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RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Liquidity and Credit Risk (Details) - CAD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
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 $ 262,000,000 $ 562,000,000
Cash collateral on asset exposure 0 0
Derivative instruments | Canadian financial institutions    
LIQUIDITY RISK AND CREDIT RISK    
Maximum credit exposure with respect to derivative instruments 21,000,000 28,000,000
Derivative instruments | United States financial institutions    
LIQUIDITY RISK AND CREDIT RISK    
Maximum credit exposure with respect to derivative instruments 51,000,000 107,000,000
Derivative instruments | European financial institutions    
LIQUIDITY RISK AND CREDIT RISK    
Maximum credit exposure with respect to derivative instruments 58,000,000 84,000,000
Derivative instruments | Asian financial institutions    
LIQUIDITY RISK AND CREDIT RISK    
Maximum credit exposure with respect to derivative instruments 0 6,000,000
Derivative instruments | Other    
LIQUIDITY RISK AND CREDIT RISK    
Maximum credit exposure with respect to derivative instruments $ 132,000,000 $ 337,000,000