ATI (ATI) 10-Q 29 Oct 20 2020 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2020	Oct. 16, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2020
Document Transition Report	false
Entity File Number	1-12001
Entity Registrant Name	ALLEGHENY TECHNOLOGIES INCORPORATED
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	25-1792394
Entity Address, Address Line One	1000 Six PPG Place
Entity Address, City or Town	Pittsburgh,
Entity Address, State or Province	PA
Entity Address, Postal Zip Code	15222-5479
City Area Code	412
Local Phone Number	394-2800
Title of 12(b) Security	Common stock, par value $0.10
Trading Symbol	ATI
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding (in shares)		126,816,444
Amendment Flag	false
Entity Central Index Key	0001018963
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Current Assets:
Cash and cash equivalents	$ 572.2	$ 490.8
Accounts receivable, net	388	554.1
Short-term contract assets	41.7	38.5
Inventories, net	1,035.9	1,155.3
Prepaid expenses and other current assets	48.5	64.3
Total current assets	2,086.3	2,303
Property, plant and equipment, net	2,466.6	2,450.1
Goodwill	238.4	525.8
Other assets	272.6	355.7
Total assets	5,063.9	5,634.6
Current Liabilities:
Accounts payable	240.7	521.2
Short-term contract liabilities	97.8	78.7
Short-term debt and current portion of long-term debt	12.2	11.5
Other current liabilities	213	237.8
Total current liabilities	563.7	849.2
Long-term debt	1,529	1,387.4
Accrued postretirement benefits	300.2	312.5
Pension liabilities	608.9	731.5
Other long-term liabilities	212.3	160.8
Total liabilities	3,214.1	3,441.4
ATI Stockholders' Equity:
Preferred stock, par value $0.10: authorized-50,000,000 shares; issued-none	0	0
Common stock, par value $0.10: authorized-500,000,000 shares; issued-126,817,461 shares at September 30, 2020 and 126,695,171 shares at December 31, 2019; outstanding-126,815,756 shares at September 30, 2020 and 126,085,348 shares at December 31, 2019	12.7	12.7
Additional paid-in capital	1,622.2	1,618
Retained earnings	1,227.4	1,679.3
Treasury stock: 1,705 shares at September 30, 2020 and 609,823 shares at December 31, 2019	0	(18.2)
Accumulated other comprehensive loss, net of tax	(1,128.6)	(1,201.7)
Total ATI stockholders’ equity	1,733.7	2,090.1
Noncontrolling interests
Noncontrolling interests	116.1	103.1
Total Equity	1,849.8	2,193.2
Total Liabilities and Equity	$ 5,063.9	$ 5,634.6
Preferred stock, par value	$ 0.10	$ 0.10
Preferred stock, authorized	50,000,000	50,000,000
Preferred stock, issued	0	0
Common stock, par value	$ 0.10	$ 0.10
Common stock, authorized	500,000,000	500,000,000

CONSOLIDATED BALANCE SHEETS (PA

CONSOLIDATED BALANCE SHEETS (PARENTHETICAL) - $ / shares	Sep. 30, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.10	$ 0.10
Preferred stock, authorized	50,000,000	50,000,000
Preferred stock, issued	0	0
Common stock, par value	$ 0.10	$ 0.10
Common stock, authorized	500,000,000	500,000,000
Common stock, issued	126,817,461	126,695,171
Entity Common Stock, Shares Outstanding (in shares)	126,815,756	126,085,348
Treasury stock	1,705	609,823

CONSOLIDATED STATEMENTS OF INCO

CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Sales	$ 598	$ 1,018.7	$ 2,323.8	$ 3,103.9
Cost of sales	559.9	859	2,076.2	2,635.4
Gross profit	38.1	159.7	247.6	468.5
Selling and administrative expenses	45.4	65.2	148.2	200.9
Impairment of goodwill	0	0	287	0
Restructuring charges	2.3	0	27	0
Operating income (loss)	(9.6)	94.5	(214.6)	267.6
Nonoperating retirement benefit expense	(11.1)	(18.5)	(33.5)	(55.2)
Interest expense, net	(25.1)	(24.2)	(68.7)	(74.9)
Debt extinguishment charge	0	0	(21.5)	0
Other income (expense), net	(0.4)	67.2	(0.8)	82.9
Income (loss) before income taxes	(46.2)	119	(339.1)	220.4
Income tax provision	0.8	3.7	104.2	10.3
Net income (loss)	(47)	115.3	(443.3)	210.1
Less: Net income attributable to noncontrolling interests	3.1	4.3	8.3	9
Net income (loss) attributable to ATI	$ (50.1)	$ 111	$ (451.6)	$ 201.1
Basic net income (loss) attributable to ATI per common share (in dollars per share)	$ (0.40)	$ 0.88	$ (3.57)	$ 1.60
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ (0.40)	$ 0.78	$ (3.57)	$ 1.44

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ (47)	$ 115.3	$ (443.3)	$ 210.1
Currency translation adjustment
Unrealized net change arising during the period	23.2	(16.3)	14	(13.8)
Derivatives
Net derivatives gain (loss) on hedge transactions	6.1	14.3	(4.2)	19.1
Reclassification to net income (loss) of net realized (gain) loss	0.4	(0.8)	6.4	0.6
Income taxes on derivative transactions	0	0	0	0
Total	6.5	13.5	2.2	19.7
Postretirement benefit plans
Amortization of net actuarial loss	21.3	21.9	63.9	65.4
Amortization to net income (loss) of net prior service credits	(0.8)	(0.7)	(2.3)	(1.9)
Income taxes on postretirement benefit plans	0	0	0	0
Total	20.5	21.2	61.6	63.5
Other comprehensive income, net of tax	50.2	18.4	77.8	69.4
Comprehensive income (loss)	3.2	133.7	(365.5)	279.5
Less: Comprehensive income (loss) attributable to noncontrolling interests	9.1	(0.3)	13	6
Comprehensive income (loss) attributable to ATI	$ (5.9)	$ 134	$ (378.5)	$ 273.5

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Operating Activities:
Net income (loss)	$ (443.3)	$ 210.1
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization	108.3	114.6
Deferred taxes	98.5	4.1
Impairment of goodwill	287	0
Debt extinguishment charge	21.5	0
Net gains from disposal of property, plant and equipment	(3.3)	(90.6)
Loss from sale of business	0	1.8
Changes in operating assets and liabilities:
Inventories	119.3	12.2
Accounts receivable	166.2	(92.5)
Accounts payable	(280.5)	(75.2)
Retirement benefits	(78.3)	(86)
Accrued liabilities and other	49.6	(8.5)
Cash provided by (used in) operating activities	45	(10)
Investing Activities:
Purchases of property, plant and equipment	(94.6)	(98.1)
Proceeds from sale of businesses, net of transaction costs	0	155.9
Proceeds from disposal of property, plant and equipment	4	91.8
Other	1.4	(0.2)
Cash provided by (used in) investing activities	(89.2)	149.4
Financing Activities:
Borrowings on long-term debt	391.4	0
Payments on long-term debt and finance leases	(209.5)	(5.3)
Net borrowings (payments) under credit facilities	(0.9)	5.1
Purchase of convertible note capped call	(19.4)	0
Debt issuance costs	(9.1)	0
Debt extinguishment charge	(19.1)	0
Shares repurchased for income tax withholding on share-based compensation and other	(7.8)	(9.9)
Cash provided by (used in) financing activities	125.6	(10.1)
Increase (decrease) in cash and cash equivalents
Increase in cash and cash equivalents	81.4	129.3
Cash and cash equivalents at beginning of period	490.8	382
Cash and cash equivalents at end of period	$ 572.2	$ 511.3

STATEMENTS OF CHANGES IN CONSOL

STATEMENTS OF CHANGES IN CONSOLIDATED EQUITY - USD ($) $ in Millions	Total	[CommonStockMember]	[AdditionalPaidInCapitalMember]	[RetainedEarningsMember]	[TreasuryStockMember]	[AccumulatedOtherComprehensiveIncomeMember]	[NoncontrollingInterestMember]
Total Stockholders' Equity at Dec. 31, 2018	$ 1,991.6	$ 12.7	$ 1,615.4	$ 1,422	$ (30.6)	$ (1,133.8)	$ 105.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	210.1			201.1			9
Other comprehensive income (loss)	69.4					72.4	(3)
Employee stock plans	8.8		(3.3)	(0.3)	12.4
Total Stockholders' Equity at Sep. 30, 2019	2,279.9	12.7	1,612.1	1,622.8	(18.2)	(1,061.4)	111.9
Total Stockholders' Equity at Jun. 30, 2019	2,138.6	12.7	1,604.6	1,511.8	(18.3)	(1,084.4)	112.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	115.3			111			4.3
Other comprehensive income (loss)	18.4					23	(4.6)
Employee stock plans	7.6		7.5	0	0.1
Total Stockholders' Equity at Sep. 30, 2019	2,279.9	12.7	1,612.1	1,622.8	(18.2)	(1,061.4)	111.9
Total Stockholders' Equity at Dec. 31, 2019	2,193.2	12.7	1,618	1,679.3	(18.2)	(1,201.7)	103.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(443.3)			(451.6)			8.3
Other comprehensive income (loss)	77.8					73.1	4.7
Equity component of convertible note	49.8		49.8
Convertible note capped call	(19.4)		(19.4)
Employee stock plans	(8.3)		(26.2)	(0.3)	18.2
Total Stockholders' Equity at Sep. 30, 2020	1,849.8	12.7	1,622.2	1,227.4	0	(1,128.6)	116.1
Total Stockholders' Equity at Jun. 30, 2020	1,842.3	12.7	1,617.8	1,277.6	0	(1,172.8)	107
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(47)			(50.1)			3.1
Other comprehensive income (loss)	50.2					44.2	6
Equity component of convertible note	1.1		1.1
Convertible note capped call	(0.4)		(0.4)
Employee stock plans	3.6		3.7	(0.1)	0
Total Stockholders' Equity at Sep. 30, 2020	$ 1,849.8	$ 12.7	$ 1,622.2	$ 1,227.4	$ 0	$ (1,128.6)	$ 116.1

Accounting Policies

Accounting Policies	9 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Accounting Policies	Accounting Policies The interim consolidated financial statements include the accounts of Allegheny Technologies Incorporated and its subsidiaries. Unless the context requires otherwise, “Allegheny Technologies”, “ATI” and “the Company” refer to Allegheny Technologies Incorporated and its subsidiaries. These unaudited consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles for interim financial information and with the instructions for Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and note disclosures required by U.S. generally accepted accounting principles for complete financial statements. In management’s opinion, all adjustments (which include only normal recurring adjustments) considered necessary for a fair presentation have been included. Certain prior year amounts have been reclassified in order to conform with fiscal year 2020 presentation. These unaudited consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto included in the Company’s 2019 Annual Report on Form 10-K. The results of operations for these interim periods are not necessarily indicative of the operating results for any future period. The December 31, 2019 financial information has been derived from the Company’s audited consolidated financial statements. Effective January 1, 2020, the Company began operating under two revised business segments: High Performance Materials & Components (HPMC) and Advanced Alloys & Solutions (AA&S). HPMC is now comprised of the Specialty Materials and Forged Products businesses, as well as the ATI Europe distribution operations. The new AA&S segment combines the Specialty Alloys & Components (SAC) business, including the primary titanium operations in Richland, WA and Albany, OR, with ATI’s former Flat Rolled Products (FRP) business segment, which included the FRP business and the 60%-owned Shanghai STAL Precision Stainless Steel Company Limited (STAL), as well as the Uniti LLC (Uniti) and Allegheny & Tsingshan Stainless (A&T Stainless) 50%-owned joint ventures that are reported in AA&S segment results under the equity method of accounting. See Note 13, Business Segments, for further information. Financial results of aerospace-grade titanium plate products also transferred from HPMC to AA&S effective January 1, 2020. Prior period segment information has been restated to conform to this operating structure. The Company’s collective bargaining agreements (CBAs) with the United Steel, Paper and Forestry, Rubber, Manufacturing, Energy, Allied & Industrial Service Workers International Union, AFL-CIO, CLC (USW) involving approximately 1,500 full-time represented employees located primarily within the AA&S segment operations expired on February 29, 2020. On March 25, 2020, the Company announced an agreement with the USW that extended the terms of the expired CBAs for one year, to February 28, 2021. New Accounting Pronouncements Adopted In March 2020, the Financial Accounting Standards Board (FASB) issued new optional accounting guidance for a limited period of time to ease the potential burden in accounting for (or recognizing the effects of) reference rate reform on financial reporting. In response to concerns about structural risks of interbank offered rates (IBORs), and, particularly, the risk of cessation of the London Interbank Offered Rate (LIBOR), regulators in several jurisdictions around the world have undertaken reference rate reform initiatives to identify alternative reference rates that are more observable or transaction based and less susceptible to manipulation. The new accounting guidance provides optional expedients and exceptions for applying generally accepted accounting principles to contracts, hedging relationships, and other transactions affected by reference rate reform if certain criteria are met. The new accounting guidance applies only to contracts, hedging relationships, and other transactions that reference LIBOR or another reference rate expected to be discontinued because of reference rate reform. The expedients and exceptions provided by the amendments generally do not apply to contract modifications made and hedging relationships entered into or evaluated after December 31, 2022. Management is continuing to evaluate the issue, and presently does not expect a transition away from LIBOR, primarily involving ATI’s domestic credit facility and an interest rate swap contract, to have any significant financial impact to ATI. In December 2019, the FASB issued new guidance to simplify the accounting for income taxes. The areas for simplification in the guidance involve the removal of certain exceptions to the general principals in the current guidance, including intraperiod allocation and the calculation of income taxes in an interim period when a year to date loss exceeds the anticipated loss for the year. The new guidance also simplifies the accounting for income taxes in the area of franchise taxes. This new guidance is effective for the Company in fiscal year 2021, with early adoption permitted. This guidance was early adopted by the Company in fiscal year 2020 without significant impact to the consolidated financial statements. In August 2018, the FASB issued new disclosure guidance on fair value measurement. This new guidance modifies the disclosure requirements on fair value measurements, including removal and modifications of various current disclosures as well as some additional disclosure requirements for Level 3 fair value measurements. Some of these disclosure changes must be applied prospectively while others retrospectively depending on the requirement. This guidance was adopted by the Company in fiscal year 2020 without an impact on the Company’s consolidated financial statements other than disclosures. In June 2016, the FASB added a new impairment model (known as the current expected credit loss (CECL) model) that is based on expected losses rather than incurred losses. Under the new guidance, an entity recognizes as an allowance its estimate of expected credit losses. The CECL model applies to trade receivables, other receivables, contract assets and most debt instruments. The CECL model does not have a minimum threshold for recognition of impairment losses, and entities will need to measure expected credit losses on assets that have a low risk of loss. This guidance was adopted by the Company in fiscal year 2020 without significant impact to the consolidated financial statements. Pending Accounting Pronouncements In August 2020, the FASB issued new accounting guidance related to accounting for convertible instruments. Under this new guidance, embedded conversion features are no longer separated from the host contract for convertible instruments with conversion features that are not required to be accounted for as derivatives, or that do not result in substantial premiums accounted for as paid-in capital. As such, a convertible debt instrument will be accounted for as a single liability measured at its amortized cost, as long as no other features require bifurcation and recognition as derivatives. By removing those separation models, the reported interest rate of convertible debt instruments typically will be closer to the coupon interest rate. The new guidance also addresses how convertible instruments are accounted for in the diluted earnings per share calculation, requiring the if-converted method, and requires enhanced disclosures about the terms of convertible instruments and contracts in an entity’s own equity. This new guidance is effective for the Company in fiscal year 2022, with early adoption permitted.

