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INCOME TAXES - U.S. Tax Reform and Alberta Tax Rate Reduction (Details) - CAD ($)
$ in Millions
 1 Months Ended 3 Months Ended 12 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Dec. 31, 2019
Dec. 31, 2017
Income Tax Contingency [Line Items]        
Provisional deferred income tax recovery (expense)       $ 816
Increase in regulatory liabilities for businesses subject to RRA       1,686
U.S. Tax Reform, additional deferred income tax recovery   $ 52    
U.S. Tax Reform, additional deferred income tax recovery subject to RRA   $ 115    
Alberta Tax Rate Reform, additional deferred income tax recovery $ 32      
Alberta Tax Rate Reform, additional deferred income tax recovery, subject to RRA     $ 83  
Other Post-Retirement Benefit Plans        
Income Tax Contingency [Line Items]        
Provisional deferred income tax recovery (expense)       $ (12)