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Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions | 3 Months Ended | 12 Months Ended
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Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Earnings Per Share Reconciliation [Abstract] |  |  |  |  |  |  |  |  |  |  |
Net income (loss) |  |  |  |  |  |  |  |  | $ 222.4 | $ (91.9) | $ (640.9)
Effect of dilutive securities: Convertible Senior Notes |  |  |  |  |  |  |  |  | 12.9 | 0.0 | 0.0
Numerator for diluted net income (loss) per common share - Income (loss) attributable to ATI after assumed conversions |  |  |  |  |  |  |  |  | $ 235.3 | $ (91.9) | $ (640.9)
Denominator for basic net income (loss) per common share - weighted average shares (shares) | 125.7 | 125.7 | 125.7 | 125.7 | 118.6 | 108.9 | 108.9 | 108.8 | 125.2 | 110.1 | 107.3
Effect of dilutive securities: Share-based compensation |  |  |  |  |  |  |  |  | 0.8 | 0.0 | 0.0
Effect of dilutive securities: Convertible Senior Notes |  |  |  |  |  |  |  |  | 19.9 | 0.0 | 0.0
Denominator for diluted net income (loss) per common share—adjusted weighted average shares and assumed conversions |  |  |  |  |  |  |  |  | 145.9 | 110.1 | 107.3
Basic income (loss) attributable to ATI per common share (in dollars per share) |  |  |  |  |  |  |  |  | $ 1.78 | $ (0.83) | $ (5.97)
Diluted income (loss) attributable to ATI per common share (in dollars per share) |  |  |  |  |  |  |  |  | $ 1.61 | $ (0.83) | $ (5.97)
Anti-dilutive securities excluded from computation of earnings per share amount |  |  |  |  |  |  |  |  | 0.0 | 20.8 | 13.1
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022 [Member] |  |  |  |  |  |  |  |  |  |  |
Earnings Per Share Reconciliation [Abstract] |  |  |  |  |  |  |  |  |  |  |
Debt instrument interest rate stated percentage | 4.75% |  |  |  | 4.75% |  |  |  | 4.75% | 4.75% |
Convertible Debt [Member] | Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022 [Member] |  |  |  |  |  |  |  |  |  |  |
Earnings Per Share Reconciliation [Abstract] |  |  |  |  |  |  |  |  |  |  |
Debt instrument interest rate stated percentage | 4.75% |  |  |  |  |  |  |  | 4.75% |  |

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