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PERSONNEL EXPENSES AND DEFERRED EMPLOYEE BENEFITS - Share Based Payments - Narrative (Details)
12 Months Ended
Dec. 31, 2019
USD ($)
shares
Dec. 31, 2018
USD ($)
shares
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2016
May 07, 2019
shares
May 09, 2018
shares
May 10, 2017
shares
Disclosure of terms and conditions of share-based payment arrangement [line items]              
Number of share options granted in share-based payment arrangement 0 0 0        
Employee Stock Option              
Disclosure of terms and conditions of share-based payment arrangement [line items]              
Compensation expense | $ $ 0 $ 0 $ 0        
Restricted Share Units (RSUs) and Performance Share Units (PSUs)              
Disclosure of terms and conditions of share-based payment arrangement [line items]              
Compensation expense | $ $ 0 $ 31,000,000 $ 31,000,000        
ArcelorMittal Global Stock Option 2009-2018 | Top of range              
Disclosure of terms and conditions of share-based payment arrangement [line items]              
Number of shares authorized for grant 100,000,000            
ArcelorMittal Global Stock Option 2009-2018 | Top of range | Employee Stock Option              
Disclosure of terms and conditions of share-based payment arrangement [line items]              
Expiration period 10 years            
CEO Office Plan | Performance Share Units (PSUs)              
Disclosure of terms and conditions of share-based payment arrangement [line items]              
Award performance period 3 years 3 years 3 years 5 years      
CEO Office Plan | Top of range | Performance Share Units (PSUs)              
Disclosure of terms and conditions of share-based payment arrangement [line items]              
Number of shares authorized for grant         2,500,000 1,500,000 3,000,000
ArcelorMittal Equity Incentive Plan | Performance Share Units (PSUs)              
Disclosure of terms and conditions of share-based payment arrangement [line items]              
Award performance period 3 years            
ArcelorMittal Equity Incentive Plan | Restricted Share Units (RSUs)              
Disclosure of terms and conditions of share-based payment arrangement [line items]              
Award vesting period (in years) 3 years            
Percentage of award vesting rights (as a percentage) 100.00%            
Pro forma | ArcelorMittal Global Stock Option 2009-2018              
Disclosure of terms and conditions of share-based payment arrangement [line items]              
Number of shares authorized for grant 33,333,333