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