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Business Segments - Narrative (Details) $ in Millions	3 Months Ended							12 Months Ended
	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2020 USD ($) position	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Gain on asset sales, net								$ 2.5	$ 89.8	$ 0.0
Restructuring and Related Cost, Expected Number of Positions Eliminated | position								1,400
Restructuring and other charges	$ 1,105.1	$ 2.3	$ 16.7	$ 8.0	$ 4.5			$ 1,132.1	4.5	0.0
Income (loss) from equity method investments								(7.0)	(10.7)	(1.0)
Total international sales								1,173.0	1,667.9	1,698.4
Sales by U.S. operations to foreign countries								812.3	1,262.6	1,303.8
LIFO-related net realizable value charge	(44.1)				(33.6)			(44.1)	(33.6)
Inventory valuation reserves	$ 126.9				$ 104.1			126.9	104.1
Closed operations and other expenses								7.4	24.0	19.5
Gain (Loss) on Disposition of Oil and Gas and Timber Property						$ 62.4	$ 29.3		91.7
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal								$ 0.0	(1.8)	0.0
Allegheny & Tsingshan Stainless
Segment Reporting Information [Line Items]
Equity method investment ownership percentage	50.00%							50.00%
Revenue from related parties								$ 18.4	14.6	4.1
Income (loss) from equity method investments								(10.6)	(19.3)	(3.9)
Allegheny & Tsingshan Stainless | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Income (loss) from equity method investments								$ (8.2)	(12.2)
Uniti [Member]
Segment Reporting Information [Line Items]
Equity method investment ownership percentage	50.00%							50.00%
Revenue from related parties								$ 36.7	31.3	49.4
Uniti [Member] | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Income (loss) from equity method investments								$ 1.2	1.5	$ 2.9
Aerospace & Defense Market Concentration Risk [Member] | Revenue Benchmark [Member] | High Performance Materials & Components
Segment Reporting Information [Line Items]
Percentage of sales								80.00%
Energy and Aerospace & Defense Market Concentration [Member] | Revenue Benchmark [Member] | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Percentage of sales								50.00%
Cast Products
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							$ (7.7)		$ (8.1)
Allegheny Technologies Inc | STAL Precision Stainless Steel Company Limited [Member]
Segment Reporting Information [Line Items]
Joint venture ownership percentage	60.00%							60.00%

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