Revenue from Contracts with Cus

Revenue from Contracts with Customers	9 Months Ended
	Sep. 30, 2020
Revenue from Contract with Customer [Abstract]
Revenue from Contracts with Customers	Revenue from Contracts with Customers Disaggregation of Revenue The Company operates in two business segments: High Performance Materials & Components (HPMC) and Advanced Alloys & Solutions (AA&S). Revenue is disaggregated within these two business segments by diversified global markets, primary geographical markets and diversified products. Comparative information of the Company’s overall revenues (in millions) by global and geographical markets for the third quarters and nine months ended September 30, 2020 and 2019 were as follows: (in millions) Third quarter ended September 30, 2020 September 30, 2019 HPMC AA&S Total HPMC AA&S Total Diversified Global Markets: Aerospace & Defense $ 171.8 $ 74.3 $ 246.1 $ 377.7 $ 125.8 $ 503.5 Energy* 24.4 106.0 130.4 36.2 181.0 217.2 Automotive 1.5 57.9 59.4 2.2 71.4 73.6 Electronics/Computers/Communications 0.2 45.7 45.9 0.1 44.4 44.5 Food Equipment & Appliances — 33.8 33.8 0.1 53.5 53.6 Construction/Mining 4.0 24.3 28.3 5.4 38.3 43.7 Medical 10.6 16.7 27.3 18.9 22.1 41.0 Other 8.8 18.0 26.8 11.2 30.4 41.6 Total $ 221.3 $ 376.7 $ 598.0 $ 451.8 $ 566.9 $ 1,018.7 (in millions) Nine Months Ended September 30, 2020 September 30, 2019 HPMC AA&S Total HPMC AA&S Total Diversified Global Markets: Aerospace & Defense $ 782.9 $ 321.3 $ 1,104.2 $ 1,223.1 $ 378.2 $ 1,601.3 Energy* 71.3 397.2 468.5 97.7 501.4 599.1 Automotive 4.5 182.2 186.7 8.9 215.2 224.1 Food Equipment & Appliances 0.1 131.3 131.4 0.2 156.2 156.4 Electronics/Computers/Communications 0.7 117.9 118.6 0.4 116.3 116.7 Construction/Mining 14.3 92.4 106.7 37.3 116.9 154.2 Medical 37.9 57.9 95.8 64.7 64.8 129.5 Other 30.6 81.3 111.9 49.2 73.4 122.6 Total $ 942.3 $ 1,381.5 $ 2,323.8 $ 1,481.5 $ 1,622.4 $ 3,103.9 *Includes the oil & gas, hydrocarbon and chemical processing, and electrical energy markets. (in millions) Third quarter ended September 30, 2020 September 30, 2019 HPMC AA&S Total HPMC AA&S Total Primary Geographical Market: United States $ 129.3 $ 219.3 $ 348.6 $ 241.1 $ 364.3 $ 605.4 Asia 19.9 103.0 122.9 44.1 120.4 164.5 Europe 60.7 34.1 94.8 134.5 51.8 186.3 Canada 4.4 8.5 12.9 12.6 13.0 25.6 South America, Middle East and other 7.0 11.8 18.8 19.5 17.4 36.9 Total $ 221.3 $ 376.7 $ 598.0 $ 451.8 $ 566.9 $ 1,018.7 (in millions) Nine Months Ended September 30, 2020 September 30, 2019 HPMC AA&S Total HPMC AA&S Total Primary Geographical Market: United States $ 522.8 $ 909.1 $ 1,431.9 $ 800.3 $ 1,066.7 $ 1,867.0 Europe 286.2 104.1 390.3 454.0 150.5 604.5 Asia 72.6 288.3 360.9 136.2 323.4 459.6 Canada 21.1 30.0 51.1 38.9 42.5 81.4 South America, Middle East and other 39.6 50.0 89.6 52.1 39.3 91.4 Total $ 942.3 $ 1,381.5 $ 2,323.8 $ 1,481.5 $ 1,622.4 $ 3,103.9 Comparative information of the Company’s major high-value and standard products based on their percentages of sales is included in the following table. Hot-Rolling and Processing Facility (HRPF) conversion service sales in the AA&S segment are excluded from this presentation. Third quarter ended September 30, 2020 September 30, 2019 HPMC AA&S Total HPMC AA&S Total Diversified Products and Services: High-Value Products Nickel-based alloys and specialty alloys 38 % 22 % 28 % 38 % 29 % 33 % Precision rolled strip products — % 28 % 18 % — % 22 % 12 % Titanium and titanium-based alloys 32 % 8 % 17 % 27 % 13 % 19 % Precision forgings, castings and components 30 % — % 11 % 35 % — % 16 % Zirconium and related alloys — % 16 % 10 % — % 9 % 5 % Total High-Value Products 100 % 74 % 84 % 100 % 73 % 85 % Standard Products Standard stainless products — % 26 % 16 % — % 27 % 15 % Total 100 % 100 % 100 % 100 % 100 % 100 % Nine Months Ended September 30, 2020 September 30, 2019 HPMC AA&S Total HPMC AA&S Total Diversified Products and Services: High-Value Products Nickel-based alloys and specialty alloys 38 % 25 % 30 % 37 % 26 % 32 % Titanium and titanium-based alloys 27 % 12 % 18 % 25 % 11 % 18 % Precision forgings, castings and components 35 % — % 14 % 38 % — % 18 % Precision rolled strip products — % 21 % 13 % — % 22 % 11 % Zirconium and related alloys — % 13 % 9 % — % 12 % 6 % Total High-Value Products 100 % 71 % 84 % 100 % 71 % 85 % Standard Products Standard stainless products — % 29 % 16 % — % 29 % 15 % Total 100 % 100 % 100 % 100 % 100 % 100 % The Company maintained a backlog of confirmed orders totaling $1.44 billion and $2.34 billion at September 30, 2020 and 2019, respectively. Due to the structure of the Company’s long-term agreements, approximately 70% of this backlog at September 30, 2020 represented booked orders with performance obligations that will be satisfied within the next 12 months. The backlog does not reflect any elements of variable consideration. Contract balances As of September 30, 2020 and December 31, 2019, accounts receivable with customers were $392.3 million and $558.7 million, respectively. The following represents the rollforward of accounts receivable - reserve for doubtful accounts and contract assets and liabilities for the nine months ended September 30, 2020 and 2019: (in millions) Accounts Receivable - Reserve for Doubtful Accounts September 30, September 30, Balance as of beginning of fiscal year $ 4.6 $ 6.0 Expense to increase the reserve 0.1 0.3 Write-off of uncollectible accounts (0.4) (1.3) Balance as of period end $ 4.3 $ 5.0 (in millions) Contract Assets Short-term September 30, September 30, Balance as of beginning of fiscal year $ 38.5 $ 51.2 Recognized in current year 55.8 57.0 Reclassified to accounts receivable (52.7) (63.1) Impairment — — Reclassification to/from long-term 0.1 — Divestiture — (7.3) Balance as of period end $ 41.7 $ 37.8 Long-term September 30, September 30, Balance as of beginning of fiscal year $ 0.1 $ 0.1 Recognized in current year — — Reclassified to accounts receivable — — Impairment — — Reclassification to/from short-term (0.1) — Balance as of period end $ — $ 0.1 (in millions) Contract Liabilities Short-term September 30, September 30, Balance as of beginning of fiscal year $ 78.7 $ 71.4 Recognized in current year 121.5 104.2 Amounts in beginning balance reclassified to revenue (42.9) (43.9) Current year amounts reclassified to revenue (65.6) (34.8) Other — 0.2 Reclassification to/from long-term 6.1 — Balance as of period end $ 97.8 $ 97.1 Long-term September 30, September 30, Balance as of beginning of fiscal year $ 25.9 $ 7.3 Recognized in current year 12.9 0.8 Amounts in beginning balance reclassified to revenue (0.8) (0.8) Current year amounts reclassified to revenue — — Other — — Reclassification to/from short-term (6.1) — Balance as of period end $ 31.9 $ 7.3 Contract costs for obtaining and fulfilling a contract were $5.6 million and $6.5 million as of September 30, 2020 and December 31, 2019, respectively, and are reported in other long-term assets on the consolidated balance sheet. Contract cost amortization expense for the three and nine months ended September 30, 2020 was $0.4 million and $1.1 million, respectively.

Inventories

Inventories	9 Months Ended
	Sep. 30, 2020
Inventory Disclosure [Abstract]
Inventories	Inventories Inventories at September 30, 2020 and December 31, 2019 were as follows (in millions): September 30, December 31, Raw materials and supplies $ 210.5 $ 164.9 Work-in-process 717.9 899.6 Finished goods 191.1 161.3 Total inventories at current cost 1,119.5 1,225.8 Adjustment from current cost to LIFO cost basis 41.1 33.6 Inventory valuation reserves (124.7) (104.1) Total inventories, net $ 1,035.9 $ 1,155.3 Inventories are stated at the lower of cost (last-in, first-out (LIFO), first-in, first-out (FIFO), and average cost methods) or market. Most of the Company’s inventory is valued utilizing the LIFO costing methodology. Inventory of the Company’s non-U.S. operations is valued using average cost or FIFO methods. Due to deflationary impacts primarily related to raw materials, the carrying value of the Company’s inventory as valued on LIFO exceeds current replacement cost, and based on a lower of cost or market value analysis, the Company maintains net realizable value (NRV) inventory valuation reserves to adjust carrying value of LIFO inventory to current replacement cost. These NRV reserves were $41.1 million at September 30, 2020 and $33.6 million at December 31, 2019. Impacts to cost of sales for changes in the LIFO costing methodology and associated NRV inventory reserves were as follows (in millions): Nine months ended September 30, 2020 2019 LIFO benefit (charge) $ 7.5 $ (1.4) NRV benefit (charge) (7.5) 1.3 Net cost of sales impact $ — $ (0.1)

Property Plant And Equipment

Property Plant And Equipment	9 Months Ended
	Sep. 30, 2020
Property, Plant and Equipment [Abstract]
Property Plant and Equipment	Property, Plant and Equipment Property, plant and equipment at September 30, 2020 and December 31, 2019 was as follows (in millions): September 30, December 31, Land $ 35.0 $ 34.6 Buildings 884.2 832.7 Equipment and leasehold improvements 3,721.7 3,671.3 4,640.9 4,538.6 Accumulated depreciation and amortization (2,174.3) (2,088.5) Total property, plant and equipment, net $ 2,466.6 $ 2,450.1

Goodwill Impairment

Goodwill Impairment	9 Months Ended
	Sep. 30, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill Impairment	Goodwill Impairment At September 30, 2020, the Company had $238.4 million of goodwill on its consolidated balance sheet, all of which relates to the HPMC segment. Goodwill decreased $287.4 million at September 30, 2020 compared to December 31, 2019, due to a $287.0 million interim impairment charge in the HPMC segment and $0.4 million from the impact of foreign currency translation on goodwill denominated in functional currencies other than the U.S. dollar. The Company performs its annual goodwill impairment evaluations in the fourth quarter of each year. During the second quarter of 2020, the Company performed an interim goodwill impairment analysis on the Forged Products reporting unit and its $460.4 million goodwill balance based on assessed potential indicators of impairment, including recent disruptions to the global commercial aerospace market resulting from the COVID-19 pandemic, and the increasing uncertainty of near-term demand requirements of aero-engine and airframe markets based on government responses to the pandemic and ongoing interactions with customers. In the previous 2019 annual goodwill impairment evaluation, this reporting unit had a fair value that exceeded carrying value by approximately 30%. For the 2020 interim impairment analysis, fair value was determined by a quantitative assessment that used a discounted cash flow technique, which represents Level 3 unobservable information in the fair value hierarchy. As a result of the second quarter 2020 interim goodwill impairment evaluation, the Company determined that the fair value of the Forged Products reporting unit was below carrying value, including goodwill, by $287.0 million. This was primarily due to changes in the timing and amount of expected cash flows resulting from lower projected revenues, profitability and cash flows due to near-term reductions in commercial aerospace market demand. Consequently, during the second quarter of 2020, the Company recorded a $287.0 million impairment charge for the partial impairment of the Forged Products reporting unit goodwill, most of which was assigned from the Company’s 2011 Ladish acquisition that was not deductible for income tax purposes. This goodwill impairment charge was excluded from 2020 HPMC business segment results. Accumulated goodwill impairment losses as of September 30, 2020 and December 31, 2019 were $528.0 million and $241.0 million, respectively . The $238.4 million of goodwill remaining as of September 30, 2020 on the Company’s consolidated balance sheet is comprised of $173.4 million at Forged Products and $65.0 million at Specialty Materials. No indicators of impairment were observed in the third quarter 2020 associated with any of the Company’s long-lived assets.

Joint Ventures

Joint Ventures	9 Months Ended
	Sep. 30, 2020
Equity Method Investments and Joint Ventures [Abstract]
Joint Ventures	Joint Ventures The financial results of majority-owned joint ventures are consolidated into the Company’s operating results and financial position, with the minority ownership interest recognized in the consolidated statements of operations as net income attributable to noncontrolling interests, and as equity attributable to the noncontrolling interests within total stockholders’ equity. Investments in which the Company exercises significant influence, but which it does not control (generally a 20% to 50% ownership interest), are accounted for under the equity method of accounting. Majority-Owned Joint Ventures STAL: The Company has a 60% interest in the Chinese joint venture known as STAL. The remaining 40% interest in STAL is owned by China Baowu Steel Group Corporation Limited, a state authorized investment company whose equity securities are publicly traded in the People’s Republic of China. STAL is part of ATI’s AA&S segment, and manufactures Precision Rolled Strip stainless products mainly for the electronics, communication equipment, computer and automotive markets located in Asia. Cash and cash equivalents held by STAL as of September 30, 2020 were $48.9 million. Next Gen Alloys LLC: The Company has a 51% interest in Next Gen Alloys LLC, a joint venture with GE Aviation for the development of a new meltless titanium alloy powder manufacturing technology. The titanium alloy powders are being developed for use in additive manufacturing applications, including 3D printing. Cash and cash equivalents held by this joint venture as of September 30, 2020 were $3.2 million. Equity Method Joint Ventures A&T Stainless: The Company has a 50% interest in A&T Stainless, a joint venture with an affiliate company of Tsingshan Group (Tsingshan) to produce 60-inch wide stainless sheet products for sale in North America. Tsingshan purchased its 50% joint venture interest in A&T Stainless in 2018 for $17.5 million, of which $12.0 million has been received by ATI. The A&T Stainless operations include the Company’s previously-idled direct roll and pickle (DRAP) facility in Midland, PA. ATI provides hot-rolling conversion services to A&T Stainless using the AA&S segment’s Hot-Rolling and Processing Facility. ATI accounts for the A&T Stainless joint venture under the equity method of accounting. In late March 2018, ATI filed for an exclusion from the Section 232 tariffs on behalf of A&T Stainless, which imports semi-finished stainless slab products from Indonesia. In April 2019, the Company learned that this exclusion request was denied by the U.S. Department of Commerce. ATI filed new requests on behalf of A&T Stainless for exclusion from the Section 232 tariffs in October 2019. These requests were denied by the U.S. Department of Commerce in the second quarter of 2020, and the 25% tariff remains in place. On March 31, 2020, ATI announced that A&T Stainless would be idling the DRAP facility in 2020, in an orderly shut down process that was completed in the third quarter. A&T Stainless recorded a $4.8 million charge in the second quarter 2020 for contractual termination benefits as a result of the idling decision. ATI’s share of A&T Stainless results were losses of $1.6 million and $10.1 million for the three and nine months ended September 30, 2020, respectively, and losses of $2.6 million and $9.9 million for the three and nine months ended September 30, 2019, respectively, which is included within other income/expense, net, on the consolidated statements of operations. These equity method results are reported in the AA&S segment, with the exception of ATI’s $2.4 million share of the charge for termination benefits. During the fourth quarter of 2019, A&T Stainless recorded a $14.2 million impairment charge on its long-lived assets, of which ATI recognized a $7.1 million equity loss for its 50% share. In addition, ATI recorded a $4.3 million reserve during the fourth quarter of 2019 on its net receivables for working capital advances and administrative services from A&T Stainless. No additional impairment charge on the long-lived assets of A&T Stainless or additional reserve on ATI’s receivables from A&T Stainless, based on ATI’s share of the estimated fair value of the joint venture’s net assets, was considered necessary during the first nine months of 2020. As of September 30, 2020 and December 31, 2019, ATI had net receivables for working capital advances and administrative services, including the $4.3 million reserve, from A&T Stainless of $14.5 million and $32.5 million, respectively. For the September 30, 2020 balance of net receivables, $0.5 million was reported in prepaid expenses and other current assets and $14.0 million in other long-term assets on the consolidated balance sheet, while for December 31, 2019, $8.3 million was reported in prepaid expenses and other current assets and $24.2 million in other long-term assets. In addition, ATI evaluated the collectability of its remaining $5.5 million receivable from Tsingshan, which is reported in other long-term assets on the consolidated balance sheet, and concluded that no impairment or loss in expected value exists at this time. Uniti: ATI has a 50% interest in the industrial titanium joint venture known as Uniti, with the remaining 50% interest held by VSMPO, a Russian producer of titanium, aluminum, and specialty steel products. Uniti is accounted for under the equity method of accounting. ATI’s share of Uniti’s income was $0.6 million and $1.1 million for the three and nine months ended September 30, 2020, respectively, and $0.2 million and $1.2 million for the three and nine months ended September 30, 2019, respectively, which is included in the AA&S segment’s operating results, and within other income/expense, net on the consolidated statements of operations.

Supplemental Financial Statemen

Supplemental Financial Statement Information - (Notes)	9 Months Ended
	Sep. 30, 2020
Other Income and Expenses [Abstract]
Supplemental Financial Statement Information	Supplemental Financial Statement Information Other income (expense), net for the three and nine months ended September 30, 2020 and 2019 was as follows: (in millions) Three months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 Rent and royalty income $ 0.1 $ 1.4 $ 0.6 $ 2.8 Gains from disposal of property, plant and equipment, net 0.8 62.3 3.3 90.6 Net equity loss on joint ventures (See Note 6) (1.0) (2.4) (6.6) (8.7) Joint venture restructuring charges (See Note 6) — — (2.4) — Net gain (loss) from sales of businesses — 5.8 — (1.9) Adjustment to indemnification for conditional ARO costs — — 4.3 — Other (0.3) 0.1 — 0.1 Total other income (expense), net $ (0.4) $ 67.2 $ (0.8) $ 82.9 Gains from disposal of property, plant and equipment, net for the nine months ended September 30, 2020 include a $2.5 million gain on the sale of certain oil and gas rights. Gains from disposal of property, plant and equipment, net for the three and nine months ended September 30, 2019 include a $62.4 million and $91.7 million gain on the sale of certain oil and gas rights, respectively. These cash gains are reported as investing activities on the consolidated statements of cash flows for the nine months ended September 30, 2020 and September 30, 2019, respectively, and are excluded from segment operating results.

