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Derivatives Instruments (Realized and Unrealized Gains (Losses) with Respect to Other derivatives) (Details)
shares in Millions, $ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2020
CAD ($)
Dec. 31, 2019
CAD ($)
Dec. 31, 2020
USD ($)
shares
Derivative Instruments, Gain (Loss) [Line Items]      
Equity derivative hedges, return of shares | shares     2.8
Combined notional amount     $ 100
Foreign exchange forwards      
Derivative Instruments, Gain (Loss) [Line Items]      
Total gains (losses) in net income $ 13 $ 0  
Foreign exchange forwards | Operating, maintenance and general expense      
Derivative Instruments, Gain (Loss) [Line Items]      
Unrealized gain on equity derivatives 0 0  
Total gains (losses) in net income 0 0  
Foreign exchange forwards | Other income (expense), net      
Derivative Instruments, Gain (Loss) [Line Items]      
Unrealized gain on equity derivatives 15 0  
Total gains (losses) in net income (2) 0  
Equity derivatives      
Derivative Instruments, Gain (Loss) [Line Items]      
Total gains (losses) in net income (4) 28  
Equity derivatives | Operating, maintenance and general expense      
Derivative Instruments, Gain (Loss) [Line Items]      
Unrealized gain on equity derivatives (1) 1  
Total gains (losses) in net income (3) 27  
Equity derivatives | Other income (expense), net      
Derivative Instruments, Gain (Loss) [Line Items]      
Unrealized gain on equity derivatives 0 0  
Total gains (losses) in net income $ 0 $ 0