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Business Segments - Additional Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring reserve	$ 20.1		$ 20.1		$ 4.5
Gain on asset sales, net	0.0	$ 68.2	$ 2.5	$ 89.8
Number of business segments \| segment			2
Goodwill	238.4		$ 238.4		$ 525.8
Adjustment to indemnification for conditional ARO costs	0.0	0.0	4.3	0.0
Gain on the sale of oil and gas rights		62.4	2.5	91.7
Loss from sale of businesses	$ 0.0	5.8	0.0	(1.9)
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal			$ 0.0	(1.8)
Cast Products [Member]
Restructuring Cost and Reserve [Line Items]
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal		6.2		6.2
Forged Products
Restructuring Cost and Reserve [Line Items]
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal		$ (0.4)		$ (8.1)
Aerospace and Defense Market Concentration [Member] \| High Performance Materials & Components \| Revenue from Contract with Customer, Segment Benchmark [Member]
Restructuring Cost and Reserve [Line Items]
Concentration Risk, Percentage			80.00%
Energy and Aerospace & Defense Market Concentration [Member] \| Advanced Alloys & Solutions [Member] \| Revenue from Contract with Customer, Segment Benchmark [Member]
Restructuring Cost and Reserve [Line Items]
Concentration Risk, Percentage			50.00%
Allegheny & Tsingshan Stainless
Restructuring Cost and Reserve [Line Items]
Equity method investment ownership percentage	50.00%		50.00%
Uniti
Restructuring Cost and Reserve [Line Items]
Equity method investment ownership percentage	50.00%		50.00%
Allegheny Technologies Inc \| Shanghai STAL Precision Stainless Steel Co Ltd \| Advanced Alloys & Solutions [Member]
Restructuring Cost and Reserve [Line Items]
Joint venture ownership percentage	60.00%		60.00%