Income Taxes	3 Months Ended
	Mar. 31, 2022
Income Tax Disclosure [Abstract]
Income Taxes	Income TaxesThe provision for income taxes for the first quarters ended March 31, 2022 and 2021 was $4.9 million and $5.5 million, respectively. Tax expense in both periods is mainly attributable to the Company’s foreign operations. The tax expense for the first quarter of 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 first quarter of 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. Both calculations excluded the results related to the Company’s Sheffield, UK operations.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 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.