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Regulatory Assets and Liabilities (FAM Roll Forward) (Details) - CAD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Fuel Adjustment Mechanism [Roll Forward]    
Regulatory Assets, Current $ 165 $ 138
Regulatory Liability, Current 251 226
Regulatory Liability, Noncurrent $ 2,359 $ 2,242