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RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - LIQUIDITY AND CREDIT RISK (Details) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
LIQUIDITY RISK AND CREDIT RISK    
Rolling time period over which the Company forecasts cash requirements 12 months  
Period of anticipated requirements for which the Company maintains sufficient liquidity through committed credit facilities 1 year  
Period after which receivables are classified as past due 30 days  
Derivative instruments    
LIQUIDITY RISK AND CREDIT RISK    
Maximum credit exposure with respect to derivative instruments $ 562 $ 385
Derivative instruments | Canadian financial institutions    
LIQUIDITY RISK AND CREDIT RISK    
Maximum credit exposure with respect to derivative instruments 28 82
Derivative instruments | United States financial institutions    
LIQUIDITY RISK AND CREDIT RISK    
Maximum credit exposure with respect to derivative instruments 107 19
Derivative instruments | European financial institutions    
LIQUIDITY RISK AND CREDIT RISK    
Maximum credit exposure with respect to derivative instruments 84 145
Derivative instruments | Asian financial institutions    
LIQUIDITY RISK AND CREDIT RISK    
Maximum credit exposure with respect to derivative instruments 6 2
Derivative instruments | Other    
LIQUIDITY RISK AND CREDIT RISK    
Maximum credit exposure with respect to derivative instruments $ 337 $ 137