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Benefit Plans and Other Postretirement Benefits, Fair value measurements of plan assets (Details) - Pension Benefits - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure      
Fair value of pension plan assets $ 632.3 $ 581.0 $ 514.1
U.S. equities - large cap      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 124.9 112.0  
U.S. equities - small/mid cap and other      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 37.7 36.4  
International equities - growth      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 67.5 56.9  
International equities - other      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 96.2 74.9  
Alternative investment funds      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 22.4 21.1  
U.S. fixed income securities - short term      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets   18.1  
U.S. fixed income securities - intermediate term      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 29.2 26.8  
Fixed Income Securities - Long Term [Member]      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 130.5 118.6  
U.S. fixed income securities - high yield      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 15.5 14.2  
International fixed income securities - other      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 49.5 24.0  
Insurance contracts      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 40.0 37.0  
Real estate funds      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 10.8 10.7  
Cash and cash equivalents      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 8.1 30.3  
Quoted Prices in Active Markets for Identical Assets (Level 1)      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 162.4 165.8  
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. equities - large cap      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 63.5 56.9  
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. equities - small/mid cap and other      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | International equities - growth      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 61.6 51.8  
Quoted Prices in Active Markets for Identical Assets (Level 1) | International equities - other      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Alternative investment funds      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. fixed income securities - short term      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets   0.0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. fixed income securities - intermediate term      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 29.2 26.8  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed Income Securities - Long Term [Member]      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. fixed income securities - high yield      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | International fixed income securities - other      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Insurance contracts      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Real estate funds      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 8.1 30.3  
Significant Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 341.8 312.5  
Significant Observable Inputs (Level 2) | U.S. equities - large cap      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 61.4 55.1  
Significant Observable Inputs (Level 2) | U.S. equities - small/mid cap and other      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 37.7 36.4  
Significant Observable Inputs (Level 2) | International equities - growth      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 5.9 5.1  
Significant Observable Inputs (Level 2) | International equities - other      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 41.3 41.0  
Significant Observable Inputs (Level 2) | Alternative investment funds      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Significant Observable Inputs (Level 2) | U.S. fixed income securities - short term      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets   18.1  
Significant Observable Inputs (Level 2) | U.S. fixed income securities - intermediate term      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Significant Observable Inputs (Level 2) | Fixed Income Securities - Long Term [Member]      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 130.5 118.6  
Significant Observable Inputs (Level 2) | U.S. fixed income securities - high yield      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 15.5 14.2  
Significant Observable Inputs (Level 2) | International fixed income securities - other      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 49.5 24.0  
Significant Observable Inputs (Level 2) | Insurance contracts      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Significant Observable Inputs (Level 2) | Real estate funds      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Significant Observable Inputs (Level 2) | Cash and cash equivalents      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 50.8 47.7 $ 45.3
Significant Unobservable Inputs (Level 3) | U.S. equities - large cap      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3) | U.S. equities - small/mid cap and other      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3) | International equities - growth      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3) | International equities - other      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3) | Alternative investment funds      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3) | U.S. fixed income securities - short term      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets   0.0  
Significant Unobservable Inputs (Level 3) | U.S. fixed income securities - intermediate term      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3) | Fixed Income Securities - Long Term [Member]      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3) | U.S. fixed income securities - high yield      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3) | International fixed income securities - other      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Significant Unobservable Inputs (Level 3) | Insurance contracts      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 40.0 37.0  
Significant Unobservable Inputs (Level 3) | Real estate funds      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 10.8 10.7  
Significant Unobservable Inputs (Level 3) | Cash and cash equivalents      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Fair Value Measured At Net Asset Value      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 77.3 55.0  
Fair Value Measured At Net Asset Value | U.S. equities - large cap      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Fair Value Measured At Net Asset Value | U.S. equities - small/mid cap and other      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Fair Value Measured At Net Asset Value | International equities - growth      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Fair Value Measured At Net Asset Value | International equities - other      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 54.9 33.9  
Fair Value Measured At Net Asset Value | Alternative investment funds      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 22.4 21.1  
Fair Value Measured At Net Asset Value | U.S. fixed income securities - short term      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets   0.0  
Fair Value Measured At Net Asset Value | U.S. fixed income securities - intermediate term      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Fair Value Measured At Net Asset Value | Fixed Income Securities - Long Term [Member]      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Fair Value Measured At Net Asset Value | U.S. fixed income securities - high yield      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Fair Value Measured At Net Asset Value | International fixed income securities - other      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Fair Value Measured At Net Asset Value | Insurance contracts      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Fair Value Measured At Net Asset Value | Real estate funds      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets 0.0 0.0  
Fair Value Measured At Net Asset Value | Cash and cash equivalents      
Defined Benefit Plan Disclosure      
Fair value of pension plan assets $ 0.0 $ 0.0