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Accounting Policies (Accounting Pronouncements) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2022 USD ($) segment	Mar. 31, 2021 USD ($)	Dec. 31, 2021 USD ($)	Jan. 01, 2021 USD ($)	Dec. 31, 2020 USD ($)
Revenue recognition [Line Items]
Number of business segments \| segment	2
Impact of new accounting principle on prior period operating income	$ 77.0	$ 31.8
Unfavorable impact on pre-tax operating results	(40.1)	(3.1)
Equity adjustment for adoption	795.7	609.3	$ 832.7		$ 641.4
Additional Paid-in Capital [Member]
Revenue recognition [Line Items]
Equity adjustment for adoption	1,602.5	1,580.5	1,596.7		1,625.5
[RetainedEarningsMember]
Revenue recognition [Line Items]
Equity adjustment for adoption	$ 103.6	$ 103.0	$ 72.7		$ 106.5
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 \| Convertible Debt
Revenue recognition [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.50%		3.50%
Accounting Standards Update 2020-06 \| Additional Paid-in Capital [Member]
Revenue recognition [Line Items]
Equity adjustment for adoption				$ (49.8)
Accounting Standards Update 2020-06 \| [RetainedEarningsMember]
Revenue recognition [Line Items]
Equity adjustment for adoption				4.4
Accounting Standards Update 2020-06 \| Allegheny Technologies 3.5% Convertible Senior Notes due 2025 \| Convertible Debt
Revenue recognition [Line Items]
Equity adjustment for adoption				$ (45.4)
Allegheny & Tsingshan Stainless
Revenue recognition [Line Items]
Equity method investment ownership percentage	50.00%
Uniti
Revenue recognition [Line Items]
Equity method investment ownership percentage	50.00%
Allegheny Technologies Inc \| Shanghai STAL Precision Stainless Steel Co Ltd \| Advanced Alloys & Solutions [Member]
Revenue recognition [Line Items]
Joint venture ownership percentage	60.00%