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GOODWILL AND OTHER INTANGIBLE ASSETS - Intangible Assets (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, beginning balance $ 272 $ 271
Additions 16 4
Disposals (16)  
Amortization and impairment losses (17) (3)
Intangible assets, ending balance 255 272
Cost    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, ending balance 425  
Accumulated amortization and impairment losses    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, ending balance 170  
Water rights    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, beginning balance 87 87
Additions 0 0
Disposals (16)  
Amortization and impairment losses 0 0
Intangible assets, ending balance 71 87
Water rights | Cost    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, ending balance 71  
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 11 12
Additions 0 0
Disposals 0  
Amortization and impairment losses (2) (1)
Intangible assets, ending balance 9 11
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 8  
Supply contracts    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, beginning balance 14 16
Additions 0 0
Disposals 0  
Amortization and impairment losses (3) (2)
Intangible assets, ending balance 11 14
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 28  
Exploration potential    
Reconciliation of changes in intangible assets other than goodwill [abstract]    
Intangible assets, beginning balance 160 156
Additions 16 4
Disposals 0  
Amortization and impairment losses (12) 0
Intangible assets, ending balance 164 $ 160
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 $ 134