Debt

Debt	9 Months Ended
	Sep. 30, 2020
Debt Disclosure [Abstract]
Debt	Debt Debt at September 30, 2020 and December 31, 2019 was as follows (in millions): September 30, December 31, Allegheny Technologies 5.875% Notes due 2023 (a) $ 500.0 $ 500.0 Allegheny Technologies 5.875% Notes due 2027 350.0 350.0 Allegheny Technologies 3.5% Convertible Senior Notes due 2025 291.4 — Allegheny Technologies 4.75% Convertible Senior Notes due 2022 84.2 287.5 Allegheny Ludlum 6.95% Debentures due 2025 150.0 150.0 Term Loan due 2024 200.0 100.0 U.S. revolving credit facility — — Foreign credit facilities 4.2 4.9 Finance leases and other 25.6 18.8 Debt issuance costs (15.2) (12.3) Equity component of convertible debt (49.0) — Total debt 1,541.2 1,398.9 Short-term debt and current portion of long-term debt 12.2 11.5 Total long-term debt $ 1,529.0 $ 1,387.4 (a) Bearing interest at 7.875% effective February 15, 2016. Revolving Credit Facility The Company has an Asset Based Lending (ABL) Credit Facility, which is collateralized by the accounts receivable and inventory of the Company’s domestic operations. The ABL facility, which matures in September 2024, includes a $500 million revolving credit facility, a letter of credit sub-facility of up to $200 million, and as of September 30, 2020, a $200 million term loan (Term Loan). In June 2020, the Company exercised its right to borrow an additional $100 million under the term loan portion of the ABL, with the same September 2024 maturity date. The Term Loan has an interest rate of 2.0% plus a LIBOR spread and can be prepaid in increments of $25 million if certain minimum liquidity conditions are satisfied. In addition, the Company has the right to request an increase of up to $200 million in the maximum amount available under the revolving credit facility for the duration of the ABL. The Company has a $50 million floating-for-fixed interest rate swap which converts a portion of the Term Loan to a 4.21% fixed interest rate. The swap matures in June 2024. The applicable interest rate for revolving credit borrowings under the ABL facility includes interest rate spreads based on available borrowing capacity that range between 1.25% and 1.75% for LIBOR-based borrowings and between 0.25% and 0.75% for base rate borrowings. The ABL facility contains a financial covenant whereby the Company must maintain a fixed charge coverage ratio of not less than 1.00:1.00 after an event of default has occurred and is continuing or if the undrawn availability under the ABL revolving credit portion of the facility is less than the greater of (i) $87.5 million, calculated as 12.5% of the then applicable maximum advance amount under the revolving credit portion of the ABL and the outstanding Term Loan balance, or (ii) $62.5 million. The Company does not meet this required fixed charge coverage ratio at September 30, 2020. As a result, the Company is unable to access this remaining 12.5%, or $87.5 million, of the ABL facility until it meets the required ratio. Additionally, the Company must demonstrate minimum liquidity, as calculated in accordance with the terms of the ABL facility, during the 90-day period immediately preceding the stated maturity date of the 4.75% Convertible Notes due 2022 and the 5.875% Notes due 2023. The ABL also contains customary affirmative and negative covenants for credit facilities of this type, including limitations on the Company’s ability to incur additional indebtedness or liens or to enter into investments, mergers and acquisitions, dispositions of assets and transactions with affiliates, some of which are more restrictive at any time during the term of the ABL when the Company’s fixed charge coverage ratio is less than 1.00:1.00 and its undrawn availability under the revolving portion of the ABL is less than the greater of (a) $150 million or (b) 30% of the sum of the maximum advance amount under the revolving credit portion of the ABL and the outstanding Term Loan balance. As of September 30, 2020, there were no outstanding borrowings under the revolving portion of the ABL facility, and $34.8 million was utilized to support the issuance of letters of credit. There were average revolving credit borrowings of $38 million bearing an average annual interest rate of 2.2% under the ABL facility for the first nine months of 2020. There were no average revolving credit borrowings for the first nine months of 2019. Convertible Notes In June 2020, the Company issued and sold $285.0 million aggregate principal amount of the 2025 Convertible Notes. The Company granted the underwriters a 13-day option to purchase up to an additional $40.0 million aggregate principal amount of 2025 Convertible Notes on the same terms and conditions to cover over-allotments, if any. The underwriters exercised a portion of this option on June 30, 2020, and the Company completed the offering and sale of an additional $6.4 million aggregate principal amount of 2025 Convertible Notes on July 2, 2020, subsequent to the end of the second quarter 2020. Interest on the 2025 Convertible Notes at the 3.5% cash coupon rate is payable semi-annually in arrears on each June 15 and December 15, commencing December 15, 2020. The Company used a portion of the net proceeds from the offering of the 2025 Convertible Notes to repurchase $203.2 million aggregate principal amount of its outstanding 4.75% Convertible Senior Notes due 2022 (the 2022 Convertible Notes), resulting in a $21.5 million debt extinguishment charge, which included a $19.1 million cash make-whole payment related to the early extinguishment of the 2022 Convertible Notes as required by the applicable indenture, and a $2.4 million charge for deferred debt issue costs. The Company also used $19.4 million of the net proceeds of the offering of the 2025 Convertible Notes to pay the cost of capped call transactions, described below, which was recorded as a reduction to additional paid-in-capital in stockholders’ equity on the consolidated balance sheet. The remainder of the net proceeds from the offering will be used for general corporate purposes. The Company does not have the right to redeem the 2025 Convertible Notes prior to June 15, 2023. On or after June 15, 2023 and prior to the 41st scheduled trading day immediately preceding the maturity date, the Company may redeem all or any portion of the 2025 Convertible Notes, at its option, at a redemption price equal to 100% of the principal amount thereof, plus any accrued and unpaid interest if the last reported sale price of ATI’s common stock has been at least 130% of the conversion price then in effect for at least 20 trading days (whether or not consecutive) during any 30 consecutive trading day period (including the last trading day of such period) ending on the trading day immediately preceding the date on which ATI provides written notice of redemption. The initial conversion rate for the 2025 Convertible Notes is 64.5745 shares of ATI common stock per $1,000 principal amount of the 2025 Convertible Notes, equivalent to an initial conversion price of approximately $15.49 per share. Prior to the close of business on the business day immediately preceding March 15, 2025, the 2025 Convertible Notes will be convertible at the option of the holders of 2025 Convertible Notes only upon the satisfaction of specified conditions and during certain periods. Thereafter, until the close of business on the second scheduled trading day immediately preceding the maturity date, the 2025 Convertible Notes will be convertible at the option of holders of 2025 Convertible Notes at any time regardless of these conditions. Conversions of the 2025 Convertible Notes may be settled in cash, shares of ATI’s common stock or a combination thereof, at ATI’s election. As a result of this flexible settlement feature of the 2025 Convertible Notes, the embedded conversion option is required to be separately accounted for as a component of stockholders’ equity. The value of the embedded conversion option was determined to be $51.4 million based on the estimated fair value of comparable senior unsecured debt without the conversion feature, using an income approach of expected present value. The equity component will be amortized as additional non-cash interest expense over the term of the 2025 Convertible Notes using the effective interest method, and is not remeasured as long as it continues to meet the conditions for equity classification. Offering costs attributable to the debt component totaling $7.5 million are being amortized to interest expense over the term of the 2025 Convertible Notes, and offering costs attributable to the equity component totaling $1.6 million were netted within stockholders’ equity. As a result, $49.8 million of the 2025 Convertible Notes was recorded in additional paid-in-capital in stockholders’ equity ($51.4 million of the gross $291.4 million, net of $1.6 million of allocated offering costs). Including debt issue cost amortization, the 2025 Convertible Notes will have reported interest expense at an 8.4% rate, higher than the 3.5% cash coupon rate. Holders of the 2025 Convertible Notes may require ATI to repurchase their 2025 Convertible Notes upon the occurrence of certain events that constitute a fundamental change under the indenture governing the 2025 Convertible Notes at a purchase price equal to 100% of the principal amount thereof, plus any accrued and unpaid interest to, but excluding, the fundamental change repurchase date. In connection with certain corporate events or if ATI issues a notice of redemption, it will, under certain circumstances, increase the conversion rate for holders who elect to convert their 2025 Convertible Notes in connection with such corporate event or during the relevant redemption period. In connection with the pricing of the 2025 Convertible Notes, ATI entered into privately negotiated capped call transactions with certain of the initial purchasers or their respective affiliates (collectively, the Counterparties). The capped call transactions are expected generally to reduce potential dilution to ATI’s common stock upon any conversion of the 2025 Convertible Notes

Derivative Financial Instrument

Derivative Financial Instruments and Hedging	9 Months Ended
	Sep. 30, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Financial Instruments and Hedging	Derivative Financial Instruments and Hedging As part of its risk management strategy, the Company, from time-to-time, utilizes derivative financial instruments to manage its exposure to changes in raw material prices, energy costs, foreign currencies, and interest rates. In accordance with applicable accounting standards, the Company accounts for most of these contracts as hedges. The Company sometimes uses futures and swap contracts to manage exposure to changes in prices for forecasted purchases of raw materials, such as nickel, and natural gas. Under these contracts, which are generally accounted for as cash flow hedges, the price of the item being hedged is fixed at the time that the contract is entered into, and the Company is obligated to make or receive a payment equal to the net change between this fixed price and the market price at the date the contract matures. The majority of ATI’s products are sold utilizing raw material surcharges and index mechanisms. However, as of September 30, 2020, the Company had entered into financial hedging arrangements, primarily at the request of its customers, related to firm orders, for an aggregate notional amount of approximately 3 million pounds of nickel with hedge dates through 2023. The aggregate notional amount hedged is less than 5% of a single year’s estimated nickel raw material purchase requirements. At September 30, 2020, the outstanding financial derivatives used to hedge the Company’s exposure to energy cost volatility included natural gas cost hedges. At September 30, 2020, the Company hedged approximately 100% of the Company’s forecasted domestic requirements for natural gas for the remainder of 2020, approximately 70% for 2021 and approximately 25% for 2022. While the majority of the Company’s direct export sales are transacted in U.S. dollars, foreign currency exchange contracts are used, from time-to-time, to limit transactional exposure to changes in currency exchange rates for those transactions denominated in a non-U.S. currency. The Company sometimes purchases foreign currency forward contracts that permit it to sell specified amounts of foreign currencies expected to be received from its export sales for pre-established U.S. dollar amounts at specified dates. The forward contracts are denominated in the same foreign currencies in which export sales are denominated. These contracts are designated as hedges of the variability in cash flows of a portion of the forecasted future export sales transactions which otherwise would expose the Company to foreign currency risk, primarily euro. In addition, the Company may also hedge forecasted capital expenditures and designate cash balances held in foreign currencies as hedges of forecasted foreign currency transactions. At September 30, 2020, the Company had no significant outstanding foreign currency forward contracts. The Company may enter into derivative interest rate contracts to maintain a reasonable balance between fixed- and floating-rate debt. The Company has a $50 million floating-for-fixed interest rate swap which converts a portion of the Term Loan to a 4.21% fixed rate. The swap matures in June 2024. The Company designated the interest rate swap as a cash flow hedge of the Company’s exposure to the variability of the payment of interest on a portion of its Term Loan borrowings. The ineffectiveness at hedge inception, determined from the fair value of the swap immediately prior to its July 2019 amendment, will be amortized to interest expense over the initial Term Loan swap maturity date of January 12, 2021. There are no credit risk-related contingent features in the Company’s derivative contracts, and the contracts contained no provisions under which the Company has posted, or would be required to post, collateral. The counterparties to the Company’s derivative contracts are substantial and creditworthy commercial banks that are recognized market makers. The Company controls its credit exposure by diversifying across multiple counterparties and by monitoring credit ratings and credit default swap spreads of its counterparties. The Company also enters into master netting agreements with counterparties when possible. The fair values of the Company’s derivative financial instruments are presented below, representing the gross amounts recognized which are not offset by counterparty or by type of item hedged. All fair values for these derivatives were measured using Level 2 information as defined by the accounting standard hierarchy, which includes quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, and inputs derived principally from or corroborated by observable market data. (In millions) Asset derivatives Balance sheet location September 30, December 31, Derivatives designated as hedging instruments: Nickel and other raw material contracts Prepaid expenses and other current assets 1.8 4.4 Natural gas contracts Prepaid expenses and other current assets 1.4 — Nickel and other raw material contracts Other assets 0.4 1.2 Natural gas contracts Other assets 0.5 — Total derivatives designated as hedging instruments 4.1 5.6 Total asset derivatives $ 4.1 $ 5.6 Liability derivatives Balance sheet location Derivatives designated as hedging instruments: Interest rate swap Other current liabilities $ 1.1 $ 0.3 Natural gas contracts Other current liabilities 0.3 2.5 Nickel and other raw material contracts Other current liabilities 0.1 2.5 Interest rate swap Other long-term liabilities 2.8 1.2 Natural gas contracts Other long-term liabilities — 1.0 Total derivatives designated as hedging instruments 4.3 7.5 Total liability derivatives $ 4.3 $ 7.5 For derivative financial instruments that are designated as cash flow hedges, the gain or loss on the derivative is reported as a component of other comprehensive income (OCI) and reclassified into earnings in the same period or periods during which the hedged item affects earnings. For derivative financial instruments that are designated as fair value hedges, changes in the fair value of these derivatives are recognized in current period results and are reported as changes within accrued liabilities and other on the consolidated statements of cash flows. There were no outstanding fair value hedges as of September 30, 2020. The Company did not use net investment hedges for the periods presented. The effects of derivative instruments in the tables below are presented net of related income taxes, excluding any impacts of changes to income tax valuation allowances affecting results of operations or other comprehensive income, when applicable (see Note 16 for further explanation). Assuming market prices remain constant with those at September 30, 2020, a pre-tax gain of $1.7 million is expected to be recognized over the next 12 months. Activity with regard to derivatives designated as cash flow hedges for the three and nine month periods ended September 30, 2020 and 2019 was as follows (in millions): Amount of Gain (Loss) Amount of Gain (Loss) Three months ended September 30, Three months ended September 30, Derivatives in Cash Flow Hedging Relationships 2020 2019 2020 2019 Nickel and other raw material contracts $ 2.1 $ 11.8 $ 0.7 $ 1.1 Natural gas contracts 2.5 (0.9) (0.8) (0.6) Foreign exchange contracts — 0.9 — 0.3 Interest rate swap 0.1 (1.2) (0.2) (0.1) Total $ 4.7 $ 10.6 $ (0.3) $ 0.7 Amount of Gain (Loss) Amount of Gain (Loss) Nine months ended September 30, Nine months ended September 30, Derivatives in Cash Flow Hedging Relationships 2020 2019 2020 2019 Nickel and other raw material contracts $ (2.1) $ 18.2 $ (1.3) $ 0.5 Natural gas contracts 1.2 (3.2) (2.8) (0.6) Foreign exchange contracts (0.1) 1.0 (0.1) — Interest rate swap (2.1) (1.7) (0.7) (0.3) Total $ (3.1) $ 14.3 $ (4.9) $ (0.4) (a) The gains (losses) reclassified from accumulated OCI into income related to the derivatives, with the exception of the interest rate swap, are presented in cost of sales in the same period or periods in which the hedged item affects earnings. The gains (losses) reclassified from accumulated OCI into income on the interest rate swap are presented in interest expense in the same period as the interest expense on the Term Loan is recognized in earnings. The disclosures of gains or losses presented above for nickel and other raw material contracts and foreign currency contracts do not take into account the anticipated underlying transactions. Since these derivative contracts represent hedges, the net effect of any gain or loss on results of operations may be fully or partially offset.

Fair Value Of Financial Instrum

Fair Value Of Financial Instruments	9 Months Ended
	Sep. 30, 2020
Fair Value Disclosures [Abstract]
Fair Value Of Financial Instruments	Fair Value of Financial Instruments The estimated fair value of financial instruments at September 30, 2020 was as follows: Fair Value Measurements at Reporting Date Using (In millions) Total Total Quoted Prices in Significant Cash and cash equivalents $ 572.2 $ 572.2 $ 572.2 $ — Derivative financial instruments: Assets 4.1 4.1 — 4.1 Liabilities 4.3 4.3 — 4.3 Debt (a) 1,605.4 1,597.7 1,367.9 229.8 The estimated fair value of financial instruments at December 31, 2019 was as follows: Fair Value Measurements at Reporting Date Using (In millions) Total Total Quoted Prices in Significant Cash and cash equivalents $ 490.8 $ 490.8 $ 490.8 $ — Derivative financial instruments: Assets 5.6 5.6 — 5.6 Liabilities 7.5 7.5 — 7.5 Debt (a) 1,411.2 1,676.5 1,552.8 123.7 (a) The total carrying amount for debt for both periods excludes debt issuance costs related to the recognized debt liability which is presented in the consolidated balance sheet as a direct reduction from the carrying amount of the debt liability. The September 30, 2020 debt carrying value includes $49.0 million for the unamortized balance of the portion of the 2025 Convertible Notes recorded in stockholders’ equity due to the flexible settlement feature of the notes (see Note 8). In accordance with accounting standards, fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants at the measurement date. Accounting standards established three levels of a fair value hierarchy that prioritizes the inputs used to measure fair value. This hierarchy requires entities to maximize the use of observable inputs and minimize the use of unobservable inputs. The three levels of inputs used to measure fair value are as follows: Level 1 – Quoted prices in active markets for identical assets or liabilities. Level 2 – Observable inputs other than quoted prices included in Level 1, such as quoted prices for similar assets and liabilities in active markets; quoted prices for identical or similar assets and liabilities in markets that are not active; or other inputs that are observable or can be corroborated by observable market data. Level 3 – Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets and liabilities. This includes certain pricing models, discounted cash flow methodologies and similar techniques that use significant unobservable inputs. The availability of observable market data is monitored to assess the appropriate classification of financial instruments within the fair value hierarchy. Changes in economic conditions or model-based valuation techniques may require the transfer of financial instruments from one fair value level to another. In such instances, the transfer is reported at the beginning of the reporting period. The following methods and assumptions were used by the Company in estimating the fair value of its financial instruments: Cash and cash equivalents: Fair value was determined using Level 1 information. Derivative financial instruments: Fair values for derivatives were measured using exchange-traded prices for the hedged items. The fair value was determined using Level 2 information, including consideration of counterparty risk and the Company’s credit risk. Short-term and long-term debt: The fair values of the Company’s publicly traded debt were based on Level 1 information. The fair values of the other short-term and long-term debt were determined using Level 2 information.

Retirement Benefits

Retirement Benefits	9 Months Ended
	Sep. 30, 2020
Retirement Benefits [Abstract]
Retirement Benefits	Retirement Benefits The Company has defined contribution retirement plans or defined benefit pension plans covering substantially all employees. Company contributions to defined contribution retirement plans are generally based on a percentage of eligible pay or based on hours worked. Benefits under the defined benefit pension plans are generally based on years of service and/or final average pay. The Company funds the U.S. pension plans in accordance with the Employee Retirement Income Security Act of 1974, as amended, and the Internal Revenue Code (IRC). The Company also sponsors several postretirement plans covering certain collectively-bargained salaried and hourly employees. The plans provide health care and life insurance benefits for eligible retirees. In most retiree health care plans, Company contributions towards premiums are capped based on the cost as of a certain date, thereby creating a defined contribution. All defined benefit pension and retiree health care plans are closed to new entrants. For the three month periods ended September 30, 2020 and 2019, the components of pension and other postretirement benefit expense for the Company’s defined benefit plans included the following (in millions): Pension Benefits Other Postretirement Benefits Three months ended September 30, Three months ended September 30, 2020 2019 2020 2019 Service cost - benefits earned during the year $ 3.2 $ 3.2 $ 0.6 $ 0.5 Interest cost on benefits earned in prior years 21.6 26.4 2.6 3.7 Expected return on plan assets (33.6) (32.8) — — Amortization of prior service cost (credit) 0.1 — (0.9) (0.7) Amortization of net actuarial loss 18.6 18.5 2.7 3.4 Total retirement benefit expense $ 9.9 $ 15.3 $ 5.0 $ 6.9 For the nine month periods ended September 30, 2020 and 2019, the components of pension and other postretirement benefit expense for the Company’s defined benefit plans included the following (in millions): Pension Benefits Other Postretirement Benefits Nine months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 Service cost - benefits earned during the year $ 9.5 $ 9.5 $ 1.7 $ 1.4 Interest cost on benefits earned in prior years 64.7 79.1 8.0 11.1 Expected return on plan assets (100.8) (98.5) — — Amortization of prior service cost (credit) 0.5 0.2 (2.8) (2.1) Amortization of net actuarial loss 55.8 55.3 8.1 10.1 Total retirement benefit expense $ 29.7 $ 45.6 $ 15.0 $ 20.5

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Income Taxes	Income TaxesThe provision for income taxes for the third quarter and nine months ended September 30, 2020 was $0.8 million, or (1.7)%, and $104.2 million, or (30.7)%, respectively, of loss before taxes. In the second quarter 2020, ATI recorded a $99.0 million deferred tax asset valuation allowance on its U.S. federal and state tax attributes. The Company entered into a three-year cumulative loss within the United States as of June 30, 2020, limiting the Company’s ability to utilize future projections when analyzing the need for a valuation allowance, therefore limiting sources of income as part of the analysis. The Company continues to be unable to make a reliable estimate of the annual effective tax rate, as significant changes in projected results for the Company’s domestic operations could produce a significant variation in its annual effective tax rate. The tax provision for the third quarter 2019 and the nine months ended September 30, 2019 was $3.7 million, or 3.1%, and $10.3 million, or 4.7%, respectively, of income before taxes, primarily due to deferred tax asset valuation allowances in the United States. The Company has analyzed the impacts of the Coronavirus Aid, Relief, and Economic Security Act (CARES Act) that was enacted on March 27, 2020 and has determined that the impact does not result in a material income tax expense or benefit. The Company continues to evaluate the overall changes between the deferred tax asset classifications of interest, pension and net operating losses resulting from the CARES Act.

