Supplemental Financial Statement Information - Schedule of Other Income, Net (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Other Income and Expenses [Abstract]
Rent and royalty income	$ 0.2	$ 0.1	$ 0.7	$ 0.6
Gains from disposal of property, plant and equipment, net	0.1	0.8	2.5	3.3
Net equity (loss) gain on joint ventures	0.4	(1.0)	0.4	(6.6)
Gain from sale of business	13.7	0.0	13.7	0.0
Adjustment to indemnification for conditional ARO costs	0.0	0.0	0.0	4.3
Other	0.1	(0.3)	0.1	0.0
Other income (expense), net	14.5	(0.4)	17.4	(0.8)
Gain on the sale of oil and gas rights				2.5
Allegheny & Tsingshan Stainless | Employee Severance [Member]
Other Income and Expenses [Abstract]
Net equity (loss) gain on joint ventures	$ 0.0	$ 0.0	$ 0.0	$ (2.4)