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Risk Management Activities - Hedging Instruments (Details)
CAD in Thousands, € in Millions, ¥ in Millions, AUD in Millions, $ in Millions, kJ in Billions
 3 Months Ended 12 Months Ended
Mar. 31, 2017
CAD
Dec. 31, 2017
CAD
MWh
kJ
Dec. 31, 2016
CAD
MWh
kJ
Dec. 31, 2015
CAD
Dec. 31, 2017
USD ($)
MWh
kJ
Dec. 31, 2017
JPY (¥)
MWh
kJ
Dec. 31, 2017
AUD
MWh
kJ
Dec. 31, 2017
EUR (€)
MWh
kJ
Mar. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
MWh
kJ
Dec. 31, 2016
JPY (¥)
MWh
kJ
Dec. 31, 2016
AUD
MWh
kJ
Disclosure of detailed information about financial instruments [line items]                        
Derivative hedge, notional amount   CAD 690,000                    
Collateral paid (Note 14)   CAD 67,000 CAD 77,000                  
Interest rate   6.65% 6.65%   6.65% 6.65% 6.65% 6.65%   6.65% 6.65% 6.65%
Notional amount   CAD 3,664,000 CAD 4,311,000                  
Fair value asset   30,000 163,000                  
Unrealized pre-tax gains (losses)   2,000 0 CAD 3,000                
Cumulative gains (loss) in derivative instruments   CAD 1,000 CAD 4,000                  
Borrowings subject to floating interest rates   4.70% 6.00% 6.00% 4.70% 4.70% 4.70% 4.70%   6.00% 6.00% 6.00%
Cash flow hedges | Currency risk                        
Disclosure of detailed information about financial instruments [line items]                        
Fair value asset     CAD 0                  
Long-term debt | Discontinued Hedge Positions [Member] | Interest rate risk                        
Disclosure of detailed information about financial instruments [line items]                        
Derivative hedge, notional amount CAD 50,000                      
Fair value asset   CAD 1,000                    
Fair value hedge, cumulative gain (loss) amortized CAD 2,000                      
Long-term debt | Discontinued Cash Flow Hedge Position [Member] | Interest rate risk                        
Disclosure of detailed information about financial instruments [line items]                        
Derivative hedge, notional amount | $                 $ 690      
Long-term debt | Net investment hedges                        
Disclosure of detailed information about financial instruments [line items]                        
Notional amount | $         $ 480         $ 630    
Long-term debt | Fair value hedges | Interest rate risk                        
Disclosure of detailed information about financial instruments [line items]                        
Notional amount | $         50         50    
Fair value asset   0 3,000                  
Foreign Currency Forward Contracts | Cash flow hedges | Currency risk                        
Disclosure of detailed information about financial instruments [line items]                        
Fair value asset     0                  
Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures CAD 1 | Cash flow hedges | Currency risk                        
Disclosure of detailed information about financial instruments [line items]                        
Notional amount sold   9,000                    
Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures CAD 2 | Cash flow hedges | Currency risk                        
Disclosure of detailed information about financial instruments [line items]                        
Notional amount sold   14,000                    
Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures | Discontinued Hedge Positions [Member] | Currency risk                        
Disclosure of detailed information about financial instruments [line items]                        
Fair value asset   11,000 12,000                  
Fair value liability   9,000 3,000                  
Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures | Cash flow hedges | Currency risk                        
Disclosure of detailed information about financial instruments [line items]                        
Notional amount sold | AUD             AUD 1         AUD 8
Notional amount purchased         7 ¥ 119   € 9     ¥ 710  
Fair value asset   0                    
Foreign Exchange Forward Contracts - foreign-denominated debt | Discontinued Hedge Positions [Member] | Currency risk                        
Disclosure of detailed information about financial instruments [line items]                        
Notional amount sold   294,000                    
Notional amount purchased | $         230              
Fair value liability   4,000                    
Foreign Exchange Forward Contracts - foreign-denominated debt | Cash flow hedges | Currency risk                        
Disclosure of detailed information about financial instruments [line items]                        
Notional amount sold     26,000                  
Notional amount purchased | $                   20    
Fair value asset   0                    
Cross-Currency Swaps - foreign-denominated debt | Discontinued Hedge Positions [Member] | Currency risk                        
Disclosure of detailed information about financial instruments [line items]                        
Notional amount sold   306,000                    
Notional amount purchased | $         $ 270              
Fair value liability   (35,000)                    
Cross-Currency Swaps - foreign-denominated debt | Cash flow hedges | Currency risk                        
Disclosure of detailed information about financial instruments [line items]                        
Notional amount sold     434,000                  
Notional amount purchased | $                   400    
Fair value asset   CAD 0                    
Cross-Currency Swaps - foreign-denominated debt 2 | Cash flow hedges | Currency risk                        
Disclosure of detailed information about financial instruments [line items]                        
Notional amount sold     CAD 306,000                  
Notional amount purchased | $                   $ 270    
Natural gas | Non-Hedges [Member] | Commodity risk management                        
Disclosure of detailed information about financial instruments [line items]                        
Notional amount sold | kJ   74,194 146,113   74,194 74,194 74,194 74,194   146,113 146,113 146,113
Notional amount purchased | kJ   103,805 173,187   103,805 103,805 103,805 103,805   173,187 173,187 173,187
Transmission | Non-Hedges [Member] | Commodity risk management                        
Disclosure of detailed information about financial instruments [line items]                        
Notional amount sold | MWh   1,000 0   1,000 1,000 1,000 1,000   0 0 0
Notional amount purchased | MWh   3,455,000 3,429,000   3,455,000 3,455,000 3,455,000 3,455,000   3,429,000 3,429,000 3,429,000
2019 | Cross-Currency Swaps - foreign-denominated debt | Cash flow hedges | Currency risk                        
Disclosure of detailed information about financial instruments [line items]                        
Fair value asset     CAD 59,000                  
2018 | Cross-Currency Swaps - foreign-denominated debt | Cash flow hedges | Currency risk                        
Disclosure of detailed information about financial instruments [line items]                        
Fair value asset     104,000                  
Minimum | Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures | Discontinued Hedge Positions [Member] | Currency risk                        
Disclosure of detailed information about financial instruments [line items]                        
Notional amount purchased   CAD 104,000 216,000                  
Maximum | Foreign Currency Forward Contracts | Cash flow hedges | Currency risk                        
Disclosure of detailed information about financial instruments [line items]                        
Fair value asset     1,000                  
Maximum | Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures | Discontinued Hedge Positions [Member] | Currency risk                        
Disclosure of detailed information about financial instruments [line items]                        
Notional amount purchased   CAD 157,000 CAD 219,000                  
U.S. Dollars | Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures | Discontinued Hedge Positions [Member] | Currency risk                        
Disclosure of detailed information about financial instruments [line items]                        
Notional amount sold | $         $ 73         $ 152    
Australia, Dollars | Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures | Discontinued Hedge Positions [Member] | Currency risk                        
Disclosure of detailed information about financial instruments [line items]                        
Notional amount sold | AUD             AUD 170         AUD 232