabilities 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 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226008-175313 Reference 2: 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=73719730&loc=d3e13495-108611 Reference 3: 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 4: 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=73719730&loc=d3e13433-108611 Reference 5: 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 6: 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 + Details
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