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GOODWILL AND OTHER INTANGIBLE ASSETS - Intangible Assets (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, beginning balance $ 226 $ 227
Additions other than through business combinations, intangible assets other than goodwill 5 1
Disposals, intangible assets and goodwill 46  
Amortization and impairment losses (16) (2)
Intangible assets, ending balance 169 226
Cost    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, ending balance 385  
Accumulated amortization and impairment losses    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, ending balance (216)  
Water rights    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, beginning balance 72 71
Additions other than through business combinations, intangible assets other than goodwill 0 1
Disposals, intangible assets and goodwill 5  
Amortization and impairment losses 0 0
Intangible assets, ending balance 67 72
Water rights | Cost    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, ending balance 67  
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 7 8
Additions other than through business combinations, intangible assets other than goodwill 0 0
Disposals, intangible assets and goodwill 0  
Amortization and impairment losses (1) (1)
Intangible assets, ending balance 6 7
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 (11)  
Supply contracts    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, beginning balance 7 8
Additions other than through business combinations, intangible assets other than goodwill 0 0
Disposals, intangible assets and goodwill 0  
Amortization and impairment losses (3) (1)
Intangible assets, ending balance 4 7
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 (35)  
Exploration potential    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, beginning balance 140 140
Additions other than through business combinations, intangible assets other than goodwill 5 0
Disposals, intangible assets and goodwill 41  
Amortization and impairment losses (12) 0
Intangible assets, ending balance 92 $ 140
Exploration potential | Cost    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, ending balance 262  
Exploration potential | Accumulated amortization and impairment losses    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, ending balance $ (170)