Retirement Benefits (Details 17) - Pension Benefits [Member] - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Multiemployer Plan Disclosure [Line Items]
Company Contributions		$ 4.5	$ 4.6	$ 4.0
Steelworkers Western Independent Shops Pension Plan [Member]
Multiemployer Plan Disclosure [Line Items]
EIN		900169564
Pension Plan Number		001
Pension Protection Act Zone Status	[1]	Green	Green
FIP / RP Status Pending / Implemented	[2]	NA
Company Contributions		$ 0.6	$ 1.2	0.7
Surcharge Imposed	[3]	No
Expiration Dates of Collective Bargaining Agreements		Feb. 29, 2020
Boilermakers-Blacksmiths National Pension Trust [Member]
Multiemployer Plan Disclosure [Line Items]
EIN		486168020
Pension Plan Number		001
Pension Protection Act Zone Status	[1]	Yellow	Yellow
FIP / RP Status Pending / Implemented	[2]	Implemented
Company Contributions		$ 2.2	$ 1.8	1.8
Surcharge Imposed	[3]	No
Expiration Dates of Collective Bargaining Agreements		Sep. 30, 2026
IAM National Pension Fund [Member]
Multiemployer Plan Disclosure [Line Items]
EIN		516031295
Pension Plan Number		002
Pension Protection Act Zone Status	[1]	Green	Green
FIP / RP Status Pending / Implemented	[2]	NA
Company Contributions		$ 1.7	$ 1.6	$ 1.5
Surcharge Imposed	[3]	No
Expiration Dates of Collective Bargaining Agreements, First Date	[4]	Apr. 22, 2018
Expiration Dates of Collective-Bargaining Agreements, Last Date	[4]	Feb. 27, 2022

[1] The most recent Pension Protection Act Zone Status available for ATI’s fiscal years 2017 and 2016 is for plan years ending in calendar years 2016 and 2015, respectively. The zone status is based on information provided to ATI and other participating employers by each plan and is certified by the plan’s actuary. A plan in the “red” zone had been determined to be in “critical status”, based on criteria established by the Code, and is generally less than 65% funded. A plan in the “yellow” zone has been determined to be in “endangered status”, based on criteria established under the Code, and is generally less than 80% funded. A plan in the “green” zone has been determined to be neither in “critical status” nor in “endangered status”, and is generally at least 80% funded.

[2] The “FIP / RP Status Pending / Implemented” column indicates whether a Funding Improvement Plan, as required under the Code by plans in the “yellow” zone, or a Rehabilitation Plan, as required under the Code to be adopted by plans in the “red” zone, is pending or has been implemented as of the end of the plan year that ended in 2017.

[3] The “Surcharge Imposed” column indicates whether ATI’s contribution rate for 2017 included an amount in addition to the contribution rate specified in the applicable collective bargaining agreement, as imposed by a plan in “critical status”, in accordance with the requirements of the Code.

[4] The Company is party to five separate bargaining agreements that require contributions to this plan. Expiration dates of these collective bargaining agreements range between April 22, 2018 and February 27, 2022.