Annual report pursuant to Section 13 and 15(d)

BENEFIT PLANS - Fair Value of Postretirement Assets (Details)

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BENEFIT PLANS - Fair Value of Postretirement Assets (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]      
Hedge funds, minimum number of days notice prior to redemption 30 days    
Hedge funds, maximum number of days notice prior to redemption 90 days    
Other Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets $ 15.4 $ 13.9 $ 17.1
Other Benefits | Large-Cap Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1.1 1.8  
Other Benefits | Small- and Mid-Cap Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.6 0.5  
Other Benefits | International Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 2.0 1.9  
Other Benefits | Fixed Income      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 9.5 7.4  
Other Benefits | Private Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.4 0.5  
Other Benefits | Hedge Funds      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1.0 1.0  
Other Benefits | Real Assets      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.5 0.6  
Other Benefits | Cash      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.3 0.2  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 2.0 2.8  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Benefits | Large-Cap Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1.1 1.8  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Benefits | Small- and Mid-Cap Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.6 0.5  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Benefits | International Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.3  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Benefits | Fixed Income      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Benefits | Private Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Benefits | Hedge Funds      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Benefits | Real Assets      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Benefits | Cash      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.3 0.2  
Significant Other Observable Inputs (Level 2) | Other Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 11.5 9.0  
Significant Other Observable Inputs (Level 2) | Other Benefits | Large-Cap Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Other Observable Inputs (Level 2) | Other Benefits | Small- and Mid-Cap Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Other Observable Inputs (Level 2) | Other Benefits | International Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 2.0 1.6  
Significant Other Observable Inputs (Level 2) | Other Benefits | Fixed Income      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 9.5 7.4  
Significant Other Observable Inputs (Level 2) | Other Benefits | Private Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Other Observable Inputs (Level 2) | Other Benefits | Hedge Funds      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Other Observable Inputs (Level 2) | Other Benefits | Real Assets      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Other Observable Inputs (Level 2) | Other Benefits | Cash      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3) | Other Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1.9 2.1  
Significant Unobservable Inputs (Level 3) | Other Benefits | Large-Cap Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3) | Other Benefits | Small- and Mid-Cap Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3) | Other Benefits | International Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3) | Other Benefits | Fixed Income      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3) | Other Benefits | Private Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.4 0.5  
Significant Unobservable Inputs (Level 3) | Other Benefits | Hedge Funds      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1.0 1.0  
Significant Unobservable Inputs (Level 3) | Other Benefits | Real Assets      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.5 0.6  
Significant Unobservable Inputs (Level 3) | Other Benefits | Cash      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets $ 0.0 $ 0.0