Income Taxes	6 Months Ended
	Jun. 30, 2020
Income Tax Disclosure [Abstract]
Income Taxes	Income TaxesThe provision for income taxes for the second quarter and six months ended June 30, 2020 was $92.6 million, or (28.3)%, and $103.4 million, or (35.3)%, respectively, of loss before taxes. In the second quarter 2020, ATI recorded a $99.0 million deferred tax asset valuation allowance on its U.S. federal and state tax attributes. The Company entered into a three-year cumulative loss within the United States as of June 30, 2020, 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. 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 domestic operations could produce a significant variation in its annual effective tax rate. The tax provision for the second quarter 2019 and the six months ended June 30, 2019 was $5.8 million, or 6.9%, and $6.6 million, or 6.5%, respectively, of income before taxes, due to deferred tax asset valuation allowances in the United States. The Company has analyzed the impacts of the Coronavirus Aid, Relief, and Economic Security Act (CARES Act) that was enacted on March 27, 2020 and has determined that the impact does not result in a material income tax expense or benefit. The Company continues to evaluate the overall changes between the deferred tax asset classifications of interest, pension and net operating losses resulting from the CARES Act.