Retirement Benefits (Details 11) - Pension Benefits [Member] - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 1,772.2	$ 2,129.6	$ 1,894.6
NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
NAV	1,189.5	1,404.4
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	212.2	323.5
Significant Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	370.5	401.7
Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Equity Securities: ATI common stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		71.3
Equity Securities: ATI common stock [Member] | NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
NAV		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		71.3
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
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
Equity Securities: Other U.S. equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	362.9	416.0
Equity Securities: Other U.S. equities [Member] | NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
NAV	219.9	267.7
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	143.0	148.3
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	377.4	432.5
Equity Securities: International equities [Member] | NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
NAV	335.7	385.9
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	41.7	46.6
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	493.7	501.1
Fixed income and cash equivalents [Member] | NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
NAV	116.9	92.6
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	6.3	10.0
Fixed income and cash equivalents [Member] | Significant Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	370.5	398.5
Fixed income and cash equivalents [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Floating rate [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	90.1	169.9
Floating rate [Member] | NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
NAV	68.9	122.6
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	21.2	47.3
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	0.0	0.0
Private equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	137.1	137.5
Private equity [Member] | NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
NAV	137.1	137.5
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	258.3	307.4
Hedge funds [Member] | NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
NAV	258.3	307.4
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	52.7	93.9
Real estate and other [Member] | NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
NAV	52.7	90.7
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	0.0	3.2
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