Income Taxes	9 Months Ended
	Sep. 30, 2023
Income Tax Disclosure [Abstract]
Income Taxes	Income TaxesThe provision for income taxes for the third quarter and nine months ended September 30, 2023 was $4.9 million and $12.9 million, respectively. The 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 both periods is mainly attributable to the Company’s foreign operations and was based on an estimated annual effective tax rate calculation which included foreign, non-valuation allowance operations combined with the U.S. jurisdiction. The 2022 calculation excluded the results related to the Company’s Sheffield, UK operations, which was sold in the second quarter of 2022. 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.