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STOCK-BASED COMPENSATION - Employee Stock Options (Details)
shares in Millions
 12 Months Ended
Dec. 31, 2019
USD ($)
shares
Dec. 31, 2019
CAD ($)
shares
Dec. 31, 2018
USD ($)
shares
Dec. 31, 2018
CAD ($)
shares
Canada        
Disclosure of terms and conditions of share-based payment arrangement [line items]        
Number of options at January 1 (shares) | shares 0.3 0.3 0.3 0.3
Number of options at December 31 (shares) | shares 0.2 0.2 0.3 0.3
Average price of stock options outstanding at January 1 (in dollars per share) | $   $ 13   $ 13
Number of share options exercised in share-based payment arrangement | shares (0.1) (0.1) 0.0 0.0
Weighted average exercise price of share options exercised in share-based payment arrangement | $   $ 16   $ 10
Average price of stock options outstanding at December 31 (in dollars per share) | $   $ 10   $ 13
United States        
Disclosure of terms and conditions of share-based payment arrangement [line items]        
Number of options at January 1 (shares) | shares 0.5 0.5 0.7 0.7
Number of options forfeited (shares) | shares 0.0 0.0 (0.1) (0.1)
Number of options cancelled/expired (shares) | shares (0.4) (0.4) (0.1) (0.1)
Number of options at December 31 (shares) | shares 0.1 0.1 0.5 0.5
Average price of stock options outstanding at January 1 (in dollars per share) | $ $ 37   $ 40  
Average price of options forfeited (in dollars per share) | $ 0   34  
Average price of options cancelled/expired (in dollars per share) | $ 39   49  
Average price of stock options outstanding at December 31 (in dollars per share) | $ $ 32   $ 37