Business Segments

Business Segments	9 Months Ended
	Sep. 30, 2020
Segment Reporting [Abstract]
Business Segments	Business SegmentsEffective January 1, 2020, the Company began operating under two revised business segments: High Performance Materials & Components (HPMC) and Advanced Alloys & Solutions (AA&S). HPMC is now comprised of the Specialty Materials and Forged Products businesses, as well as our ATI Europe distribution operations. The updated HPMC segment intensifies its primary focus on maximizing aero-engine materials and components growth, with approximately 80% of its revenue derived from the aerospace and defense markets. The new AA&S segment combines our Specialty Alloys & Components (SAC) business, including the primary titanium operations in Richland, WA and Albany, OR, with ATI’s former Flat Rolled Products (FRP) business segment, which included the FRP business, the 60%-owned STAL joint venture, and the Uniti and A&T Stainless 50%-owned joint ventures that are reported in AA&S segment results under the equity method of accounting. AA&S is focused on delivering high-value flat products primarily to the energy, aerospace, and defense end-markets, which comprise approximately 50% of its revenue. AA&S was created to align melting technologies with hot-rolling capabilities to produce products with faster flow times and lower costs. Financial results of our aerospace-grade titanium plate products also transferred from HPMC to AA&S effective January 1, 2020. All segment reporting information for 2020 and prior periods below reflect these two revised business segments. The measure of segment operating profit, which is used to analyze the performance and results of the business segments, excludes all effects of LIFO inventory accounting and any related changes in net realizable value inventory reserves which offset the Company’s aggregate net debit LIFO valuation balance, income taxes, corporate expenses, net interest expense, closed operations and other expenses, charges for goodwill impairment (see Note 5), restructuring and other charges (see Note 14), debt extinguishment charges and non-operating gains and losses. Management believes segment operating profit, as defined, provides an appropriate measure of controllable operating results at the business segment level. Following is certain financial information with respect to the Company’s business segments for the periods indicated (in millions): Three months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 Total sales: High Performance Materials & Components $ 230.1 $ 471.6 $ 995.3 $ 1,540.7 Advanced Alloys & Solutions 393.3 626.2 1,487.1 1,811.2 623.4 1,097.8 2,482.4 3,351.9 Intersegment sales: High Performance Materials & Components 8.8 19.8 53.0 59.2 Advanced Alloys & Solutions 16.6 59.3 105.6 188.8 25.4 79.1 158.6 248.0 Sales to external customers: High Performance Materials & Components 221.3 451.8 942.3 1,481.5 Advanced Alloys & Solutions 376.7 566.9 1,381.5 1,622.4 $ 598.0 $ 1,018.7 $ 2,323.8 $ 3,103.9 Three months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 Segment operating profit (loss): High Performance Materials & Components $ (2.7) $ 68.4 $ 63.6 $ 198.5 Advanced Alloys & Solutions (4.1) 30.8 38.1 76.9 Total segment operating profit (loss) (6.8) 99.2 101.7 275.4 LIFO and net realizable value reserves — — — (0.1) Corporate expenses (10.9) (16.0) (31.4) (50.6) Closed operations and other expense (1.1) (8.2) (5.3) (19.2) Restructuring and other charges (See Note 14) (2.3) — (27.0) — Impairment of goodwill (See Note 5) — — (287.0) — Joint venture restructuring charges (See Note 6) — — (2.4) — Gain on asset sales, net — 68.2 2.5 89.8 Debt extinguishment charge (See Note 8) — — (21.5) — Interest expense, net (25.1) (24.2) (68.7) (74.9) Income (loss) before income taxes $ (46.2) $ 119.0 $ (339.1) $ 220.4 Corporate expenses were lower for the third quarter and first nine months of 2020 compared to 2019 primarily due to lower incentive compensation expense based on expected performance versus targeted metrics, and lower expenses resulting from cost reduction actions. Closed operations and other expenses were lower in the third quarter and first nine months of 2020 compared to 2019, reflecting lower legal and retirement benefit expense of closed operations. Closed operations and other expenses for the first nine months of 2020 also benefited by $4.3 million from settlements of contract indemnity obligations, along with lower environmental expense of closed operations compared to the prior year period. The $2.5 million gain on asset sales, net for the nine months ended September 30, 2020 consists of a gain on the sale of certain oil and gas rights (see Note 7). The $68.2 million net gain on asset sales for the third quarter ended September 30, 2019 consists of a $62.4 million gain on the sale of certain oil and gas rights and a $6.2 million gain on the sale of the Company’s Cast Products business, partially offset by a $0.4 million loss on the sale of two non-core forging facilities, located in Portland, IN and Lebanon, KY. The $89.8 million net gain on asset sales for the nine months ended September 30, 2019 consists of a

Restructuring

Restructuring	3 Months Ended
	Sep. 30, 2020
Restructuring and Related Activities [Abstract]
Restructuring	Restructuring For the third quarter and nine months ended September 30, 2020, the Company recorded restructuring charges of $2.3 million and $27.0 million, respectively, which are presented as restructuring charges in the consolidated statements of operations and are excluded from segment operating results. These charges were a result of workforce right-sizing actions to better match the Company’s cost structure to expected demand, primarily as a result of economic challenges created by the COVID-19 pandemic. For the third quarter of 2020, these charges are comprised of severance obligations for the elimination of approximately 100 positions at several operating locations and the corporate office. For the nine months ended September 30, 2020, these charges also include severance obligations for the elimination of approximately 550 positions for both involuntary reductions and voluntary retirement incentive programs related to both salary and hourly employees in the HPMC segment in the second quarter of 2020 and the reduction of approximately 90 positions for a voluntary retirement incentive program completed in the first quarter of 2020 for eligible salaried employees, building on the previously announced restructuring program in the fourth quarter of 2019. Reserves for restructuring charges at September 30, 2020 consist of severance costs incurred in the fourth quarter 2019 and the first nine months of 2020, which are expected to be substantially paid by mid-2021. Restructuring reserves activity is as follows: Severance and Employee Benefit Costs Balance at December 31, 2019 $ 4.5 Additions 27.0 Payments (11.4) Balance at September 30, 2020 $ 20.1

Per Share Information

Per Share Information	9 Months Ended
	Sep. 30, 2020
Earnings Per Share [Abstract]
Per Share Information	Per Share Information The following table sets forth the computation of basic and diluted income (loss) per common share: Three months ended Nine months ended (In millions, except per share amounts) September 30, September 30, 2020 2019 2020 2019 Numerator: Numerator for basic income (loss) per common share – Net income (loss) attributable to ATI $ (50.1) $ 111.0 $ (451.6) $ 201.1 Effect of dilutive securities: 4.75% Convertible Senior Notes due 2022 — 3.2 — 9.7 3.5% Convertible Senior Notes due 2025 — — — — Numerator for diluted income (loss) per common share – Net income (loss) attributable to ATI after assumed conversions $ (50.1) $ 114.2 $ (451.6) $ 210.8 Denominator: Denominator for basic net income (loss) per common share – weighted average shares 126.6 125.9 126.5 125.7 Effect of dilutive securities: Share-based compensation — 0.9 — 0.8 4.75% Convertible Senior Notes due 2022 — 19.9 — 19.9 3.5% Convertible Senior Notes due 2025 — — — — Denominator for diluted net income (loss) per common share – adjusted weighted average shares and assumed conversions 126.6 146.7 126.5 146.4 Basic net income (loss) attributable to ATI per common share $ (0.40) $ 0.88 $ (3.57) $ 1.60 Diluted net income (loss) attributable to ATI per common share $ (0.40) $ 0.78 $ (3.57) $ 1.44 Common stock that would be issuable upon the assumed conversion of the 2022 Convertible Notes and the 2025 Convertible Notes and other option equivalents and contingently issuable shares are excluded from the computation of contingently issuable shares, and therefore, from the denominator for diluted earnings per share, if the effect of inclusion is anti-dilutive. There were 25.0 million and 21.9 million anti-dilutive shares for the three and nine month periods ended September 30, 2020, respectively. There were no anti-dilutive shares for the three and nine months periods ended September 30, 2019.

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income (Loss)	9 Months Ended
	Sep. 30, 2020
Equity [Abstract]
Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) The changes in accumulated other comprehensive income (loss) (AOCI) by component, net of tax, for the three month period ended September 30, 2020 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, June 30, 2020 $ (1,051.8) $ (84.5) $ (3.7) $ (32.8) $ (1,172.8) OCI before reclassifications — 17.2 4.7 — 21.9 Amounts reclassified from AOCI (a) 15.6 (b) — (c) 0.3 (d) 6.4 22.3 Net current-period OCI 15.6 17.2 5.0 6.4 44.2 Balance, September 30, 2020 $ (1,036.2) $ (67.3) $ 1.3 $ (26.4) $ (1,128.6) Attributable to noncontrolling interests: Balance, June 30, 2020 $ — $ 8.5 $ — $ — $ 8.5 OCI before reclassifications — 6.0 — — 6.0 Amounts reclassified from AOCI — (b) — — — — Net current-period OCI — 6.0 — — 6.0 Balance, September 30, 2020 $ — $ 14.5 $ — $ — $ 14.5 The changes in AOCI by component, net of tax, for the nine month period ended September 30, 2020 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, December 31, 2019 $ (1,083.1) $ (76.6) $ (0.5) $ (41.5) $ (1,201.7) OCI before reclassifications — 9.3 (3.1) — 6.2 Amounts reclassified from AOCI (a) 46.9 (b) — (c) 4.9 (d) 15.1 66.9 Net current-period OCI 46.9 9.3 1.8 15.1 73.1 Balance, September 30, 2020 $ (1,036.2) $ (67.3) $ 1.3 $ (26.4) $ (1,128.6) Attributable to noncontrolling interests: Balance, December 31, 2019 $ — $ 9.8 $ — $ — $ 9.8 OCI before reclassifications — 4.7 — — 4.7 Amounts reclassified from AOCI — (b) — — — — Net current-period OCI — 4.7 — — 4.7 Balance, September 30, 2020 $ — $ 14.5 $ — $ — $ 14.5 (a) Amounts were included in net periodic benefit cost for pension and other postretirement benefit plans (see Note 11). (b) No amounts were reclassified to earnings. (c) Amounts related to derivatives are included in cost of goods sold or interest expense in the period or periods the hedged item affects earnings (see Note 9). (d) Represents the net change in deferred tax asset valuation allowances on changes in AOCI balances between the balance sheet dates. The changes in AOCI by component, net of tax, for the three month period ended September 30, 2019 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, June 30, 2019 $ (973.4) $ (73.0) $ — $ (38.0) $ (1,084.4) OCI before reclassifications — (11.7) 10.6 — (1.1) Amounts reclassified from AOCI (a) 16.1 (b) — (c) (0.7) (d) 8.7 24.1 Net current-period OCI 16.1 (11.7) 9.9 8.7 23.0 Balance, September 30, 2019 $ (957.3) $ (84.7) $ 9.9 $ (29.3) $ (1,061.4) Attributable to noncontrolling interests: Balance, June 30, 2019 $ — $ 12.7 $ — $ — $ 12.7 OCI before reclassifications — (4.6) — — (4.6) Amounts reclassified from AOCI — (b) — — — — Net current-period OCI — (4.6) — — $ (4.6) Balance, September 30, 2019 $ — $ 8.1 $ — $ — $ 8.1 The changes in AOCI by component, net of tax, for the nine month period ended September 30, 2019 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, December 31, 2018 $ (1,005.8) $ (73.9) $ (4.8) $ (49.3) $ (1,133.8) OCI before reclassifications — (10.8) 14.3 — 3.5 Amounts reclassified from AOCI (a) 48.5 (b) — (c) 0.4 (d) 20.0 68.9 Net current-period OCI 48.5 (10.8) 14.7 20.0 72.4 Balance, September 30, 2019 $ (957.3) $ (84.7) $ 9.9 $ (29.3) $ (1,061.4) Attributable to noncontrolling interests: Balance, December 31, 2018 $ — $ 11.1 $ — $ — $ 11.1 OCI before reclassifications — (3.0) — — (3.0) Amounts reclassified from AOCI — (b) — — — — Net current-period OCI — (3.0) — — $ (3.0) Balance, September 30, 2019 $ — $ 8.1 $ — $ — $ 8.1 (a) Amounts were included in net periodic benefit cost for pension and other postretirement benefit plans (see Note 11). (b) No amounts were reclassified to earnings. (c) Amounts related to derivatives are included in cost of goods sold or interest expense in the period or periods the hedged item affects earnings (see Note 9). (d) Represents the net change in deferred tax asset valuation allowances on changes in AOCI balances between the balance sheet dates. Other comprehensive income (loss) amounts (OCI) reported above by category are net of applicable income tax expense (benefit) for each year presented. Income tax expense (benefit) on OCI items is recorded as a change in a deferred tax asset or liability. Amounts recognized in OCI include the impact of any deferred tax asset valuation allowances, when applicable. Foreign currency translation adjustments, including those pertaining to noncontrolling interests, are generally not adjusted for income taxes as they relate to indefinite investments in non-U.S. subsidiaries. Reclassifications out of AOCI for the three and nine month periods ended September 30, 2020 and 2019 were as follows: Amount reclassified from AOCI Details about AOCI Components (In millions) Three months ended September 30, 2020 Three months ended September 30, 2019 Nine months ended September 30, 2020 Nine months ended September 30, 2019 Affected line item in the Postretirement benefit plans Prior service credit $ 0.8 $ 0.7 $ 2.3 $ 1.9 (a) Actuarial losses (21.3) (21.9) (63.9) (65.4) (a) (20.5) (21.2) (61.6) (63.5) (c) Total before tax (4.9) (5.1) (14.7) (15.0) Tax benefit (d) $ (15.6) $ (16.1) $ (46.9) $ (48.5) Net of tax Derivatives Nickel and other raw material contracts $ 0.9 $ 1.5 $ (1.7) $ 0.7 (b) Natural gas contracts (1.1) (0.8) (3.7) (0.8) (b) Foreign exchange contracts — 0.4 (0.1) — (b) Interest rate swap (0.2) (0.3) (0.9) (0.5) (b) (0.4) 0.8 (6.4) (0.6) (c) Total before tax (0.1) 0.1 (1.5) (0.2) Tax (benefit) expense (d) $ (0.3) $ 0.7 $ (4.9) $ (0.4) Net of tax (a) Amounts are reported in nonoperating retirement benefit expense (see Note 11). (b) Amounts related to derivatives, with the exception of the interest rate swap, are included in cost of goods sold in the period or periods the hedged item affects earnings. Amounts related to the interest rate swap are included in interest expense in the same period as the interest expense on the Term Loan is recognized in earnings (see Note 8). (c) For pretax items, positive amounts are income and negative amounts are expense in terms of the impact to net income. Tax effects are presented in conformity with ATI’s presentation in the consolidated statements of operations. (d) These amounts exclude the impact of any deferred tax asset valuation allowances, when applicable.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies The Company is subject to various domestic and international environmental laws and regulations that govern the discharge of pollutants and disposal of wastes, and which may require that it investigate and remediate the effects of the release or disposal of materials at sites associated with past and present operations. The Company could incur substantial cleanup costs, fines, and civil or criminal sanctions, third party property damage or personal injury claims as a result of violations or liabilities under these laws or noncompliance with environmental permits required at its facilities. The Company is currently involved in the investigation and remediation of a number of its current and former sites, as well as third party sites. Environmental liabilities are recorded when the Company’s liability is probable and the costs are reasonably estimable. In many cases, however, the Company is not able to determine whether it is liable or, if liability is probable, to reasonably estimate the loss or range of loss. Estimates of the Company’s liability remain subject to additional uncertainties, including the nature and extent of site contamination, available remediation alternatives, the extent of corrective actions that may be required, and the number, participation, and financial condition of other potentially responsible parties (PRPs). The Company adjusts its accruals to reflect new information as appropriate. Future adjustments could have a material adverse effect on the Company’s consolidated results of operations in a given period, but the Company cannot reliably predict the amounts of such future adjustments. At September 30, 2020, the Company’s reserves for environmental remediation obligations totaled approximately $14 million, of which $4 million was included in other current liabilities. The reserve includes estimated probable future costs of $3 million for federal Superfund and comparable state-managed sites; $9 million for formerly owned or operated sites for which the Company has remediation or indemnification obligations; $1 million for owned or controlled sites at which Company operations have been discontinued; and $1 million for sites utilized by the Company in its ongoing operations. The timing of expenditures depends on a number of factors that vary by site. The Company expects that it will expend present accruals over many years and that remediation of all sites with which it has been identified will be completed within thirty years. The Company continues to evaluate whether it may be able to recover a portion of past and future costs for environmental liabilities from third parties and to pursue such recoveries where appropriate. Based on currently available information, it is reasonably possible that costs for recorded matters may exceed the Company’s recorded reserves by as much as $15 million. Future investigation or remediation activities may result in the discovery of additional hazardous materials, potentially higher levels of contamination than discovered during prior investigation, and may impact costs of the success or lack thereof in remedial solutions. Therefore, future developments, administrative actions or liabilities relating to environmental matters could have a material adverse effect on the Company’s consolidated financial condition or results of operations. A number of other lawsuits, claims and proceedings have been or may be asserted against the Company relating to the conduct of its currently and formerly owned businesses, including those pertaining to product liability, environmental, health and safety matters and occupational disease (including as each relates to alleged asbestos exposure), as well as patent infringement, commercial, government contracting, construction, employment, employee and retiree benefits, taxes, environmental, and stockholder and corporate governance matters. While the outcome of litigation cannot be predicted with certainty, and some of these lawsuits, claims or proceedings may be determined adversely to the Company, management does not believe that the disposition of any such pending matters is likely to have a material adverse effect on the Company’s financial condition or liquidity, although the resolution in any reporting period of one or more of these matters could have a material adverse effect on the Company’s consolidated results of operations for that period. Allegheny Technologies Incorporated and its subsidiary, ATI Titanium LLC (“ATI Titanium”), are parties to a lawsuit captioned US Magnesium, LLC v. ATI Titanium LLC (Case No. 2:17-cv-00923-DB) and filed in federal district court in Salt Lake City, UT, pertaining to a Supply and Operating Agreement between US Magnesium LLC (“USM”) and ATI Titanium entered into in 2006 (the “Supply Agreement”). In 2016, ATI Titanium notified USM that it would suspend performance under the Supply Agreement in reliance on certain terms and conditions included in the Supply Agreement. USM subsequently filed a claim challenging ATI Titanium’s right to suspend performance under the Supply Agreement, claiming that such suspension was a material breach of the Supply Agreement and seeking monetary damages, and ATI Titanium filed a counterclaim for breach of contract against USM. In 2018, USM obtained leave of the court to add Allegheny Technologies Incorporated as a separate party defendant, and ATI Titanium filed a motion to dismiss the claim against Allegheny Technologies Incorporated, which the court denied on April 19, 2019. The case currently is scheduled for trial in mid-2021, and while ATI intends to vigorously defend against and pursue these claims, it cannot predict their outcomes at this time.

