Subsequent Event - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jul. 31, 2021 USD ($)	Feb. 28, 2021 employee	Dec. 31, 2020 USD ($)
Subsequent Event [Line Items]
Income tax provision	$ 22.0	$ 0.8	$ 31.5	$ 104.2
Accrued postretirement benefits	269.4		$ 269.4				$ 326.7
Entity number of employees covered by USW Collective Bargaining Agreements that expired February 28 2021 | employee						1,100
Collective-Bargaining Arrangement, Other
Subsequent Event [Line Items]
Income tax provision	$ 15.5
Other Postretirement Benefit Plan, Defined Benefit | Collective-Bargaining Arrangement, Other
Subsequent Event [Line Items]
Accrued postretirement benefits					$ 43.0
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax					$ (21.9)