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Summary of Significant Accounting Policies - Pending Accounting Pronouncements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Jan. 01, 2021	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Equity component of convertible note	$ 49.8
Change in shareholders' equity from ASU 2020-06 adoption	641.4		$ 2,193.2	$ 1,991.6	$ 1,844.5
Accounting Standards Update 2020-06
Debt Instrument [Line Items]
Change in shareholders' equity from ASU 2020-06 adoption		$ (45.4)
Additional Paid-In Capital
Debt Instrument [Line Items]
Equity component of convertible note	49.8
Change in shareholders' equity from ASU 2020-06 adoption	1,625.5		1,618.0	1,615.4	1,596.3
Additional Paid-In Capital \| Accounting Standards Update 2020-06
Debt Instrument [Line Items]
Change in shareholders' equity from ASU 2020-06 adoption		(49.8)
Retained Earnings
Debt Instrument [Line Items]
Change in shareholders' equity from ASU 2020-06 adoption	$ 106.5		$ 1,679.3	$ 1,422.0	$ 1,184.3
Retained Earnings \| Accounting Standards Update 2020-06
Debt Instrument [Line Items]
Change in shareholders' equity from ASU 2020-06 adoption		4.4
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%
Allegheny Technologies, Convertible Senior Notes, 3.5%, Due 2025 [Member] \| Convertible Debt [Member] \| Accounting Standards Update 2020-06
Debt Instrument [Line Items]
Long-term debt		$ 45.4