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GOODWILL AND OTHER INTANGIBLE ASSETS - Intangible Assets (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, beginning balance $ 227 $ 255
Additions other than through business combinations, intangible assets other than goodwill 1  
Amortization (2) (28)
Intangible assets, ending balance 226 227
Cost    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, ending balance 426  
Accumulated amortization and impairment losses    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, ending balance 200  
Water rights    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, beginning balance 71 71
Additions other than through business combinations, intangible assets other than goodwill 1  
Amortization 0 0
Intangible assets, ending balance 72 71
Water rights | Cost    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, ending balance 72  
Water rights | Accumulated amortization and impairment losses    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, ending balance 0  
Technology    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, beginning balance 8 9
Additions other than through business combinations, intangible assets other than goodwill 0  
Amortization (1) (1)
Intangible assets, ending balance 7 8
Technology | Cost    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, ending balance 17  
Technology | Accumulated amortization and impairment losses    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, ending balance 10  
Supply contracts    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, beginning balance 8 11
Additions other than through business combinations, intangible assets other than goodwill 0  
Amortization (1) (3)
Intangible assets, ending balance 7 8
Supply contracts | Cost    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, ending balance 39  
Supply contracts | Accumulated amortization and impairment losses    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, ending balance 32  
Exploration potential    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, beginning balance 140 164
Additions other than through business combinations, intangible assets other than goodwill 0  
Amortization 0 (24)
Intangible assets, ending balance 140 $ 140
Exploration potential | Cost    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, ending balance 298  
Exploration potential | Accumulated amortization and impairment losses    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, ending balance $ 158