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Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		10 Months Ended	42 Months Ended
Joint Ventures (Details) - USD ($) $ in Millions	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2018	Sep. 30, 2021	Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 472.5		$ 472.5				$ 645.9
Income (loss) from equity investments	0.0	$ (1.9)	0.0	$ (5.6)
Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 1.7		$ 1.7
Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%		50.00%
Sales to noncontrolling interests					$ 12.0
Allegheny & Tsingshan Stainless \| Employee Severance [Member]
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	$ 0.0	(2.4)	$ 0.0	(2.4)
Allegheny & Tsingshan Stainless \| Forecast
Schedule of Equity Method Investments [Line Items]
Sales to noncontrolling interests						$ 17.5
Uniti
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%		50.00%
Tsingshan Group \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from Affiliates	$ 5.5		$ 5.5
Advanced Alloys & Solutions [Member] \| Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	47.2		47.2
Advanced Alloys & Solutions [Member] \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	(0.4)	(4.6)	(0.9)	(8.5)
Due from Affiliates	13.2		13.2				14.0
Advanced Alloys & Solutions [Member] \| Uniti
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	0.4	$ 0.3	0.9	$ 0.5
Prepaid expenses and other current assets \| Advanced Alloys & Solutions [Member] \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from Affiliates	0.5		0.5				0.5
Other Noncurrent Assets [Member] \| Advanced Alloys & Solutions [Member] \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from Affiliates	$ 12.7		$ 12.7				$ 13.5
Allegheny Technologies Inc \| Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	51.00%		51.00%
Allegheny Technologies Inc \| Advanced Alloys & Solutions [Member] \| Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	60.00%		60.00%
China Baowu Steel Group Corporation Limited \| Advanced Alloys & Solutions [Member] \| China Baowu Steel Group Corporation Limited \| Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	40.00%		40.00%
Tsingshan Group \| Tsingshan Group \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50.00%		50.00%
VSMPO \| VSMPO \| Uniti
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50.00%		50.00%