Accounting Policies (Policies)

Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Basis Of Accounting	The interim consolidated financial statements include the accounts of Allegheny Technologies Incorporated and its subsidiaries. Unless the context requires otherwise, “Allegheny Technologies”, “ATI” and “the Company” refer to Allegheny Technologies Incorporated and its subsidiaries.These unaudited consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles for interim financial information and with the instructions for Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and note disclosures required by U.S. generally accepted accounting principles for complete financial statements. In management’s opinion, all adjustments (which include only normal recurring adjustments) considered necessary for a fair presentation have been included. Certain prior year amounts have been reclassified in order to conform with fiscal year 2020 presentation. These unaudited consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto included in the Company’s 2019 Annual Report on Form 10-K. The results of operations for these interim periods are not necessarily indicative of the operating results for any future period. The December 31, 2019 financial information has been derived from the Company’s audited consolidated financial statements.
New Accounting Pronouncements Adopted	New Accounting Pronouncements Adopted In March 2020, the Financial Accounting Standards Board (FASB) issued new optional accounting guidance for a limited period of time to ease the potential burden in accounting for (or recognizing the effects of) reference rate reform on financial reporting. In response to concerns about structural risks of interbank offered rates (IBORs), and, particularly, the risk of cessation of the London Interbank Offered Rate (LIBOR), regulators in several jurisdictions around the world have undertaken reference rate reform initiatives to identify alternative reference rates that are more observable or transaction based and less susceptible to manipulation. The new accounting guidance provides optional expedients and exceptions for applying generally accepted accounting principles to contracts, hedging relationships, and other transactions affected by reference rate reform if certain criteria are met. The new accounting guidance applies only to contracts, hedging relationships, and other transactions that reference LIBOR or another reference rate expected to be discontinued because of reference rate reform. The expedients and exceptions provided by the amendments generally do not apply to contract modifications made and hedging relationships entered into or evaluated after December 31, 2022. Management is continuing to evaluate the issue, and presently does not expect a transition away from LIBOR, primarily involving ATI’s domestic credit facility and an interest rate swap contract, to have any significant financial impact to ATI. In December 2019, the FASB issued new guidance to simplify the accounting for income taxes. The areas for simplification in the guidance involve the removal of certain exceptions to the general principals in the current guidance, including intraperiod allocation and the calculation of income taxes in an interim period when a year to date loss exceeds the anticipated loss for the year. The new guidance also simplifies the accounting for income taxes in the area of franchise taxes. This new guidance is effective for the Company in fiscal year 2021, with early adoption permitted. This guidance was early adopted by the Company in fiscal year 2020 without significant impact to the consolidated financial statements. In August 2018, the FASB issued new disclosure guidance on fair value measurement. This new guidance modifies the disclosure requirements on fair value measurements, including removal and modifications of various current disclosures as well as some additional disclosure requirements for Level 3 fair value measurements. Some of these disclosure changes must be applied prospectively while others retrospectively depending on the requirement. This guidance was adopted by the Company in fiscal year 2020 without an impact on the Company’s consolidated financial statements other than disclosures. In June 2016, the FASB added a new impairment model (known as the current expected credit loss (CECL) model) that is based on expected losses rather than incurred losses. Under the new guidance, an entity recognizes as an allowance its estimate of expected credit losses. The CECL model applies to trade receivables, other receivables, contract assets and most debt instruments. The CECL model does not have a minimum threshold for recognition of impairment losses, and entities will need to measure expected credit losses on assets that have a low risk of loss. This guidance was adopted by the Company in fiscal year 2020 without significant impact to the consolidated financial statements.
Pending Accounting Pronouncements	Pending Accounting Pronouncements In August 2020, the FASB issued new accounting guidance related to accounting for convertible instruments. Under this new guidance, embedded conversion features are no longer separated from the host contract for convertible instruments with conversion features that are not required to be accounted for as derivatives, or that do not result in substantial premiums accounted for as paid-in capital. As such, a convertible debt instrument will be accounted for as a single liability measured at its amortized cost, as long as no other features require bifurcation and recognition as derivatives. By removing those separation models, the reported interest rate of convertible debt instruments typically will be closer to the coupon interest rate. The new guidance also addresses how convertible instruments are accounted for in the diluted earnings per share calculation, requiring the if-converted method, and requires enhanced disclosures about the terms of convertible instruments and contracts in an entity’s own equity. This new guidance is effective for the Company in fiscal year 2022, with early adoption permitted.
Inventory	Inventories are stated at the lower of cost (last-in, first-out (LIFO), first-in, first-out (FIFO), and average cost methods) or market. Most of the Company’s inventory is valued utilizing the LIFO costing methodology. Inventory of the Company’s non-U.S. operations is valued using average cost or FIFO methods. Due to deflationary impacts primarily related to raw materials, the carrying value of the Company’s inventory as valued on LIFO exceeds current replacement cost, and based on a lower of cost or market value analysis, the Company maintains net realizable value (NRV) inventory valuation reserves to adjust carrying value of LIFO inventory to current replacement cost.
Derivatives	As part of its risk management strategy, the Company, from time-to-time, utilizes derivative financial instruments to manage its exposure to changes in raw material prices, energy costs, foreign currencies, and interest rates. In accordance with applicable accounting standards, the Company accounts for most of these contracts as hedges. The Company sometimes uses futures and swap contracts to manage exposure to changes in prices for forecasted purchases of raw materials, such as nickel, and natural gas. Under these contracts, which are generally accounted for as cash flow hedges, the price of the item being hedged is fixed at the time that the contract is entered into, and the Company is obligated to make or receive a payment equal to the net change between this fixed price and the market price at the date the contract matures. The majority of ATI’s products are sold utilizing raw material surcharges and index mechanisms. However, as of September 30, 2020, the Company had entered into financial hedging arrangements, primarily at the request of its customers, related to firm orders, for an aggregate notional amount of approximately 3 million pounds of nickel with hedge dates through 2023. The aggregate notional amount hedged is less than 5% of a single year’s estimated nickel raw material purchase requirements. At September 30, 2020, the outstanding financial derivatives used to hedge the Company’s exposure to energy cost volatility included natural gas cost hedges. At September 30, 2020, the Company hedged approximately 100% of the Company’s forecasted domestic requirements for natural gas for the remainder of 2020, approximately 70% for 2021 and approximately 25% for 2022. While the majority of the Company’s direct export sales are transacted in U.S. dollars, foreign currency exchange contracts are used, from time-to-time, to limit transactional exposure to changes in currency exchange rates for those transactions denominated in a non-U.S. currency. The Company sometimes purchases foreign currency forward contracts that permit it to sell specified amounts of foreign currencies expected to be received from its export sales for pre-established U.S. dollar amounts at specified dates. The forward contracts are denominated in the same foreign currencies in which export sales are denominated. These contracts are designated as hedges of the variability in cash flows of a portion of the forecasted future export sales transactions which otherwise would expose the Company to foreign currency risk, primarily euro. In addition, the Company may also hedge forecasted capital expenditures and designate cash balances held in foreign currencies as hedges of forecasted foreign currency transactions. At September 30, 2020, the Company had no significant outstanding foreign currency forward contracts. The Company may enter into derivative interest rate contracts to maintain a reasonable balance between fixed- and floating-rate debt. The Company has a $50 million floating-for-fixed interest rate swap which converts a portion of the Term Loan to a 4.21% fixed rate. The swap matures in June 2024. The Company designated the interest rate swap as a cash flow hedge of the Company’s exposure to the variability of the payment of interest on a portion of its Term Loan borrowings. The ineffectiveness at hedge inception, determined from the fair value of the swap immediately prior to its July 2019 amendment, will be amortized to interest expense over the initial Term Loan swap maturity date of January 12, 2021. There are no credit risk-related contingent features in the Company’s derivative contracts, and the contracts contained no provisions under which the Company has posted, or would be required to post, collateral. The counterparties to the Company’s derivative contracts are substantial and creditworthy commercial banks that are recognized market makers. The Company controls its credit exposure by diversifying across multiple counterparties and by monitoring credit ratings and credit default swap spreads of its counterparties. The Company also enters into master netting agreements with counterparties when possible.
Retirement Benefits	The Company has defined contribution retirement plans or defined benefit pension plans covering substantially all employees. Company contributions to defined contribution retirement plans are generally based on a percentage of eligible pay or based on hours worked. Benefits under the defined benefit pension plans are generally based on years of service and/or final average pay. The Company funds the U.S. pension plans in accordance with the Employee Retirement Income Security Act of 1974, as amended, and the Internal Revenue Code (IRC). The Company also sponsors several postretirement plans covering certain collectively-bargained salaried and hourly employees. The plans provide health care and life insurance benefits for eligible retirees. In most retiree health care plans, Company contributions towards premiums are capped based on the cost as of a certain date, thereby creating a defined contribution. All defined benefit pension and retiree health care plans are closed to new entrants.
Commitments And Contingencies	Environmental liabilities are recorded when the Company’s liability is probable and the costs are reasonably estimable. In many cases, however, the Company is not able to determine whether it is liable or, if liability is probable, to reasonably estimate the loss or range of loss. Estimates of the Company’s liability remain subject to additional uncertainties, including the nature and extent of site contamination, available remediation alternatives, the extent of corrective actions that may be required, and the number, participation, and financial condition of other potentially responsible parties (PRPs). The Company adjusts its accruals to reflect new information as appropriate. Future adjustments could have a material adverse effect on the Company’s consolidated results of operations in a given period, but the Company cannot reliably predict the amounts of such future adjustments.

Inventory (Policies)

Inventory (Policies)	9 Months Ended
	Sep. 30, 2020
Inventory Disclosure [Abstract]
Inventory	Inventories are stated at the lower of cost (last-in, first-out (LIFO), first-in, first-out (FIFO), and average cost methods) or market. Most of the Company’s inventory is valued utilizing the LIFO costing methodology. Inventory of the Company’s non-U.S. operations is valued using average cost or FIFO methods. Due to deflationary impacts primarily related to raw materials, the carrying value of the Company’s inventory as valued on LIFO exceeds current replacement cost, and based on a lower of cost or market value analysis, the Company maintains net realizable value (NRV) inventory valuation reserves to adjust carrying value of LIFO inventory to current replacement cost.

Derivative Instruments and Hedg

Derivative Instruments and Hedging Activities (Policies)	9 Months Ended
	Sep. 30, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives	As part of its risk management strategy, the Company, from time-to-time, utilizes derivative financial instruments to manage its exposure to changes in raw material prices, energy costs, foreign currencies, and interest rates. In accordance with applicable accounting standards, the Company accounts for most of these contracts as hedges. The Company sometimes uses futures and swap contracts to manage exposure to changes in prices for forecasted purchases of raw materials, such as nickel, and natural gas. Under these contracts, which are generally accounted for as cash flow hedges, the price of the item being hedged is fixed at the time that the contract is entered into, and the Company is obligated to make or receive a payment equal to the net change between this fixed price and the market price at the date the contract matures. The majority of ATI’s products are sold utilizing raw material surcharges and index mechanisms. However, as of September 30, 2020, the Company had entered into financial hedging arrangements, primarily at the request of its customers, related to firm orders, for an aggregate notional amount of approximately 3 million pounds of nickel with hedge dates through 2023. The aggregate notional amount hedged is less than 5% of a single year’s estimated nickel raw material purchase requirements. At September 30, 2020, the outstanding financial derivatives used to hedge the Company’s exposure to energy cost volatility included natural gas cost hedges. At September 30, 2020, the Company hedged approximately 100% of the Company’s forecasted domestic requirements for natural gas for the remainder of 2020, approximately 70% for 2021 and approximately 25% for 2022. While the majority of the Company’s direct export sales are transacted in U.S. dollars, foreign currency exchange contracts are used, from time-to-time, to limit transactional exposure to changes in currency exchange rates for those transactions denominated in a non-U.S. currency. The Company sometimes purchases foreign currency forward contracts that permit it to sell specified amounts of foreign currencies expected to be received from its export sales for pre-established U.S. dollar amounts at specified dates. The forward contracts are denominated in the same foreign currencies in which export sales are denominated. These contracts are designated as hedges of the variability in cash flows of a portion of the forecasted future export sales transactions which otherwise would expose the Company to foreign currency risk, primarily euro. In addition, the Company may also hedge forecasted capital expenditures and designate cash balances held in foreign currencies as hedges of forecasted foreign currency transactions. At September 30, 2020, the Company had no significant outstanding foreign currency forward contracts. The Company may enter into derivative interest rate contracts to maintain a reasonable balance between fixed- and floating-rate debt. The Company has a $50 million floating-for-fixed interest rate swap which converts a portion of the Term Loan to a 4.21% fixed rate. The swap matures in June 2024. The Company designated the interest rate swap as a cash flow hedge of the Company’s exposure to the variability of the payment of interest on a portion of its Term Loan borrowings. The ineffectiveness at hedge inception, determined from the fair value of the swap immediately prior to its July 2019 amendment, will be amortized to interest expense over the initial Term Loan swap maturity date of January 12, 2021. There are no credit risk-related contingent features in the Company’s derivative contracts, and the contracts contained no provisions under which the Company has posted, or would be required to post, collateral. The counterparties to the Company’s derivative contracts are substantial and creditworthy commercial banks that are recognized market makers. The Company controls its credit exposure by diversifying across multiple counterparties and by monitoring credit ratings and credit default swap spreads of its counterparties. The Company also enters into master netting agreements with counterparties when possible.

Commitment and Contingencies (P

Commitment and Contingencies (Policies)	9 Months Ended
	Sep. 30, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments And Contingencies	Environmental liabilities are recorded when the Company’s liability is probable and the costs are reasonably estimable. In many cases, however, the Company is not able to determine whether it is liable or, if liability is probable, to reasonably estimate the loss or range of loss. Estimates of the Company’s liability remain subject to additional uncertainties, including the nature and extent of site contamination, available remediation alternatives, the extent of corrective actions that may be required, and the number, participation, and financial condition of other potentially responsible parties (PRPs). The Company adjusts its accruals to reflect new information as appropriate. Future adjustments could have a material adverse effect on the Company’s consolidated results of operations in a given period, but the Company cannot reliably predict the amounts of such future adjustments.

