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Pension and Savings Plans - Fair Value of Assets in the Master Trust by Asset (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | --- | ---
Level 3 |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | $ 1.2 | $ 4.1 | $ 6.2 | $ 11.8
Pension Plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 847.5 | 855.2 | $ 764.8 |
Pension Plan | Fair Value, Inputs, Level 1, 2 and 3 |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 805.4 | 812.1 |  |
Pension Plan | Fair Value, Inputs, Level 1, 2 and 3 | Corporate bonds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 623.3 | 598.3 |  |
Pension Plan | Fair Value, Inputs, Level 1, 2 and 3 | U.S. government securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 142.6 | 154.9 |  |
Pension Plan | Fair Value, Inputs, Level 1, 2 and 3 | International government securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 18.7 | 18.2 |  |
Pension Plan | Fair Value, Inputs, Level 1, 2 and 3 | Asset-backed securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 4.7 | 3.4 |  |
Pension Plan | Fair Value, Inputs, Level 1, 2 and 3 | Cash funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 14.9 | 33.2 |  |
Pension Plan | Fair Value, Inputs, Level 1, 2 and 3 | Real estate interests |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 1.2 | 4.1 |  |
Pension Plan | Level 1 |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 136.3 | 169.1 |  |
Pension Plan | Level 1 | Corporate bonds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Pension Plan | Level 1 | U.S. government securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 121.4 | 135.9 |  |
Pension Plan | Level 1 | International government securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Pension Plan | Level 1 | Asset-backed securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Pension Plan | Level 1 | Cash funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 14.9 | 33.2 |  |
Pension Plan | Level 1 | Real estate interests |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Pension Plan | Level 2 |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 667.9 | 638.9 |  |
Pension Plan | Level 2 | Corporate bonds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 623.3 | 598.3 |  |
Pension Plan | Level 2 | U.S. government securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 21.2 | 19.0 |  |
Pension Plan | Level 2 | International government securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 18.7 | 18.2 |  |
Pension Plan | Level 2 | Asset-backed securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 4.7 | 3.4 |  |
Pension Plan | Level 2 | Cash funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Pension Plan | Level 2 | Real estate interests |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Pension Plan | Level 3 |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 1.2 | 4.1 |  |
Pension Plan | Level 3 | Corporate bonds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Pension Plan | Level 3 | U.S. government securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Pension Plan | Level 3 | International government securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Pension Plan | Level 3 | Asset-backed securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Pension Plan | Level 3 | Cash funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Pension Plan | Level 3 | Real estate interests |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | 1.2 | 4.1 |  |
Pension Plan | Fair Value Measured at Net Asset Value Per Share | Private mutual funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined benefit plan, assets | $ 42.1 | $ 43.1 |  |

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