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Accounting Policies (Accounting Pronouncements) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($) segment	Mar. 31, 2020 USD ($)	Jan. 01, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Revenue recognition [Line Items]
Number of business segments | segment	2
Impact of new accounting principle on prior period operating income	$ 31.8	$ 68.4
Unfavorable impact on pre-tax operating results	(3.1)	(34.4)
Equity adjustment for adoption	609.3	2,192.6		$ 641.4	$ 2,193.2
Additional Paid-in Capital [Member]
Revenue recognition [Line Items]
Equity adjustment for adoption	1,580.5	1,592.1		1,625.5	1,618.0
[RetainedEarningsMember]
Revenue recognition [Line Items]
Equity adjustment for adoption	$ 103.0	$ 1,700.4		$ 106.5	$ 1,679.3
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%

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