Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Sales	$ 1,125.5us-gaap_SalesRevenueGoodsNet	$ 987.3us-gaap_SalesRevenueGoodsNet
Operating profit (loss):	83.8ati_SegmentOperatingProfitLoss	43.5ati_SegmentOperatingProfitLoss
Corporate expenses	(11.8)ati_CorporateExpenses	(11.5)ati_CorporateExpenses
Interest expense, net	(26.7)ati_InterestExpenseNet	(29.1)ati_InterestExpenseNet
Closed company and other expenses	(5.4)ati_OtherExpense	(5.0)ati_OtherExpense
Retirement benefit expense	(19.3)ati_RetirementBenefitExpense	(23.9)ati_RetirementBenefitExpense
Income (loss) from continuing operations before income tax provision (benefit)	20.6	(26.0)
Interest Costs Capitalized	0.6us-gaap_InterestCostsCapitalized	2.3us-gaap_InterestCostsCapitalized
High Performance Materials & Components
Segment Reporting Information [Line Items]
Total sales:	542.8	484.4
Operating profit (loss):	75.9	69.1
Flat Rolled Products
Segment Reporting Information [Line Items]
Total sales:	582.7	502.9
Operating profit (loss):	7.9	(25.6)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales:	1,169.6	1,033.7
Operating Segments [Member] \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Total sales:	564.9	504.1
Operating Segments [Member] \| Flat Rolled Products
Segment Reporting Information [Line Items]
Total sales:	604.7	529.6
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Total sales:	44.1	46.4
Intersegment Eliminations [Member] \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Total sales:	22.1	19.7
Intersegment Eliminations [Member] \| Flat Rolled Products
Segment Reporting Information [Line Items]
Total sales:	$ 22.0	$ 26.7

corporate office expenses not allocable to business segments

No definition available.

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- Definition

Net interest expense - the cost of borrowed funds as interest charged against earnings during the period net of interest income derived from investments in debt securities and on cash and cash equivalents.

+ References

No definition available.

+ Details

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- Definition

Other expense, net of gains on asset sales primarily includes charges incurred in connection with closed operations and other non-operating income or expense

+ References

No definition available.

+ Details

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- Definition

Represents defined benefit plan pension expense, and other postretirement benefit expense for both defined benefit and defined contribution plans

+ References

No definition available.

+ Details

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- Definition

Business segment operating profit, which excludes costs for restructuring charges, retirement benefit income or expense, corproate expenses, interest expense, debt extinguishment costs and costs associated with closed operations. These costs are excluded for segment reporting to provide a profit measure based on what management considers to be controllable costs at the business segment level

+ References

No definition available.

+ Details

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- Definition

Sum of operating profit and nonoperating income or expense before Income or Loss from equity method investments, income taxes, extraordinary items, and noncontrolling interest.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(h)(1)(i))

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

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- Definition

Amount of interest capitalized during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Allowance for Funds Used during Construction

-URI http://asc.fasb.org/extlink&oid=6504829

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391

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- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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- Definition

Aggregate revenue during the period from the sale of goods in the normal course of business, after deducting returns, allowances and discounts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1(a))

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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