Income Taxes	9 Months Ended
	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Income Taxes	Income TaxesThe provision for income taxes for the third quarter and nine months ended September 30, 2020 was $0.8 million, or (1.7)%, and $104.2 million, or (30.7)%, 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 third quarter 2019 and the nine months ended September 30, 2019 was $3.7 million, or 3.1%, and $10.3 million, or 4.7%, respectively, of income before taxes, primarily 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.