Supplemental Financial Statement Information - Schedule of Other Income, Net (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Other Income and Expenses [Abstract]
Rent and royalty income	$ 0.1		$ 1.4	$ 0.6	$ 2.8
Gains from disposal of property, plant and equipment, net	0.8		62.3	3.3	90.6
Net equity (loss) gain on joint ventures	(1.0)		(2.4)	(6.6)	(8.7)
Loss from sale of businesses	0.0		5.8	0.0	(1.9)
Adjustment to indemnification for conditional ARO costs	0.0		0.0	4.3	0.0
Other	(0.3)		0.1	0.0	0.1
Other income (expense), net	(0.4)		67.2	(0.8)	82.9
Allegheny & Tsingshan Stainless | Employee Severance [Member]
Other Income and Expenses [Abstract]
Net equity (loss) gain on joint ventures	$ 0.0	$ (2.4)	$ 0.0	$ (2.4)	$ 0.0