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EMPLOYEE BENEFITS - Summary of Plan Assets (Details) - Postretirement Benefits - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Plan assets $ 206 $ 198 $ 176
Fair Value, Inputs, Level 1, 2 and 3      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Plan assets 205 204  
Funds - Municipal/Provincial Bonds      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Plan assets $ 13 0  
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List] us-gaap:FairValueInputsLevel1Member    
Funds - Corporate Bonds Fund      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Plan assets $ 5 0  
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List] us-gaap:FairValueInputsLevel1Member    
Federal Money Market Fund      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Plan assets $ 11 204  
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List] us-gaap:FairValueInputsLevel1Member    
Corporate Bonds      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Plan assets $ 102 0  
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List] us-gaap:FairValueInputsLevel2Member    
Government/Municipal Bonds      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Plan assets $ 8 0  
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List] us-gaap:FairValueInputsLevel2Member    
Equity Funds      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Plan assets $ 66 0  
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List] us-gaap:FairValueInputsLevel2Member    
Trust Assets and Liabilities, Net      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Plan assets $ 1 $ (6)