XML 87 R50.htm IDEA: XBRL DOCUMENT
v3.19.3.a.u2

Summary of Significant Accounting Policies - Risks and Uncertainties and Use of Estimates (Details) | 12 Months Ended
--- | ---
Dec. 31, 2019 employee
Segment Reporting Information [Line Items] |
Number of employees | 8,100
Percent of employees located outside of the U.S. | 17.00%
Percent of employees covered by collective bargaining agreements | 40.00%
Number of employees covered by USW collective bargaining agreement expiring February 29, 2020 | 1,500

X
- Definition Employees, Percent Covered by Collective Bargaining Agreements + References No definition available. + Details
Name: | ati_EmployeesPercentCoveredbyCollectiveBargainingAgreements
Namespace Prefix: | ati_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Employees, Percent Non-U.S. + References No definition available. + Details
Name: | ati_EmployeesPercentNonU.S.
Namespace Prefix: | ati_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Entity Number Of Employees Covered By USW Collective Bargaining Agreements Expiring February 29 2020 + References No definition available. + Details
Name: | ati_EntityNumberOfEmployeesCoveredByUSWCollectiveBargainingAgreementsExpiringFebruary292020
Namespace Prefix: | ati_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of persons employed by the Entity + References No definition available. + Details
Name: | dei_EntityNumberOfEmployees
Namespace Prefix: | dei_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_SegmentReportingInformationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration