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Retirement Benefits (Details 11) - Pension Benefits [Member] - USD ($) $ in Millions | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | $ 1,894.6 | $ 1,962.3 | $ 2,204.4
NAV | 1,269.9 | 1,243.1 |
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 191.2 | 287.5 |
Significant Observable Inputs (Level 2) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 334.2 | 126.1 |
Significant Unobservable Inputs (Level 3) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 99.3 | 305.6 | 361.3
Equity Securities: ATI common stock [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 47.1 | 33.2 |
NAV | 0.0 | 0.0 |
Equity Securities: ATI common stock [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 47.1 | 33.2 |
Equity Securities: ATI common stock [Member] | Significant Observable Inputs (Level 2) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0.0 | 0.0 |
Equity Securities: ATI common stock [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0.0 | 0.0 |
Equity Securities: Other U.S. equities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 311.1 | 522.0 |
NAV | 204.0 | 267.9 |
Equity Securities: Other U.S. equities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 107.1 | 254.1 |
Equity Securities: Other U.S. equities [Member] | Significant Observable Inputs (Level 2) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0.0 | 0.0 |
Equity Securities: Other U.S. equities [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0.0 | 0.0 |
Equity Securities: International equities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 375.7 | 239.8 |
NAV | 339.2 | 239.8 |
Equity Securities: International equities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 36.5 | 0.0 |
Equity Securities: International equities [Member] | Significant Observable Inputs (Level 2) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0.0 | 0.0 |
Equity Securities: International equities [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0.0 | 0.0 |
Fixed income and cash equivalents [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 412.2 | 369.7 |
NAV | 82.3 | 249.0 |
Fixed income and cash equivalents [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0.5 | 0.2 |
Fixed income and cash equivalents [Member] | Significant Observable Inputs (Level 2) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 329.2 | 120.2 |
Fixed income and cash equivalents [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0.2 | 0.3 | 0.3
Floating rate [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 164.0 | 358.0 |
NAV | 64.9 | 52.7 |
Floating rate [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0.0 | 0.0 |
Floating rate [Member] | Significant Observable Inputs (Level 2) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0.0 | 0.0 |
Floating rate [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 99.1 | 305.3 | $ 361.0
Private equity [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 204.1 | 201.2 |
NAV | 204.1 | 201.2 |
Private equity [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0.0 | 0.0 |
Private equity [Member] | Significant Observable Inputs (Level 2) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0.0 | 0.0 |
Private equity [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0.0 | 0.0 |
Hedge funds [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 283.9 | 51.9 |
NAV | 283.9 | 51.9 |
Hedge funds [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0.0 | 0.0 |
Hedge funds [Member] | Significant Observable Inputs (Level 2) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0.0 | 0.0 |
Hedge funds [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0.0 | 0.0 |
Real estate and other [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 96.5 | 186.5 |
NAV | 91.5 | 180.6 |
Real estate and other [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0.0 | 0.0 |
Real estate and other [Member] | Significant Observable Inputs (Level 2) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 5.0 | 5.9 |
Real estate and other [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | $ 0.0 | $ 0.0 |

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