Income Taxes	9 Months Ended
	Sep. 30, 2022
Income Tax Disclosure [Abstract]
Income Taxes	Income TaxesThe provision for income taxes for the third quarter and nine months ended September 30, 2022 was $3.0 million and $11.3 million, respectively. Tax expense in 2022 is mainly attributable to the Company’s foreign operations. The tax expense for the third quarter and nine months ended September 30, 2022 was based on an estimated annual effective tax rate calculation which included foreign, non-valuation allowance, operations combined with the U.S. jurisdiction. The provision for income taxes for the third quarter and nine months ended September 30, 2021 was $22.0 million and $31.5 million, respectively. The 2021 tax expense includes $15.5 million of discrete tax expense related to the postretirement medical benefits gain discussed in Note 12, in accordance with ATI’s accounting policy for recognizing deferred tax amounts stranded in accumulated other comprehensive income. The third quarter and nine months ended September 30, 2021 utilized an annual effective tax rate calculation for its foreign, non-valuation allowance operations, combined with actual year-to-date tax expense related to its U.S. jurisdiction. In the second quarter 2020, the Company entered into a three-year cumulative loss within the United States, limiting the Company’s ability to utilize future projections when analyzing the need for a deferred tax asset valuation allowance, therefore limiting sources of income as part of the analysis. ATI continues to maintain valuation allowances on its U.S. federal and state deferred tax assets, as well as for certain foreign jurisdictions.