Retirement Benefits (Details 11) - Pension Benefits [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 1,894.6	$ 2,129.6	$ 1,962.3
NAV	1,369.0	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	191.2	323.5
Significant Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	334.2	401.7
Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.2	0.0	194.6
Transfer from level 3 to NAV	0.0
Equity Securities: ATI common stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	47.1	71.3
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	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	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	416.0
NAV	204.0	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	107.1	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	375.7	432.5
NAV	339.2	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	36.5	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	412.2	501.1
NAV	82.3	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	0.5	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	329.2	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	0.2	0.0	0.3
Transfer from level 3 to NAV	0.0
Floating rate [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	225.5	169.9
NAV	225.5	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	0.0	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	$ 194.3
Transfer from level 3 to NAV	0.0
Private equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	142.6	137.5
NAV	142.6	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	283.9	307.4
NAV	283.9	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	96.5	93.9
NAV	91.5	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	5.0	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
Restatement Adjustment [Member] | Floating rate [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Transfer from level 3 to NAV	99.0
Restatement Adjustment [Member] | Private equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Decrease in assets due to transfer from asset category	$ 62.0