Business Segments (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Sales	$ 1,018.7	$ 1,020.2	$ 3,103.9	$ 3,008.7
Operating profit (loss)	99.2	105.5	275.4	325.9
LIFO and net realizable value reserves	0.0	0.0	(0.1)	0.0
Corporate expenses	(16.0)	(14.8)	(50.6)	(40.9)
Closed operations and other expenses	(8.2)	(3.4)	(19.2)	(16.6)
Gain on joint venture deconsolidation	0.0	0.0	0.0	15.9
Gain on asset sales, net	68.2	0.0	89.8	0.0
Interest expense, net	(24.2)	(24.8)	(74.9)	(75.8)
Income from Continuing Operations before Income Taxes, Noncontrolling Interest	119.0	62.5	220.4	208.5
Operating Segments
Segment Reporting Information [Line Items]
Sales	1,069.4	1,067.7	3,256.1	3,137.2
Operating Segments \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	577.3	615.2	1,875.0	1,806.3
Operating profit (loss)	78.8	76.0	250.3	259.4
Operating Segments \| Flat Rolled Products
Segment Reporting Information [Line Items]
Sales	492.1	452.5	1,381.1	1,330.9
Operating profit (loss)	20.4	29.5	25.1	66.5
Internal Customers \| Operating Segments
Segment Reporting Information [Line Items]
Sales	50.7	47.5	152.2	128.5
Internal Customers \| Operating Segments \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	25.6	29.7	79.7	68.2
Internal Customers \| Operating Segments \| Flat Rolled Products
Segment Reporting Information [Line Items]
Sales	25.1	17.8	72.5	60.3
External Customers \| Operating Segments
Segment Reporting Information [Line Items]
Sales	1,018.7	1,020.2	3,103.9	3,008.7
External Customers \| Operating Segments \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	551.7	585.5	1,795.3	1,738.1
External Customers \| Operating Segments \| Flat Rolled Products
Segment Reporting Information [Line Items]
Sales	$ 467.0	$ 434.7	$ 1,308.6	$ 1,270.6

corporate office expenses not allocable to business segments

No definition available.

Name:	ati_CorporateExpenses
Namespace Prefix:	ati_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

Name:	ati_InterestExpenseNet
Namespace Prefix:	ati_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Lifo Provision Benefit and Change in Debit LIFO NRV reserve, net

+ References

No definition available.

+ Details

Name:	ati_LifoProvisionBenefitandChangeinDebitLIFONRVreservenet
Namespace Prefix:	ati_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

Name:	ati_OtherExpense
Namespace Prefix:	ati_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

Name:	ati_SegmentOperatingProfitLoss
Namespace Prefix:	ati_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) from deconsolidation of subsidiary and derecognition of group of assets constituting transfer of business or nonprofit activity, excluding conveyance of oil and gas mineral rights and transfer of good or service in contract with customer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 1B
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109239629&loc=SL4582445-111684

+ Details

Name:	us-gaap_DeconsolidationGainOrLossAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) from the difference between the sale price or salvage price and the book value of an asset that was sold or retired, and gain (loss) from the write down of assets from their carrying value to fair value.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

+ Details

Name:	us-gaap_GainLossOnSalesOfAssetsAndAssetImpairmentCharges
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 20
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=68072869&loc=d3e41242-110953

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(10))
-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(8))
-URI http://asc.fasb.org/extlink&oid=116637232&loc=SL114874131-224263

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22694-107794

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(15))
-URI http://asc.fasb.org/extlink&oid=116637391&loc=SL114874048-224260

+ Details

Name:	us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045

+ Details

Name:	us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_SegmentReportingInformationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	srt_ConsolidationItemsAxis=us-gaap_OperatingSegmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=ati_HighPerformanceMaterialsComponentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=ati_FlatRolledProductsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MajorCustomersAxis=ati_InternalCustomersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MajorCustomersAxis=ati_ExternalCustomersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: