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Business Segments - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($) position	Dec. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Gain on asset sales and sale of business, net	$ 13.8	$ 2.5	$ 84.6
Restructuring and Related Cost, Expected Number of Positions Eliminated | position		1,400
Restructuring and other credits (charges)	10.5	$ (1,132.1)	(4.5)
Income (loss) from equity method investments	0.1	(7.0)	(10.7)
Total international sales	1,264.9	1,173.0	1,667.9
Sales by U.S. operations to foreign countries	846.3	812.3	1,262.6
Inventory valuation reserves	65.4	82.8
Closed operations and other expenses	(4.8)	(7.4)	(24.0)
Gain (Loss) on Disposition of Oil and Gas and Timber Property			91.7
Net gain (loss) on sale of businesses	$ 13.8	0.0	(7.1)
Allegheny & Tsingshan Stainless
Segment Reporting Information [Line Items]
Equity method investment ownership percentage	50.00%
Revenue from related parties	$ 0.0	18.4	14.6
Income (loss) from equity method investments	(0.9)	(10.6)	(19.3)
Allegheny & Tsingshan Stainless | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Income (loss) from equity method investments	$ (0.9)	(8.2)	(12.2)
Uniti [Member]
Segment Reporting Information [Line Items]
Equity method investment ownership percentage	50.00%
Revenue from related parties	$ 45.8	36.7	31.3
Uniti [Member] | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Income (loss) from equity method investments	$ 1.0	$ 1.2	1.5
Aerospace & Defense Market Concentration Risk [Member] | Revenue Benchmark [Member] | High Performance Materials & Components
Segment Reporting Information [Line Items]
Percentage of sales	75.00%
Energy and Aerospace & Defense Market Concentration [Member] | Revenue Benchmark [Member] | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Percentage of sales	45.00%
Commercial Jet Engines Concentration | Revenue Benchmark [Member] | High Performance Materials & Components
Segment Reporting Information [Line Items]
Percentage of sales	40.00%
Cast Products
Segment Reporting Information [Line Items]
Net gain (loss) on sale of businesses			6.2
Forged Products
Segment Reporting Information [Line Items]
Net gain (loss) on sale of businesses			$ (13.3)
Allegheny Technologies Inc | STAL Precision Stainless Steel Company Limited [Member]
Segment Reporting Information [Line Items]
Joint venture ownership percentage	60.00%

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