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Summary of Significant Accounting Policies - New and Pending Accounting Pronouncements (Details) - USD ($) $ in Millions	Dec. 31, 2021	Jan. 01, 2021	Dec. 31, 2020	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Total stockholders’ equity	$ 832.7		$ 641.4	$ 2,193.2		$ 1,991.6
Cumulative effect of adoption of new accounting standard
Debt Instrument [Line Items]
Total stockholders’ equity			(45.4)		$ 5.1
Accounting Standards Update 2020-06
Debt Instrument [Line Items]
Total stockholders’ equity		$ (45.4)
Additional Paid-In Capital
Debt Instrument [Line Items]
Total stockholders’ equity	1,596.7		1,625.5	1,618.0		$ 1,615.4
Additional Paid-In Capital | Cumulative effect of adoption of new accounting standard
Debt Instrument [Line Items]
Total stockholders’ equity			(49.8)
Retained Earnings
Debt Instrument [Line Items]
Total stockholders’ equity	$ 72.7		106.5	$ 1,679.3
Retained Earnings | Cumulative effect of adoption of new accounting standard
Debt Instrument [Line Items]
Total stockholders’ equity			$ 4.4		$ 5.1
Allegheny Technologies, Convertible Senior Notes, 3.5%, Due 2025 [Member] | Convertible Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.50%		3.50%

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Contractual interest rate for funds borrowed, under the debt agreement.

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