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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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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 30
-Section 35
-Paragraph 50
-URI http://asc.fasb.org/extlink&oid=64937038&loc=d3e12355-114930

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