Revenue from Contracts with C_2

Revenue from Contracts with Customers (Tables)	9 Months Ended
	Sep. 30, 2020
Revenue from Contract with Customer [Abstract]
Schedule of Disaggregation of Revenue	Comparative information of the Company’s overall revenues (in millions) by global and geographical markets for the third quarters and nine months ended September 30, 2020 and 2019 were as follows: (in millions) Third quarter ended September 30, 2020 September 30, 2019 HPMC AA&S Total HPMC AA&S Total Diversified Global Markets: Aerospace & Defense $ 171.8 $ 74.3 $ 246.1 $ 377.7 $ 125.8 $ 503.5 Energy* 24.4 106.0 130.4 36.2 181.0 217.2 Automotive 1.5 57.9 59.4 2.2 71.4 73.6 Electronics/Computers/Communications 0.2 45.7 45.9 0.1 44.4 44.5 Food Equipment & Appliances — 33.8 33.8 0.1 53.5 53.6 Construction/Mining 4.0 24.3 28.3 5.4 38.3 43.7 Medical 10.6 16.7 27.3 18.9 22.1 41.0 Other 8.8 18.0 26.8 11.2 30.4 41.6 Total $ 221.3 $ 376.7 $ 598.0 $ 451.8 $ 566.9 $ 1,018.7 (in millions) Nine Months Ended September 30, 2020 September 30, 2019 HPMC AA&S Total HPMC AA&S Total Diversified Global Markets: Aerospace & Defense $ 782.9 $ 321.3 $ 1,104.2 $ 1,223.1 $ 378.2 $ 1,601.3 Energy* 71.3 397.2 468.5 97.7 501.4 599.1 Automotive 4.5 182.2 186.7 8.9 215.2 224.1 Food Equipment & Appliances 0.1 131.3 131.4 0.2 156.2 156.4 Electronics/Computers/Communications 0.7 117.9 118.6 0.4 116.3 116.7 Construction/Mining 14.3 92.4 106.7 37.3 116.9 154.2 Medical 37.9 57.9 95.8 64.7 64.8 129.5 Other 30.6 81.3 111.9 49.2 73.4 122.6 Total $ 942.3 $ 1,381.5 $ 2,323.8 $ 1,481.5 $ 1,622.4 $ 3,103.9 *Includes the oil & gas, hydrocarbon and chemical processing, and electrical energy markets. (in millions) Third quarter ended September 30, 2020 September 30, 2019 HPMC AA&S Total HPMC AA&S Total Primary Geographical Market: United States $ 129.3 $ 219.3 $ 348.6 $ 241.1 $ 364.3 $ 605.4 Asia 19.9 103.0 122.9 44.1 120.4 164.5 Europe 60.7 34.1 94.8 134.5 51.8 186.3 Canada 4.4 8.5 12.9 12.6 13.0 25.6 South America, Middle East and other 7.0 11.8 18.8 19.5 17.4 36.9 Total $ 221.3 $ 376.7 $ 598.0 $ 451.8 $ 566.9 $ 1,018.7 (in millions) Nine Months Ended September 30, 2020 September 30, 2019 HPMC AA&S Total HPMC AA&S Total Primary Geographical Market: United States $ 522.8 $ 909.1 $ 1,431.9 $ 800.3 $ 1,066.7 $ 1,867.0 Europe 286.2 104.1 390.3 454.0 150.5 604.5 Asia 72.6 288.3 360.9 136.2 323.4 459.6 Canada 21.1 30.0 51.1 38.9 42.5 81.4 South America, Middle East and other 39.6 50.0 89.6 52.1 39.3 91.4 Total $ 942.3 $ 1,381.5 $ 2,323.8 $ 1,481.5 $ 1,622.4 $ 3,103.9 Comparative information of the Company’s major high-value and standard products based on their percentages of sales is included in the following table. Hot-Rolling and Processing Facility (HRPF) conversion service sales in the AA&S segment are excluded from this presentation. Third quarter ended September 30, 2020 September 30, 2019 HPMC AA&S Total HPMC AA&S Total Diversified Products and Services: High-Value Products Nickel-based alloys and specialty alloys 38 % 22 % 28 % 38 % 29 % 33 % Precision rolled strip products — % 28 % 18 % — % 22 % 12 % Titanium and titanium-based alloys 32 % 8 % 17 % 27 % 13 % 19 % Precision forgings, castings and components 30 % — % 11 % 35 % — % 16 % Zirconium and related alloys — % 16 % 10 % — % 9 % 5 % Total High-Value Products 100 % 74 % 84 % 100 % 73 % 85 % Standard Products Standard stainless products — % 26 % 16 % — % 27 % 15 % Total 100 % 100 % 100 % 100 % 100 % 100 % Nine Months Ended September 30, 2020 September 30, 2019 HPMC AA&S Total HPMC AA&S Total Diversified Products and Services: High-Value Products Nickel-based alloys and specialty alloys 38 % 25 % 30 % 37 % 26 % 32 % Titanium and titanium-based alloys 27 % 12 % 18 % 25 % 11 % 18 % Precision forgings, castings and components 35 % — % 14 % 38 % — % 18 % Precision rolled strip products — % 21 % 13 % — % 22 % 11 % Zirconium and related alloys — % 13 % 9 % — % 12 % 6 % Total High-Value Products 100 % 71 % 84 % 100 % 71 % 85 % Standard Products Standard stainless products — % 29 % 16 % — % 29 % 15 % Total 100 % 100 % 100 % 100 % 100 % 100 %
Schedule of Accounts Receivable - Reserve for Doubtful Accounts	(in millions) Accounts Receivable - Reserve for Doubtful Accounts September 30, September 30, Balance as of beginning of fiscal year $ 4.6 $ 6.0 Expense to increase the reserve 0.1 0.3 Write-off of uncollectible accounts (0.4) (1.3) Balance as of period end $ 4.3 $ 5.0
Schedule of Contract Assets and Liabilities	(in millions) Contract Assets Short-term September 30, September 30, Balance as of beginning of fiscal year $ 38.5 $ 51.2 Recognized in current year 55.8 57.0 Reclassified to accounts receivable (52.7) (63.1) Impairment — — Reclassification to/from long-term 0.1 — Divestiture — (7.3) Balance as of period end $ 41.7 $ 37.8 Long-term September 30, September 30, Balance as of beginning of fiscal year $ 0.1 $ 0.1 Recognized in current year — — Reclassified to accounts receivable — — Impairment — — Reclassification to/from short-term (0.1) — Balance as of period end $ — $ 0.1 (in millions) Contract Liabilities Short-term September 30, September 30, Balance as of beginning of fiscal year $ 78.7 $ 71.4 Recognized in current year 121.5 104.2 Amounts in beginning balance reclassified to revenue (42.9) (43.9) Current year amounts reclassified to revenue (65.6) (34.8) Other — 0.2 Reclassification to/from long-term 6.1 — Balance as of period end $ 97.8 $ 97.1 Long-term September 30, September 30, Balance as of beginning of fiscal year $ 25.9 $ 7.3 Recognized in current year 12.9 0.8 Amounts in beginning balance reclassified to revenue (0.8) (0.8) Current year amounts reclassified to revenue — — Other — — Reclassification to/from short-term (6.1) — Balance as of period end $ 31.9 $ 7.3

Inventories (Tables)

Inventories (Tables)	9 Months Ended
	Sep. 30, 2020
Inventory Disclosure [Abstract]
Schedule of Inventories	Inventories at September 30, 2020 and December 31, 2019 were as follows (in millions): September 30, December 31, Raw materials and supplies $ 210.5 $ 164.9 Work-in-process 717.9 899.6 Finished goods 191.1 161.3 Total inventories at current cost 1,119.5 1,225.8 Adjustment from current cost to LIFO cost basis 41.1 33.6 Inventory valuation reserves (124.7) (104.1) Total inventories, net $ 1,035.9 $ 1,155.3
Schedule of Inventory Valuation Impact on Income	Impacts to cost of sales for changes in the LIFO costing methodology and associated NRV inventory reserves were as follows (in millions): Nine months ended September 30, 2020 2019 LIFO benefit (charge) $ 7.5 $ (1.4) NRV benefit (charge) (7.5) 1.3 Net cost of sales impact $ — $ (0.1)

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	9 Months Ended
	Sep. 30, 2020
Property, Plant and Equipment [Abstract]
Property Plant And Equipment	Property, plant and equipment at September 30, 2020 and December 31, 2019 was as follows (in millions): September 30, December 31, Land $ 35.0 $ 34.6 Buildings 884.2 832.7 Equipment and leasehold improvements 3,721.7 3,671.3 4,640.9 4,538.6 Accumulated depreciation and amortization (2,174.3) (2,088.5) Total property, plant and equipment, net $ 2,466.6 $ 2,450.1

Supplemental Financial Statem_2

Supplemental Financial Statement Information - (Tables)	9 Months Ended
	Sep. 30, 2020
Other Income and Expenses [Abstract]
Schedule of Other Expense, Net	Other income (expense), net for the three and nine months ended September 30, 2020 and 2019 was as follows: (in millions) Three months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 Rent and royalty income $ 0.1 $ 1.4 $ 0.6 $ 2.8 Gains from disposal of property, plant and equipment, net 0.8 62.3 3.3 90.6 Net equity loss on joint ventures (See Note 6) (1.0) (2.4) (6.6) (8.7) Joint venture restructuring charges (See Note 6) — — (2.4) — Net gain (loss) from sales of businesses — 5.8 — (1.9) Adjustment to indemnification for conditional ARO costs — — 4.3 — Other (0.3) 0.1 — 0.1 Total other income (expense), net $ (0.4) $ 67.2 $ (0.8) $ 82.9

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2020
Debt Disclosure [Abstract]
Schedule of Debt Instruments	Debt at September 30, 2020 and December 31, 2019 was as follows (in millions): September 30, December 31, Allegheny Technologies 5.875% Notes due 2023 (a) $ 500.0 $ 500.0 Allegheny Technologies 5.875% Notes due 2027 350.0 350.0 Allegheny Technologies 3.5% Convertible Senior Notes due 2025 291.4 — Allegheny Technologies 4.75% Convertible Senior Notes due 2022 84.2 287.5 Allegheny Ludlum 6.95% Debentures due 2025 150.0 150.0 Term Loan due 2024 200.0 100.0 U.S. revolving credit facility — — Foreign credit facilities 4.2 4.9 Finance leases and other 25.6 18.8 Debt issuance costs (15.2) (12.3) Equity component of convertible debt (49.0) — Total debt 1,541.2 1,398.9 Short-term debt and current portion of long-term debt 12.2 11.5 Total long-term debt $ 1,529.0 $ 1,387.4 (a) Bearing interest at 7.875% effective February 15, 2016.

Derivative Financial Instrume_2

Derivative Financial Instruments and Hedging (Tables)	9 Months Ended
	Sep. 30, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule Of Derivative Instruments In Statement Of Financial Position Fair Value	The fair values of the Company’s derivative financial instruments are presented below, representing the gross amounts recognized which are not offset by counterparty or by type of item hedged. All fair values for these derivatives were measured using Level 2 information as defined by the accounting standard hierarchy, which includes quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, and inputs derived principally from or corroborated by observable market data. (In millions) Asset derivatives Balance sheet location September 30, December 31, Derivatives designated as hedging instruments: Nickel and other raw material contracts Prepaid expenses and other current assets 1.8 4.4 Natural gas contracts Prepaid expenses and other current assets 1.4 — Nickel and other raw material contracts Other assets 0.4 1.2 Natural gas contracts Other assets 0.5 — Total derivatives designated as hedging instruments 4.1 5.6 Total asset derivatives $ 4.1 $ 5.6 Liability derivatives Balance sheet location Derivatives designated as hedging instruments: Interest rate swap Other current liabilities $ 1.1 $ 0.3 Natural gas contracts Other current liabilities 0.3 2.5 Nickel and other raw material contracts Other current liabilities 0.1 2.5 Interest rate swap Other long-term liabilities 2.8 1.2 Natural gas contracts Other long-term liabilities — 1.0 Total derivatives designated as hedging instruments 4.3 7.5 Total liability derivatives $ 4.3 $ 7.5
Schedule Of Derivative Instruments Gain Loss In Statement Of Financial Performance	Activity with regard to derivatives designated as cash flow hedges for the three and nine month periods ended September 30, 2020 and 2019 was as follows (in millions): Amount of Gain (Loss) Amount of Gain (Loss) Three months ended September 30, Three months ended September 30, Derivatives in Cash Flow Hedging Relationships 2020 2019 2020 2019 Nickel and other raw material contracts $ 2.1 $ 11.8 $ 0.7 $ 1.1 Natural gas contracts 2.5 (0.9) (0.8) (0.6) Foreign exchange contracts — 0.9 — 0.3 Interest rate swap 0.1 (1.2) (0.2) (0.1) Total $ 4.7 $ 10.6 $ (0.3) $ 0.7 Amount of Gain (Loss) Amount of Gain (Loss) Nine months ended September 30, Nine months ended September 30, Derivatives in Cash Flow Hedging Relationships 2020 2019 2020 2019 Nickel and other raw material contracts $ (2.1) $ 18.2 $ (1.3) $ 0.5 Natural gas contracts 1.2 (3.2) (2.8) (0.6) Foreign exchange contracts (0.1) 1.0 (0.1) — Interest rate swap (2.1) (1.7) (0.7) (0.3) Total $ (3.1) $ 14.3 $ (4.9) $ (0.4)

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2020
Fair Value Disclosures [Abstract]
Fair Value By Balance Sheet Grouping	The estimated fair value of financial instruments at September 30, 2020 was as follows: Fair Value Measurements at Reporting Date Using (In millions) Total Total Quoted Prices in Significant Cash and cash equivalents $ 572.2 $ 572.2 $ 572.2 $ — Derivative financial instruments: Assets 4.1 4.1 — 4.1 Liabilities 4.3 4.3 — 4.3 Debt (a) 1,605.4 1,597.7 1,367.9 229.8 The estimated fair value of financial instruments at December 31, 2019 was as follows: Fair Value Measurements at Reporting Date Using (In millions) Total Total Quoted Prices in Significant Cash and cash equivalents $ 490.8 $ 490.8 $ 490.8 $ — Derivative financial instruments: Assets 5.6 5.6 — 5.6 Liabilities 7.5 7.5 — 7.5 Debt (a) 1,411.2 1,676.5 1,552.8 123.7 (a) The total carrying amount for debt for both periods excludes debt issuance costs related to the recognized debt liability which is presented in the consolidated balance sheet as a direct reduction from the carrying amount of the debt liability. The September 30, 2020 debt carrying value includes $49.0 million for the unamortized balance of the portion of the 2025 Convertible Notes recorded in stockholders’ equity due to the flexible settlement feature of the notes (see Note 8).

Retirement Benefits (Tables)

Retirement Benefits (Tables)	9 Months Ended
	Sep. 30, 2020
Retirement Benefits [Abstract]
Schedule Of Defined Benefit Plans Disclosures [Table Text Block]	For the three month periods ended September 30, 2020 and 2019, the components of pension and other postretirement benefit expense for the Company’s defined benefit plans included the following (in millions): Pension Benefits Other Postretirement Benefits Three months ended September 30, Three months ended September 30, 2020 2019 2020 2019 Service cost - benefits earned during the year $ 3.2 $ 3.2 $ 0.6 $ 0.5 Interest cost on benefits earned in prior years 21.6 26.4 2.6 3.7 Expected return on plan assets (33.6) (32.8) — — Amortization of prior service cost (credit) 0.1 — (0.9) (0.7) Amortization of net actuarial loss 18.6 18.5 2.7 3.4 Total retirement benefit expense $ 9.9 $ 15.3 $ 5.0 $ 6.9 For the nine month periods ended September 30, 2020 and 2019, the components of pension and other postretirement benefit expense for the Company’s defined benefit plans included the following (in millions): Pension Benefits Other Postretirement Benefits Nine months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 Service cost - benefits earned during the year $ 9.5 $ 9.5 $ 1.7 $ 1.4 Interest cost on benefits earned in prior years 64.7 79.1 8.0 11.1 Expected return on plan assets (100.8) (98.5) — — Amortization of prior service cost (credit) 0.5 0.2 (2.8) (2.1) Amortization of net actuarial loss 55.8 55.3 8.1 10.1 Total retirement benefit expense $ 29.7 $ 45.6 $ 15.0 $ 20.5

Business Segments (Tables)

Business Segments (Tables)	9 Months Ended
	Sep. 30, 2020
Segment Reporting [Abstract]
Schedule Of Segment Reporting Information By Segment	Following is certain financial information with respect to the Company’s business segments for the periods indicated (in millions): Three months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 Total sales: High Performance Materials & Components $ 230.1 $ 471.6 $ 995.3 $ 1,540.7 Advanced Alloys & Solutions 393.3 626.2 1,487.1 1,811.2 623.4 1,097.8 2,482.4 3,351.9 Intersegment sales: High Performance Materials & Components 8.8 19.8 53.0 59.2 Advanced Alloys & Solutions 16.6 59.3 105.6 188.8 25.4 79.1 158.6 248.0 Sales to external customers: High Performance Materials & Components 221.3 451.8 942.3 1,481.5 Advanced Alloys & Solutions 376.7 566.9 1,381.5 1,622.4 $ 598.0 $ 1,018.7 $ 2,323.8 $ 3,103.9 Three months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 Segment operating profit (loss): High Performance Materials & Components $ (2.7) $ 68.4 $ 63.6 $ 198.5 Advanced Alloys & Solutions (4.1) 30.8 38.1 76.9 Total segment operating profit (loss) (6.8) 99.2 101.7 275.4 LIFO and net realizable value reserves — — — (0.1) Corporate expenses (10.9) (16.0) (31.4) (50.6) Closed operations and other expense (1.1) (8.2) (5.3) (19.2) Restructuring and other charges (See Note 14) (2.3) — (27.0) — Impairment of goodwill (See Note 5) — — (287.0) — Joint venture restructuring charges (See Note 6) — — (2.4) — Gain on asset sales, net — 68.2 2.5 89.8 Debt extinguishment charge (See Note 8) — — (21.5) — Interest expense, net (25.1) (24.2) (68.7) (74.9) Income (loss) before income taxes $ (46.2) $ 119.0 $ (339.1) $ 220.4

Restructuring (Tables)

Restructuring (Tables)	9 Months Ended
	Sep. 30, 2020
Restructuring and Related Activities [Abstract]
Roll Forward of Restructuring Reserve Activity	Restructuring reserves activity is as follows: Severance and Employee Benefit Costs Balance at December 31, 2019 $ 4.5 Additions 27.0 Payments (11.4) Balance at September 30, 2020 $ 20.1

Per Share Information (Tables)

Per Share Information (Tables)	9 Months Ended
	Sep. 30, 2020
Earnings Per Share [Abstract]
Schedule Of Earnings Per Share Diluted By Common Class	The following table sets forth the computation of basic and diluted income (loss) per common share: Three months ended Nine months ended (In millions, except per share amounts) September 30, September 30, 2020 2019 2020 2019 Numerator: Numerator for basic income (loss) per common share – Net income (loss) attributable to ATI $ (50.1) $ 111.0 $ (451.6) $ 201.1 Effect of dilutive securities: 4.75% Convertible Senior Notes due 2022 — 3.2 — 9.7 3.5% Convertible Senior Notes due 2025 — — — — Numerator for diluted income (loss) per common share – Net income (loss) attributable to ATI after assumed conversions $ (50.1) $ 114.2 $ (451.6) $ 210.8 Denominator: Denominator for basic net income (loss) per common share – weighted average shares 126.6 125.9 126.5 125.7 Effect of dilutive securities: Share-based compensation — 0.9 — 0.8 4.75% Convertible Senior Notes due 2022 — 19.9 — 19.9 3.5% Convertible Senior Notes due 2025 — — — — Denominator for diluted net income (loss) per common share – adjusted weighted average shares and assumed conversions 126.6 146.7 126.5 146.4 Basic net income (loss) attributable to ATI per common share $ (0.40) $ 0.88 $ (3.57) $ 1.60 Diluted net income (loss) attributable to ATI per common share $ (0.40) $ 0.78 $ (3.57) $ 1.44

