Annual report pursuant to Section 13 and 15(d)

BENEFIT PLANS (Fair Value of Pension Assets) (Details)

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BENEFIT PLANS (Fair Value of Pension Assets) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
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    
Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets $ 561.1 $ 515.3 $ 518.9
Large-Cap Equity | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 125.5 128.1  
Small- and Mid-Cap Equity | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 39.0 31.4  
International Equity | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 105.1 80.8  
Fixed Income | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 204.0 174.2  
Private Equity      
Defined Benefit Plan Disclosure [Line Items]      
Commitments related to investments 10.7 12.3  
Private Equity | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 24.1 33.5  
Hedge Funds | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 35.0 33.7  
Real Assets      
Defined Benefit Plan Disclosure [Line Items]      
Commitments related to investments 0.5 0.5  
Real Assets | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 18.7 19.3  
Cash | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 9.7 14.3  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 187.4 189.4  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Large-Cap Equity | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 125.5 128.1  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Small- and Mid-Cap Equity | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 39.0 31.4  
Quoted Prices in Active Markets for Identical Assets (Level 1) | International Equity | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 13.2 15.6  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed Income | Pension Benefits      
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) | Private Equity | Pension Benefits      
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) | Hedge Funds | Pension Benefits      
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) | Real Assets | Pension Benefits      
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) | Cash | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 9.7 14.3  
Significant Other Observable Inputs (Level 2) | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 295.9 239.4  
Significant Other Observable Inputs (Level 2) | Large-Cap Equity | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Other Observable Inputs (Level 2) | Small- and Mid-Cap Equity | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Other Observable Inputs (Level 2) | International Equity | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 91.9 65.2  
Significant Other Observable Inputs (Level 2) | Fixed Income | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 204.0 174.2  
Significant Other Observable Inputs (Level 2) | Private Equity | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Other Observable Inputs (Level 2) | Hedge Funds | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Other Observable Inputs (Level 2) | Real Assets | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Other Observable Inputs (Level 2) | Cash | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3) | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 77.8 86.5  
Significant Unobservable Inputs (Level 3) | Large-Cap Equity | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3) | Small- and Mid-Cap Equity | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3) | International Equity | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3) | Fixed Income | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3) | Private Equity | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 24.1 33.5  
Significant Unobservable Inputs (Level 3) | Hedge Funds | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 35.0 33.7  
Significant Unobservable Inputs (Level 3) | Real Assets | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 18.7 19.3  
Significant Unobservable Inputs (Level 3) | Cash | Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets $ 0.0 $ 0.0