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Accounting Policies (Accounting Pronouncements) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) segment	Sep. 30, 2020 USD ($)	Jun. 30, 2021 USD ($)	Jan. 01, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Revenue recognition [Line Items]
Number of business segments \| segment			2
Impact of new accounting principle on prior period operating income	$ 29.9	$ (9.6)	$ 50.4	$ (214.6)
Unfavorable impact on pre-tax operating results	(77.2)	46.2	(39.9)	339.1
Equity adjustment for adoption	680.2	1,849.8	680.2	1,849.8	$ 606.1		$ 641.4	$ 1,842.3	$ 2,193.2
Additional Paid-in Capital [Member]
Revenue recognition [Line Items]
Equity adjustment for adoption	1,592.1	1,622.2	1,592.1	1,622.2	1,587.5		1,625.5	1,617.8	1,618.0
[RetainedEarningsMember]
Revenue recognition [Line Items]
Equity adjustment for adoption	$ 102.5	$ 1,227.4	$ 102.5	$ 1,227.4	$ 53.8		$ 106.5	$ 1,277.6	$ 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%				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%		50.00%
Uniti
Revenue recognition [Line Items]
Equity method investment ownership percentage	50.00%		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%		60.00%