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v3.21.2

Subsequent Event - Narrative (Details) $ in Millions | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2021 USD ($) | Jun. 30, 2021 USD ($) | Jun. 30, 2020 USD ($) | Jun. 30, 2021 USD ($) | Jun. 30, 2020 USD ($) | Feb. 28, 2021 employee | Dec. 31, 2020 USD ($)
Subsequent Event [Line Items] |  |  |  |  |  |  |
Income tax provision |  | $ 4.0 | $ 92.6 | $ 9.5 | $ 103.4 |  |
Accrued postretirement benefits |  | 317.1 |  | 317.1 |  |  | $ 326.7
Entity number of employees covered by USW Collective Bargaining Agreements that expired February 28 2021 | employee |  |  |  |  |  | 1,100 |
Other Postretirement Benefit Plan, Defined Benefit | Collective-Bargaining Arrangement, Other |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
Accrued postretirement benefits |  | 43.0 |  | 43.0 |  |  |
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax |  | $ (22.0) |  | $ (22.0) |  |  |
Forecast | Collective-Bargaining Arrangement, Other |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
Income tax provision | $ 15.0 |  |  |  |  |  |
Forecast | Other Postretirement Benefit Plan, Defined Benefit | Collective-Bargaining Arrangement, Other |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement | $ 65.0 |  |  |  |  |  |

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