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Income (Loss) (Tables)	9 Months Ended
	Sep. 30, 2020
Equity [Abstract]
Schedule Of Accumulated Other Comprehensive Income Loss	The changes in accumulated other comprehensive income (loss) (AOCI) by component, net of tax, for the three month period ended September 30, 2020 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, June 30, 2020 $ (1,051.8) $ (84.5) $ (3.7) $ (32.8) $ (1,172.8) OCI before reclassifications — 17.2 4.7 — 21.9 Amounts reclassified from AOCI (a) 15.6 (b) — (c) 0.3 (d) 6.4 22.3 Net current-period OCI 15.6 17.2 5.0 6.4 44.2 Balance, September 30, 2020 $ (1,036.2) $ (67.3) $ 1.3 $ (26.4) $ (1,128.6) Attributable to noncontrolling interests: Balance, June 30, 2020 $ — $ 8.5 $ — $ — $ 8.5 OCI before reclassifications — 6.0 — — 6.0 Amounts reclassified from AOCI — (b) — — — — Net current-period OCI — 6.0 — — 6.0 Balance, September 30, 2020 $ — $ 14.5 $ — $ — $ 14.5 The changes in AOCI by component, net of tax, for the nine month period ended September 30, 2020 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, December 31, 2019 $ (1,083.1) $ (76.6) $ (0.5) $ (41.5) $ (1,201.7) OCI before reclassifications — 9.3 (3.1) — 6.2 Amounts reclassified from AOCI (a) 46.9 (b) — (c) 4.9 (d) 15.1 66.9 Net current-period OCI 46.9 9.3 1.8 15.1 73.1 Balance, September 30, 2020 $ (1,036.2) $ (67.3) $ 1.3 $ (26.4) $ (1,128.6) Attributable to noncontrolling interests: Balance, December 31, 2019 $ — $ 9.8 $ — $ — $ 9.8 OCI before reclassifications — 4.7 — — 4.7 Amounts reclassified from AOCI — (b) — — — — Net current-period OCI — 4.7 — — 4.7 Balance, September 30, 2020 $ — $ 14.5 $ — $ — $ 14.5 (a) Amounts were included in net periodic benefit cost for pension and other postretirement benefit plans (see Note 11). (b) No amounts were reclassified to earnings. (c) Amounts related to derivatives are included in cost of goods sold or interest expense in the period or periods the hedged item affects earnings (see Note 9). (d) Represents the net change in deferred tax asset valuation allowances on changes in AOCI balances between the balance sheet dates. The changes in AOCI by component, net of tax, for the three month period ended September 30, 2019 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, June 30, 2019 $ (973.4) $ (73.0) $ — $ (38.0) $ (1,084.4) OCI before reclassifications — (11.7) 10.6 — (1.1) Amounts reclassified from AOCI (a) 16.1 (b) — (c) (0.7) (d) 8.7 24.1 Net current-period OCI 16.1 (11.7) 9.9 8.7 23.0 Balance, September 30, 2019 $ (957.3) $ (84.7) $ 9.9 $ (29.3) $ (1,061.4) Attributable to noncontrolling interests: Balance, June 30, 2019 $ — $ 12.7 $ — $ — $ 12.7 OCI before reclassifications — (4.6) — — (4.6) Amounts reclassified from AOCI — (b) — — — — Net current-period OCI — (4.6) — — $ (4.6) Balance, September 30, 2019 $ — $ 8.1 $ — $ — $ 8.1 The changes in AOCI by component, net of tax, for the nine month period ended September 30, 2019 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, December 31, 2018 $ (1,005.8) $ (73.9) $ (4.8) $ (49.3) $ (1,133.8) OCI before reclassifications — (10.8) 14.3 — 3.5 Amounts reclassified from AOCI (a) 48.5 (b) — (c) 0.4 (d) 20.0 68.9 Net current-period OCI 48.5 (10.8) 14.7 20.0 72.4 Balance, September 30, 2019 $ (957.3) $ (84.7) $ 9.9 $ (29.3) $ (1,061.4) Attributable to noncontrolling interests: Balance, December 31, 2018 $ — $ 11.1 $ — $ — $ 11.1 OCI before reclassifications — (3.0) — — (3.0) Amounts reclassified from AOCI — (b) — — — — Net current-period OCI — (3.0) — — $ (3.0) Balance, September 30, 2019 $ — $ 8.1 $ — $ — $ 8.1 (a) Amounts were included in net periodic benefit cost for pension and other postretirement benefit plans (see Note 11). (b) No amounts were reclassified to earnings. (c) Amounts related to derivatives are included in cost of goods sold or interest expense in the period or periods the hedged item affects earnings (see Note 9).
Reclassification out of Accumulated Other Comprehensive Income	Reclassifications out of AOCI for the three and nine month periods ended September 30, 2020 and 2019 were as follows: Amount reclassified from AOCI Details about AOCI Components (In millions) Three months ended September 30, 2020 Three months ended September 30, 2019 Nine months ended September 30, 2020 Nine months ended September 30, 2019 Affected line item in the Postretirement benefit plans Prior service credit $ 0.8 $ 0.7 $ 2.3 $ 1.9 (a) Actuarial losses (21.3) (21.9) (63.9) (65.4) (a) (20.5) (21.2) (61.6) (63.5) (c) Total before tax (4.9) (5.1) (14.7) (15.0) Tax benefit (d) $ (15.6) $ (16.1) $ (46.9) $ (48.5) Net of tax Derivatives Nickel and other raw material contracts $ 0.9 $ 1.5 $ (1.7) $ 0.7 (b) Natural gas contracts (1.1) (0.8) (3.7) (0.8) (b) Foreign exchange contracts — 0.4 (0.1) — (b) Interest rate swap (0.2) (0.3) (0.9) (0.5) (b) (0.4) 0.8 (6.4) (0.6) (c) Total before tax (0.1) 0.1 (1.5) (0.2) Tax (benefit) expense (d) $ (0.3) $ 0.7 $ (4.9) $ (0.4) Net of tax (a) Amounts are reported in nonoperating retirement benefit expense (see Note 11). (b) Amounts related to derivatives, with the exception of the interest rate swap, are included in cost of goods sold in the period or periods the hedged item affects earnings. Amounts related to the interest rate swap are included in interest expense in the same period as the interest expense on the Term Loan is recognized in earnings (see Note 8). (c) For pretax items, positive amounts are income and negative amounts are expense in terms of the impact to net income. Tax effects are presented in conformity with ATI’s presentation in the consolidated statements of operations. (d) These amounts exclude the impact of any deferred tax asset valuation allowances, when applicable.

Accounting Policies (Accounting

Accounting Policies (Accounting Pronouncements) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2020USD ($)segment	Sep. 30, 2019USD ($)	Jan. 01, 2021USD ($)	Jun. 30, 2020USD ($)	Dec. 31, 2019USD ($)employee	Jun. 30, 2019USD ($)	Dec. 31, 2018USD ($)
Revenue recognition [Line Items]
Number of business segments | segment			2
Impact of new accounting principle on prior period operating income	$ (9.6)	$ 94.5	$ (214.6)	$ 267.6
Unfavorable impact on pre-tax operating results	46.2	(119)	339.1	(220.4)
Entity Number Of Employees Covered By USW Collective Bargaining Agreements Expiring February 29 2020 | employee							1,500
Equity component of convertible note	1.1		49.8
Equity adjustment for adoption	1,849.8	2,279.9	1,849.8	2,279.9		$ 1,842.3	$ 2,193.2	$ 2,138.6	$ 1,991.6
[AdditionalPaidInCapitalMember]
Revenue recognition [Line Items]
Equity component of convertible note	1.1		49.8
Equity adjustment for adoption	1,622.2	1,612.1	1,622.2	1,612.1		1,617.8	1,618	1,604.6	1,615.4
[RetainedEarningsMember]
Revenue recognition [Line Items]
Equity adjustment for adoption	$ 1,227.4	$ 1,622.8	$ 1,227.4	$ 1,622.8		$ 1,277.6	$ 1,679.3	$ 1,511.8	$ 1,422
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt
Revenue recognition [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.50%		3.50%
Forecast | Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt | [RetainedEarningsMember] | Cumulative Effect, Period of Adoption, Adjustment
Revenue recognition [Line Items]
Equity adjustment for adoption					$ (4.6)
Allegheny & Tsingshan Stainless
Revenue recognition [Line Items]
Equity method investment ownership percentage	50.00%		50.00%
Uniti
Revenue recognition [Line Items]
Equity method investment ownership percentage	50.00%		50.00%
Allegheny Technologies Inc | Shanghai STAL Precision Stainless Steel Co Ltd | Advanced Alloys & Solutions [Member]
Revenue recognition [Line Items]
Joint venture ownership percentage	60.00%		60.00%

Revenue from Contracts with C_3

Revenue from Contracts with Customers - Narrative (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2020USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2020USD ($)segment	Sep. 30, 2019USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Revenue recognition [Line Items]
Inventories, net	$ 1,035,900,000		$ 1,035,900,000		$ 1,155,300,000
Adjustment from current cost to LIFO cost basis	(41,100,000)		(41,100,000)		(33,600,000)
Sales	598,000,000	$ 1,018,700,000	2,323,800,000	$ 3,103,900,000
Cost of sales	559,900,000	859,000,000	2,076,200,000	2,635,400,000
Short-term contract assets	41,700,000	37,800,000	41,700,000	37,800,000	38,500,000	$ 51,200,000
Long-term contract assets	0	100,000	0	100,000	100,000	100,000
Other current liabilities	213,000,000		213,000,000		237,800,000
Other long-term liabilities	212,300,000		212,300,000		160,800,000
Short-term contract liabilities	97,800,000	97,100,000	97,800,000	97,100,000	78,700,000	71,400,000
Long-term contract liabilities	31,900,000	7,300,000	$ 31,900,000	7,300,000	25,900,000	$ 7,300,000
Number of business segments | segment			2
Revenue, Performance Obligation [Abstract]
Confirmed order backlog	1,440,000,000	2,340,000,000	$ 1,440,000,000	2,340,000,000
Confirmed orders with current performance obligations	0.70		0.70
Accounts receivable with customers	392,300,000		392,300,000		558,700,000
Contract costs for obtaining and fulfilling contracts	5,600,000		5,600,000		$ 6,500,000
Amortization of contract costs	$ 400,000	$ 400,000	$ 1,100,000	$ 1,000,000

Revenue from Contracts with C_4

Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue		$ 598	$ 1,018.7	$ 2,323.8	$ 3,103.9
Percent of revenue		100.00%	100.00%	100.00%	100.00%
Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of revenue		84.00%	85.00%	84.00%	85.00%
Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		28.00%	33.00%	30.00%	32.00%
Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue		11.00%	16.00%	14.00%	18.00%
Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		17.00%	19.00%	18.00%	18.00%
Precision rolled strip products
Disaggregation of Revenue [Line Items]
Percent of revenue		18.00%	12.00%	13.00%	11.00%
Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		10.00%	5.00%	9.00%	6.00%
Standard stainless products [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue		16.00%	15.00%	16.00%	15.00%
United States
Disaggregation of Revenue [Line Items]
Revenue		$ 348.6	$ 605.4	$ 1,431.9	$ 1,867
Europe
Disaggregation of Revenue [Line Items]
Revenue		94.8	186.3	390.3	604.5
Asia
Disaggregation of Revenue [Line Items]
Revenue		122.9	164.5	360.9	459.6
Canada
Disaggregation of Revenue [Line Items]
Revenue		12.9	25.6	51.1	81.4
South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		18.8	36.9	89.6	91.4
Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		246.1	503.5	1,104.2	1,601.3
Energy*
Disaggregation of Revenue [Line Items]
Revenue	[1]	130.4	217.2	468.5	599.1
Automotive
Disaggregation of Revenue [Line Items]
Revenue		59.4	73.6	186.7	224.1
Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue		45.9	44.5	118.6	116.7
Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		28.3	43.7	106.7	154.2
Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue		33.8	53.6	131.4	156.4
Medical
Disaggregation of Revenue [Line Items]
Revenue		27.3	41	95.8	129.5
Other
Disaggregation of Revenue [Line Items]
Revenue		26.8	41.6	111.9	122.6
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		623.4	1,097.8	2,482.4	3,351.9
Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue		$ 230.1	$ 471.6	$ 995.3	$ 1,540.7
Percent of revenue		100.00%	100.00%	100.00%	100.00%
Operating Segments | High Performance Materials & Components | Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of revenue		100.00%	100.00%	100.00%	100.00%
Operating Segments | High Performance Materials & Components | Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		38.00%	38.00%	38.00%	37.00%
Operating Segments | High Performance Materials & Components | Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue		30.00%	35.00%	35.00%	38.00%
Operating Segments | High Performance Materials & Components | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		32.00%	27.00%	27.00%	25.00%
Operating Segments | High Performance Materials & Components | Precision rolled strip products
Disaggregation of Revenue [Line Items]
Percent of revenue		0.00%	0.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		0.00%	0.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | Standard stainless products [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue		0.00%	0.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | United States
Disaggregation of Revenue [Line Items]
Revenue		$ 129.3	$ 241.1	$ 522.8	$ 800.3
Operating Segments | High Performance Materials & Components | Europe
Disaggregation of Revenue [Line Items]
Revenue		60.7	134.5	286.2	454
Operating Segments | High Performance Materials & Components | Asia
Disaggregation of Revenue [Line Items]
Revenue		19.9	44.1	72.6	136.2
Operating Segments | High Performance Materials & Components | Canada
Disaggregation of Revenue [Line Items]
Revenue		4.4	12.6	21.1	38.9
Operating Segments | High Performance Materials & Components | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		7	19.5	39.6	52.1
Operating Segments | High Performance Materials & Components | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		171.8	377.7	782.9	1,223.1
Operating Segments | High Performance Materials & Components | Energy*
Disaggregation of Revenue [Line Items]
Revenue	[1]	24.4	36.2	71.3	97.7
Operating Segments | High Performance Materials & Components | Automotive
Disaggregation of Revenue [Line Items]
Revenue		1.5	2.2	4.5	8.9
Operating Segments | High Performance Materials & Components | Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue		0.2	0.1	0.7	0.4
Operating Segments | High Performance Materials & Components | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		4	5.4	14.3	37.3
Operating Segments | High Performance Materials & Components | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue		0	0.1	0.1	0.2
Operating Segments | High Performance Materials & Components | Medical
Disaggregation of Revenue [Line Items]
Revenue		10.6	18.9	37.9	64.7
Operating Segments | High Performance Materials & Components | Other
Disaggregation of Revenue [Line Items]
Revenue		8.8	11.2	30.6	49.2
Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue		$ 393.3	$ 626.2	$ 1,487.1	$ 1,811.2
Percent of revenue		100.00%	100.00%	100.00%	100.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of revenue		74.00%	73.00%	71.00%	71.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		22.00%	29.00%	25.00%	26.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue		0.00%	0.00%	0.00%	0.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		8.00%	13.00%	12.00%	11.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Precision rolled strip products
Disaggregation of Revenue [Line Items]
Percent of revenue		28.00%	22.00%	21.00%	22.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		16.00%	9.00%	13.00%	12.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Standard stainless products [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue		26.00%	27.00%	29.00%	29.00%
Operating Segments | Advanced Alloys & Solutions [Member] | United States
Disaggregation of Revenue [Line Items]
Revenue		$ 219.3	$ 364.3	$ 909.1	$ 1,066.7
Operating Segments | Advanced Alloys & Solutions [Member] | Europe
Disaggregation of Revenue [Line Items]
Revenue		34.1	51.8	104.1	150.5
Operating Segments | Advanced Alloys & Solutions [Member] | Asia
Disaggregation of Revenue [Line Items]
Revenue		103	120.4	288.3	323.4
Operating Segments | Advanced Alloys & Solutions [Member] | Canada
Disaggregation of Revenue [Line Items]
Revenue		8.5	13	30	42.5
Operating Segments | Advanced Alloys & Solutions [Member] | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		11.8	17.4	50	39.3
Operating Segments | Advanced Alloys & Solutions [Member] | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		74.3	125.8	321.3	378.2
Operating Segments | Advanced Alloys & Solutions [Member] | Energy*
Disaggregation of Revenue [Line Items]
Revenue	[1]	106	181	397.2	501.4
Operating Segments | Advanced Alloys & Solutions [Member] | Automotive
Disaggregation of Revenue [Line Items]
Revenue		57.9	71.4	182.2	215.2
Operating Segments | Advanced Alloys & Solutions [Member] | Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue		45.7	44.4	117.9	116.3
Operating Segments | Advanced Alloys & Solutions [Member] | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		24.3	38.3	92.4	116.9
Operating Segments | Advanced Alloys & Solutions [Member] | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue		33.8	53.5	131.3	156.2
Operating Segments | Advanced Alloys & Solutions [Member] | Medical
Disaggregation of Revenue [Line Items]
Revenue		16.7	22.1	57.9	64.8
Operating Segments | Advanced Alloys & Solutions [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue		18	30.4	81.3	73.4
External Customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		598	1,018.7	2,323.8	3,103.9
External Customers | Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue		221.3	451.8	942.3	1,481.5
External Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue		$ 376.7	$ 566.9	$ 1,381.5	$ 1,622.4
[1]	Includes the oil & gas, hydrocarbon and chemical processing, and electrical energy markets.

