Subsequent Event	6 Months Ended
Jun. 30, 2021
Subsequent Events [Abstract]
Subsequent Event	Subsequent EventThe Company’s collective bargaining agreement (CBA) with the United Steel, Paper and Forestry, Rubber, Manufacturing, Energy, Allied & Industrial Service Workers International Union, AFL-CIO, CLC (USW) involving approximately 1,100 active full-time represented employees located primarily within the AA&S segment operations, as well as a number of inactive employees, expired on February 28, 2021. USW-represented employees continued to work under the terms of the expired CBA until March 30, 2021 when they engaged in a strike. On July 14, 2021, ATI announced that a new four-year labor agreement was ratified with the USW, ending the strike. As a result of this new CBA, ATI expects to recognize an approximately $65 million pretax gain in the third quarter of 2021 related to a plan termination that eliminated certain postretirement medical benefit liabilities, comprised of $43 million of long-term postretirement benefit liabilities as of June 30, 2021 and $22 million of amounts recorded in accumulated other comprehensive income at that date. Discrete tax effects related to this event are expected to be approximately $15 million of income tax expense.

Subsequent Event

6 Months Ended

Jun. 30, 2021

Subsequent EventThe Company’s collective bargaining agreement (CBA) with the United Steel, Paper and Forestry, Rubber, Manufacturing, Energy, Allied & Industrial Service Workers International Union, AFL-CIO, CLC (USW) involving approximately 1,100 active full-time represented employees located primarily within the AA&S segment operations, as well as a number of inactive employees, expired on February 28, 2021. USW-represented employees continued to work under the terms of the expired CBA until March 30, 2021 when they engaged in a strike. On July 14, 2021, ATI announced that a new four-year labor agreement was ratified with the USW, ending the strike. As a result of this new CBA, ATI expects to recognize an approximately $65 million pretax gain in the third quarter of 2021 related to a plan termination that eliminated certain postretirement medical benefit liabilities, comprised of $43 million of long-term postretirement benefit liabilities as of June 30, 2021 and $22 million of amounts recorded in accumulated other comprehensive income at that date. Discrete tax effects related to this event are expected to be approximately $15 million of income tax expense.