Income Taxes	3 Months Ended
	Mar. 31, 2021
Income Tax Disclosure [Abstract]
Income Taxes	Income TaxesThe provision for income taxes for the first quarter ended March 31, 2021 was $5.5 million. 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. Throughout 2020, the Company calculated the provision for income taxes based upon actual year to date results due to the uncertainty of future projections as a result of the global pandemic. For 2021, the Company continues to be unable to make a reliable estimate of the annual effective tax rate, as significant changes in projected results for the Company’s U.S. operations could produce a significant variation in its annual effective tax rate. As a result, the Company recorded income tax for the quarter ended March 31, 2021 based upon an annual effective tax rate for its foreign, non-valuation allowance operations, combined with actual year-to-date tax expense related to its U.S. jurisdictions. The current year tax expense is mainly attributable to the Company’s foreign operations. The tax provision for the first quarter 2020 ended March 31, 2020 was $10.8 million, or 31.4%, of income before taxes, based on the actual year to date effective rate for this period.