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Pension Benefits - Fair Value Plan Assets Measurements (Details) - Pension Plans, Defined Benefit - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | $ 405.4 | $ 664.6 | $ 525.3
Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 249.7 | 400.2 |
Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 82.0 | 128.9 |
Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 | $ 0.3
United States Government and Government Agencies and Authorities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 68.3 | 158.4 |
United States Government and Government Agencies and Authorities | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 68.3 | 158.4 |
United States Government and Government Agencies and Authorities | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
United States Government and Government Agencies and Authorities | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
States and Political Subdivisions |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.7 | 0.1 |
States and Political Subdivisions | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
States and Political Subdivisions | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.7 | 0.1 |
States and Political Subdivisions | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Corporate Bonds and Notes |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 81.3 | 107.3 |
Corporate Bonds and Notes | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Corporate Bonds and Notes | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 81.3 | 107.3 |
Corporate Bonds and Notes | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Other Industries |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 64.8 | 161.5 |
Other Industries | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 64.8 | 140.0 |
Other Industries | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 21.5 |
Other Industries | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Collective Investment Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 32.1 | 71.8 |
Collective Investment Funds | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Collective Investment Funds | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Collective Investment Funds | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Bond Exchange Traded Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 108.6 | 92.8 |
Bond Exchange Traded Funds | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 108.6 | 92.8 |
Bond Exchange Traded Funds | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Bond Exchange Traded Funds | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Limited Liability Companies and Limited Partnerships |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 41.6 | 63.7 |
Limited Liability Companies and Limited Partnerships | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Limited Liability Companies and Limited Partnerships | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Limited Liability Companies and Limited Partnerships | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Short-term Investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 7.4 | 10.0 |
Short-term Investments | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 7.4 | 10.0 |
Short-term Investments | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Short-term Investments | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Receivables and Other |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.6 | (1.0) |
Receivables and Other | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.6 | (1.0) |
Receivables and Other | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Receivables and Other | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Measured at Net Asset Value |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 73.7 | 135.5 |
Measured at Net Asset Value | United States Government and Government Agencies and Authorities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Measured at Net Asset Value | States and Political Subdivisions |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Measured at Net Asset Value | Corporate Bonds and Notes |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Measured at Net Asset Value | Other Industries |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Measured at Net Asset Value | Collective Investment Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 32.1 | 71.8 |
Measured at Net Asset Value | Bond Exchange Traded Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Measured at Net Asset Value | Limited Liability Companies and Limited Partnerships |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 41.6 | 63.7 |
Measured at Net Asset Value | Short-term Investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | 0.0 | 0.0 |
Measured at Net Asset Value | Receivables and Other |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension plan assets | $ 0.0 | $ 0.0 |

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