Annual report pursuant to Section 13 and 15(d)

BENEFIT PLANS - Fair Value of Postretirement Assets (Details)

v3.20.4
BENEFIT PLANS - Fair Value of Postretirement Assets (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
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 $ 15.8 $ 15.4 $ 13.9
Other Benefits | U.S. Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1.0 1.7  
Other Benefits | International Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.9 2.0  
Other Benefits | Fixed Income      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 12.2 9.5  
Other Benefits | Private Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.3 0.4  
Other Benefits | Hedge Funds      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.4 1.0  
Other Benefits | Real Assets      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.4 0.5  
Other Benefits | Cash      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.6 0.3  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1.6 2.0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Benefits | U.S. Equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1.0 1.7  
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 0.0  
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 0.6 0.3  
Significant Other Observable Inputs (Level 2) | Other Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 13.1 11.5  
Significant Other Observable Inputs (Level 2) | Other Benefits | U.S. 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 0.9 2.0  
Significant Other Observable Inputs (Level 2) | Other Benefits | Fixed Income      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 12.2 9.5  
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 1.1 1.9  
Significant Unobservable Inputs (Level 3) | Other Benefits | U.S. 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 0.3 0.4  
Significant Unobservable Inputs (Level 3) | Other Benefits | Hedge Funds      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.4 1.0  
Significant Unobservable Inputs (Level 3) | Other Benefits | Real Assets      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.4 0.5  
Significant Unobservable Inputs (Level 3) | Other Benefits | Cash      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets $ 0.0 $ 0.0