Summary of Significant Accounting Policies - New and Pending Accounting Pronouncements (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Jan. 01, 2021	Dec. 31, 2020	Dec. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest	$ 1,157.2	$ 832.7		$ 641.4	$ 2,193.2
Additional Paid-In Capital
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest	1,668.1	1,596.7		1,625.5	1,618.0
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest	$ 176.9	$ 72.7		$ 106.5	$ 1,679.3
Accounting Standards Update 2020-06 | Additional Paid-In Capital
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest			$ (49.8)
Accounting Standards Update 2020-06 | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest			4.4
ATI Convertible Senior Notes, 3.5%, Due 2025 [Member] | Convertible Debt [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.50%	3.50%
ATI Inc., Convertible Senior Notes, 3.5%, Due 2025 | Convertible Debt [Member] | Accounting Standards Update 2020-06
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest			$ (45.4)