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STOCK-BASED COMPENSATION - Employee Stock Options (Details)
shares in Millions
 12 Months Ended
Dec. 31, 2020
shares
CAD ($)
Dec. 31, 2020
shares
USD ($)
CAD ($)
Dec. 31, 2019
shares
CAD ($)
Dec. 31, 2019
shares
CAD ($)
USD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
CAD ($)
Dec. 31, 2018
USD ($)
Canada                
Disclosure of terms and conditions of share-based payment arrangement [line items]                
Number of options at January 1 (shares) 0.2 0.2 0.3 0.3        
Number of share options exercised in share-based payment arrangement (0.1) (0.1) (0.1) (0.1)        
Number of options at December 31 (shares) 0.1 0.1 0.2 0.2        
Weighted average exercise price of share options exercisable in share-based payment arrangement | $ $ 10 $ 10 $ 10 $ 10     $ 13  
Weighted average exercise price of share options exercised in share-based payment arrangement | $ $ 10   $ 16          
United States                
Disclosure of terms and conditions of share-based payment arrangement [line items]                
Number of options at January 1 (shares) 0.1 0.1 0.5 0.5        
Number of options cancelled/expired (shares) (0.1) (0.1) (0.4) (0.4)        
Number of options at December 31 (shares) 0.0 0.0 0.1 0.1        
Weighted average exercise price of share options exercisable in share-based payment arrangement | $         $ 0 $ 32   $ 37
Weighted average exercise price of share options expired in share-based payment arrangement | $   $ 32   $ 39