Business Segments (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
Segment Reporting Information [Line Items]
Sales	$ 598.0		$ 1,018.7	$ 2,323.8	$ 3,103.9
Operating profit (loss)	(6.8)		99.2	101.7	275.4
LIFO and net realizable value reserves	0.0		0.0	0.0	(0.1)
Corporate expenses	(10.9)		(16.0)	(31.4)	(50.6)
Closed operations and other expense	(1.1)		(8.2)	(5.3)	(19.2)
Restructuring and other charges	(2.3)		0.0	(27.0)	0.0
Impairment of goodwill	0.0		0.0	(287.0)	0.0
Income (loss) from equity investments	(1.0)		(2.4)	(6.6)	(8.7)
Gain on asset sales, net	0.0		68.2	2.5	89.8
Debt extinguishment charge	0.0		0.0	(21.5)	0.0
Interest expense, net	(25.1)		(24.2)	(68.7)	(74.9)
Income (loss) before income taxes	(46.2)		119.0	(339.1)	220.4
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal				0.0	(1.8)
Allegheny & Tsingshan Stainless | Employee Severance [Member]
Segment Reporting Information [Line Items]
Income (loss) from equity investments	0.0	$ (2.4)	0.0	(2.4)	0.0
Cast Products [Member]
Segment Reporting Information [Line Items]
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal			6.2		6.2
Forged Products
Segment Reporting Information [Line Items]
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal			(0.4)		(8.1)
Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless
Segment Reporting Information [Line Items]
Income (loss) from equity investments	(1.6)		(2.6)	(10.1)	(9.9)
Operating Segments
Segment Reporting Information [Line Items]
Sales	623.4		1,097.8	2,482.4	3,351.9
Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	230.1		471.6	995.3	1,540.7
Operating profit (loss)	(2.7)		68.4	63.6	198.5
Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Sales	393.3		626.2	1,487.1	1,811.2
Operating profit (loss)	(4.1)		30.8	38.1	76.9
Internal Customers | Operating Segments
Segment Reporting Information [Line Items]
Sales	25.4		79.1	158.6	248.0
Internal Customers | Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	8.8		19.8	53.0	59.2
Internal Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Sales	16.6		59.3	105.6	188.8
External Customers | Operating Segments
Segment Reporting Information [Line Items]
Sales	598.0		1,018.7	2,323.8	3,103.9
External Customers | Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	221.3		451.8	942.3	1,481.5
External Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Sales	$ 376.7		$ 566.9	$ 1,381.5	$ 1,622.4