Revenue from Contracts with C_5

Revenue from Contracts with Customers - Accounts Receivable Reserve for Doubtful Accounts (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of beginning of fiscal year	$ 4.6	$ 6
Expense to increase the reserve	0.1	0.3
Write-off of uncollectible accounts	(0.4)	(1.3)
Balance as of period end	$ 4.3	$ 5

Revenue from Contracts with C_6

Revenue from Contracts with Customers - Contract Assets and Liabilities (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Contract Assets, Current [Roll Forward]
Balance as of beginning of fiscal year	$ 38.5	$ 51.2
Recognized in current year	55.8	57
Reclassified to accounts receivable	(52.7)	(63.1)
Impairment	0	0
Reclassification to/from long-term	0.1	0
Divestiture	0	(7.3)
Balance as of period end	41.7	37.8
Contract Assets, Noncurrent [Roll Forward]
Balance as of beginning of fiscal year	0.1	0.1
Recognized in current year	0	0
Reclassified to accounts receivable	0	0
Impairment	0	0
Reclassification to/from short-term	(0.1)	0
Balance as of period end	0	0.1
Contract Liabilities, Current [Roll Forward]
Balance as of beginning of fiscal year	78.7	71.4
Recognized in current year	121.5	104.2
Amounts in beginning balance reclassified to revenue	(42.9)	(43.9)
Current year amounts reclassified to revenue	(65.6)	(34.8)
Other	0	0.2
Reclassification to/from long-term	6.1	0
Balance as of period end	97.8	97.1
Contract Liabilities, Noncurrent [Roll Forward]
Balance as of beginning of fiscal year	25.9	7.3
Recognized in current year	12.9	0.8
Amounts in beginning balance reclassified to revenue	(0.8)	(0.8)
Current year amounts reclassified to revenue	0	0
Other	0	0
Reclassification to/from short-term	(6.1)	0
Balance as of period end	$ 31.9	$ 7.3

Inventories (Details)

Inventories (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Inventory [Line Items]
Raw materials and supplies	$ 210.5		$ 210.5		$ 164.9
Work-in-process	717.9		717.9		899.6
Finished goods	191.1		191.1		161.3
Total inventories at current cost	1,119.5		1,119.5		1,225.8
Adjustment from current cost to LIFO cost basis	41.1		41.1		33.6
Inventory valuation reserves	(124.7)		(124.7)		(104.1)
Total inventories, net	1,035.9		1,035.9		1,155.3
LIFO benefit (charge)			7.5	$ (1.4)
NRV benefit (charge)			(7.5)	1.3
Net cost of sales impact	0	$ 0	0	$ (0.1)
Net Realizable Value Reserve [Member]
Inventory [Line Items]
Inventory valuation reserves	$ (41.1)		$ (41.1)		$ (33.6)

Property, Plant and Equipment_2

Property, Plant and Equipment (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Land	$ 35	$ 34.6
Buildings	884.2	832.7
Equipment and leasehold improvements	3,721.7	3,671.3
Property Plant And Equipment, gross	4,640.9	4,538.6
Accumulated depreciation and amortization	(2,174.3)	(2,088.5)
Total property, plant and equipment, net	2,466.6	$ 2,450.1
Construction in progress	$ 223.1

Goodwill Impairment - Narrative

Goodwill Impairment - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Mar. 31, 2020	Dec. 31, 2019
Goodwill [Line Items]
Goodwill	$ 238.4		$ 238.4			$ 525.8
Decrease of goodwill during period			(287.4)
Impairment of goodwill	0	$ 0	287	$ 0
Goodwill, foreign currency translation loss			(0.4)
Accumulated goodwill impairment loss	528		528			$ 241
Forged Products
Goodwill [Line Items]
Goodwill	173.4		173.4		$ 460.4
Reporting unit, percentage of fair value in excess of carrying amount						30.00%
Specialty Materials
Goodwill [Line Items]
Goodwill	$ 65		$ 65

Joint Ventures (Details)

Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended		10 Months Ended	34 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 572.2		$ 490.8		$ 572.2
Income (loss) from equity investments	(1)			$ (2.4)	(6.6)	$ (8.7)
Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 3.2				$ 3.2
Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%				50.00%
Sales to noncontrolling interests							$ 12
Charges for termination benefits		$ 4.8
Impairment of Long-Lived Assets Held-for-use			14.2
Due From Affiliates, Allowance For Credit Loss			4.3
Allegheny & Tsingshan Stainless | Employee Severance [Member]
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	$ 0	$ (2.4)		0	$ (2.4)	0
Uniti
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%				50.00%
Tsingshan Group | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from Affiliates	$ 5.5				$ 5.5
Advanced Alloys & Solutions [Member] | Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	48.9				48.9
Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	(1.6)			(2.6)	(10.1)	(9.9)
Due from Affiliates	14.5		32.5		14.5
Advanced Alloys & Solutions [Member] | Uniti
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	0.6			$ 0.2	1.1	$ 1.2
Forecast | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Sales to noncontrolling interests								$ 17.5
Property, Plant and Equipment [Member] | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments			(7.1)
Prepaid expenses and other current assets | Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from Affiliates	0.5		8.3		0.5
Other Noncurrent Assets [Member] | Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from Affiliates	$ 14		$ 24.2		$ 14
Allegheny Technologies Inc | Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	51.00%				51.00%
Allegheny Technologies Inc | Advanced Alloys & Solutions [Member] | Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	60.00%				60.00%
China Baowu Steel Group Corporation Limited | Advanced Alloys & Solutions [Member] | China Baowu Steel Group Corporation Limited | Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	40.00%				40.00%
Tsingshan Group | Tsingshan Group | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50.00%				50.00%
VSMPO | VSMPO | Uniti
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50.00%				50.00%

Supplemental Financial Statem_3

Supplemental Financial Statement Information - Schedule of Other Income, Net (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Other Income and Expenses [Abstract]
Rent and royalty income	$ 0.1		$ 1.4	$ 0.6	$ 2.8
Gains from disposal of property, plant and equipment, net	0.8		62.3	3.3	90.6
Net equity (loss) gain on joint ventures	(1)		(2.4)	(6.6)	(8.7)
Loss from sale of businesses	0		5.8	0	(1.9)
Adjustment to indemnification for conditional ARO costs	0		0	4.3	0
Other	(0.3)		0.1	0	0.1
Other income (expense), net	(0.4)		67.2	(0.8)	82.9
Allegheny & Tsingshan Stainless | Employee Severance [Member]
Other Income and Expenses [Abstract]
Net equity (loss) gain on joint ventures	$ 0	$ (2.4)	$ 0	$ (2.4)	$ 0

Supplemental Financial Statem_4

Supplemental Financial Statement Information - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Other Income and Expenses [Abstract]
Gain on the sale of oil and gas rights	$ 62.4	$ 2.5	$ 91.7

Debt Schedule of Debt (Details)

Debt Schedule of Debt (Details) - USD ($)		9 Months Ended	12 Months Ended
		Sep. 30, 2020	Dec. 31, 2019	Jul. 02, 2020	Jun. 30, 2020	Feb. 15, 2016	Dec. 31, 2013
Debt Instrument [Line Items]
Debt Issuance Costs, Net		$ (15,200,000)	$ (12,300,000)
Total Debt		1,541,200,000	1,398,900,000
Short-term debt and current portion of long-term debt		12,200,000	11,500,000
Long-term debt		1,529,000,000	1,387,400,000
Allegheny Technologies 5.875% Notes due 2023 (a)
Debt Instrument [Line Items]
Debt, Gross	[1]	$ 500,000,000	$ 500,000,000
Debt Instrument, Issuer		Allegheny Technologies
Interest rate		7.875%				7.875%	5.875%
Debt Instrument, Maturity Date		Aug. 15, 2023	Aug. 15, 2023
Allegheny Technologies 5.875% Notes due 2027
Debt Instrument [Line Items]
Debt, Gross		$ 350,000,000	$ 350,000,000
Debt Instrument, Issuer		Allegheny Technologies
Interest rate		5.875%
Debt Instrument, Maturity Date		Dec. 1, 2027	Dec. 1, 2027
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt
Debt Instrument [Line Items]
Debt, Gross		$ 291,400,000	$ 0	$ 6,400,000	$ 285,000,000
Equity component of convertible debt		$ (49,000,000)	0	$ (51,400,000)
Debt Instrument, Issuer		Allegheny Technologies
Interest rate		3.50%
Debt Instrument, Maturity Date		Jun. 15, 2025
Allegheny Technologies 4.75% Convertible Senior Notes due 2022
Debt Instrument [Line Items]
Debt, Gross		$ 84,200,000	$ 287,500,000
Debt Instrument, Issuer		Allegheny Technologies
Interest rate		4.75%
Debt Instrument, Maturity Date		Jul. 1, 2022	Jul. 1, 2022
Allegheny Ludlum 6.95% Debentures due 2025
Debt Instrument [Line Items]
Debt, Gross		$ 150,000,000	$ 150,000,000
Debt Instrument, Issuer		Allegheny Ludlum
Interest rate		6.95%
Debt Instrument, Maturity Date		Dec. 15, 2025	Dec. 15, 2025
2024 Term Loan
Debt Instrument [Line Items]
Debt, Gross		$ 200,000,000	$ 100,000,000
Debt Instrument, Issuer		Allegheny Technologies
Interest rate		4.21%
Debt Instrument, Maturity Date		Sep. 30, 2024	Sep. 30, 2024
Domestic Bank Group $500 million asset-based credit facility
Debt Instrument [Line Items]
Short-term Debt		$ 0	$ 0
Foreign credit facilities
Debt Instrument [Line Items]
Short-term Debt		4,200,000	4,900,000
Other
Debt Instrument [Line Items]
Debt, Gross		$ 25,600,000	$ 18,800,000
[1]	Bearing interest at 7.875% effective February 15, 2016.

Debt - Revolving Credit Facilit

Debt - Revolving Credit Facility Narrative (Details) - USD ($)		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2024	Dec. 31, 2019	Feb. 15, 2016	Dec. 31, 2013
Allegheny Technologies 5.875% Notes due 2023 (a)
Debt Instrument [Line Items]
Outstanding borrowings	[1]	$ 500,000,000			$ 500,000,000
Interest rate		7.875%				7.875%	5.875%
Domestic Bank Group $500 million asset-based credit facility
Debt Instrument [Line Items]
Amount utilized to support the issuance of letters of credit		$ 34,800,000
Average borrowings during period		$ 38,000,000	$ 0
Interest rate during period		2.20%
Short-term Debt		$ 0			0
Domestic Bank Group $500 million asset-based credit facility | Revolving credit facility
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 500,000,000
Minimum fixed charge coverage ratio allowed in event of default		1
Borrowing capacity unavailable due to fixed charge coverage ratio covenant, maximum		$ 87,500,000
Percentage of borrowing capacity unavailable due to fixed charge coverage ratio covenant		12.50%
Borrowing capacity unavailable due to fixed charge coverage ratio covenant, minimum		$ 62,500,000
Minimum required liquidity number of days prior to maturity of Senior Notes		90 days
Debt covenant, undrawn availability requirement, amount		$ 150,000,000
Debt covenant, undrawn availability requirement, percent of total available liquidity		30.00%
Domestic Bank Group $500 million asset-based credit facility | Revolving credit facility | Minimum | LIBOR based borrowings
Debt Instrument [Line Items]
Variable interest rate (percentage)		1.25%
Domestic Bank Group $500 million asset-based credit facility | Revolving credit facility | Minimum | Base rate borrowings
Debt Instrument [Line Items]
Variable interest rate (percentage)		0.25%
Domestic Bank Group $500 million asset-based credit facility | Revolving credit facility | Maximum | LIBOR based borrowings
Debt Instrument [Line Items]
Variable interest rate (percentage)		1.75%
Domestic Bank Group $500 million asset-based credit facility | Revolving credit facility | Maximum | Base rate borrowings
Debt Instrument [Line Items]
Variable interest rate (percentage)		0.75%
Domestic Bank Group $500 million asset-based credit facility | Letter of credit sub-facility
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 200,000,000
2024 Term Loan
Debt Instrument [Line Items]
Outstanding borrowings		200,000,000			100,000,000
Minimum prepayment increments allowed		$ 25,000,000
Interest rate		4.21%
2024 Term Loan | Interest Rate Swap
Debt Instrument [Line Items]
Notional amount of derivative		$ 50,000,000
2024 Term Loan | LIBOR based borrowings
Debt Instrument [Line Items]
Variable interest rate (percentage)		2.00%
Delayed-Draw Term Loan
Debt Instrument [Line Items]
Outstanding borrowings		$ 100,000,000
Allegheny Technologies 4.75% Convertible Senior Notes due 2022
Debt Instrument [Line Items]
Outstanding borrowings		$ 84,200,000			$ 287,500,000
Interest rate		4.75%
Forecast | Domestic Bank Group $500 million asset-based credit facility | Revolving credit facility
Debt Instrument [Line Items]
Line of credit facility, increase to maximum borrowing capacity				$ 200,000,000
[1]	Bearing interest at 7.875% effective February 15, 2016.

Debt - Convertible Notes Narrat

Debt - Convertible Notes Narrative (Details)	1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 30, 2020USD ($)tradingDays	Sep. 30, 2020USD ($)$ / shares	Sep. 30, 2019USD ($)	Sep. 30, 2020USD ($)$ / shares	Sep. 30, 2019USD ($)	Jul. 02, 2020USD ($)	Dec. 31, 2019USD ($)
Debt Instrument [Line Items]
Debt extinguishment charge		$ 0	$ 0	$ 21,500,000	$ 0
Payment for Debt Extinguishment or Debt Prepayment Cost				19,100,000	0
Debt issuance costs				9,100,000	0
Purchase of convertible note capped call				(19,400,000)	$ 0
Debt instrument, convertible, redemption price as a percent of principal amount	100.00%
Debt instrument, convertible, threshold percentage of stock price trigger	130.00%
Debt instrument, convertible, threshold trading days | tradingDays	20
Debt instrument, convertible, threshold consecutive trading days | tradingDays	30
Debt issuance costs		15,200,000		15,200,000			$ 12,300,000
Equity component of convertible note		$ 1,100,000		$ 49,800,000
Cap price (in dollars per share) | $ / shares		$ 19.76		$ 19.76
[AdditionalPaidInCapitalMember]
Debt Instrument [Line Items]
Equity component of convertible note		$ 1,100,000		$ 49,800,000
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt
Debt Instrument [Line Items]
Interest rate		3.50%		3.50%
Debt instrument, face amount, additional amount offered to underwriters	$ 40,000,000
Debt instrument, convertible, conversion ratio		0.0645745
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares		$ 15.49		$ 15.49
Debt instrument, interest rate, effective percentage		8.40%		8.40%
Debt, Gross	$ 285,000,000	$ 291,400,000		$ 291,400,000		$ 6,400,000	0
Equity component of convertible debt		49,000,000		49,000,000		$ 51,400,000	0
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt | Long-term Debt [Member]
Debt Instrument [Line Items]
Debt issuance costs		7,500,000		7,500,000
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt | Additional Paid in Capital
Debt Instrument [Line Items]
Debt issuance costs		$ 1,600,000		$ 1,600,000
Allegheny Technologies 4.75% Convertible Senior Notes due 2022
Debt Instrument [Line Items]
Interest rate		4.75%		4.75%
Debt, Gross		$ 84,200,000		$ 84,200,000			$ 287,500,000
Allegheny Technologies 4.75% Convertible Senior Notes due 2022 | Convertible Debt
Debt Instrument [Line Items]
Debt instrument, repurchased face amount		$ 203,200,000		203,200,000
Write off of Deferred Debt Issuance Cost				$ 2,400,000

Derivative Financial Instrume_3

Derivative Financial Instruments and Hedging (Details) € in Millions, lb in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2020USD ($)lb	Sep. 30, 2019USD ($)	Sep. 30, 2020EUR (€)
Derivative Instruments Gain Loss [Line Items]
Percentage of estimated annual nickel requirements			5.00%
Percentage Of Forecasted Natural Gas Usage Hedged for 2020			100.00%
Percentage Of Forecasted Natural Gas Usage Hedged for 2021			70.00%
Percentage Of Forecasted Natural Gas Usage Hedged for 2022			25.00%
Net derivatives gain (loss) on hedge transactions	$ 6.1	$ 14.3	$ (4.2)	$ 19.1
Reclassification of Cash Flow Hedge Gain (Loss) [Abstract]
Cash Flow Hedge Gain (Loss) To Be Reclassified Within Twelve Months			1.7
Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Net derivatives gain (loss) on hedge transactions	4.7	10.6	(3.1)	14.3
Cost of Sales and Interest Expense [Member] | Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	(0.3)	0.7	$ (4.9)	(0.4)
Nickel
Derivative Instruments Gain Loss [Line Items]
Notional amount of nickel hedge (in pounds of nickel) | lb			3
Interest Rate Swap | Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Net derivatives gain (loss) on hedge transactions	0.1	(1.2)	$ (2.1)	(1.7)
Interest Rate Swap | Interest Expense [Member] | Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	(0.2)	(0.1)	(0.7)	(0.3)
Nickel and other raw material contracts | Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Net derivatives gain (loss) on hedge transactions	2.1	11.8	(2.1)	18.2
Nickel and other raw material contracts | Cost Of Sales | Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	0.7	1.1	(1.3)	0.5
Natural gas contracts | Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Net derivatives gain (loss) on hedge transactions	2.5	(0.9)	1.2	(3.2)
Natural gas contracts | Cost Of Sales | Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	(0.8)	(0.6)	(2.8)	(0.6)
Foreign exchange contracts | Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Net derivatives gain (loss) on hedge transactions	0	0.9	(0.1)	1
Foreign exchange contracts | Cost Of Sales | Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	$ 0	$ 0.3	$ (0.1)	$ 0
Designated as Hedging Instrument | Foreign exchange forward | Maturity Dates Through 2020 [Member] | Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Notional amount of derivative | €					€ 0
2024 Term Loan
Derivative Instruments Gain Loss [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.21%		4.21%		4.21%
2024 Term Loan | Interest Rate Swap
Derivative Instruments Gain Loss [Line Items]
Notional amount of derivative	$ 50		$ 50

Derivative Financial Instrume_4

Derivative Financial Instruments and Hedging (Details2) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Asset	$ 4.1	$ 5.6
Derivative Fair Value Of Derivative Liability	4.3	7.5
Designated as Hedging Instrument
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Asset	4.1	5.6
Derivative Fair Value Of Derivative Liability	4.3	7.5
Designated as Hedging Instrument | Nickel and other raw material contracts | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Asset	1.8	4.4
Designated as Hedging Instrument | Nickel and other raw material contracts | Other assets
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Asset	0.4	1.2
Designated as Hedging Instrument | Nickel and other raw material contracts | Accrued liabilities
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Liability	0.1	2.5
Designated as Hedging Instrument | Interest Rate Swap | Accrued liabilities
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Liability	1.1	0.3
Designated as Hedging Instrument | Interest Rate Swap | Other long-term liabilities
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Liability	2.8	1.2
Designated as Hedging Instrument | Natural gas contracts | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Asset	1.4	0
Designated as Hedging Instrument | Natural gas contracts | Other assets
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Asset	0.5	0
Designated as Hedging Instrument | Natural gas contracts | Accrued liabilities
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Liability	0.3	2.5
Designated as Hedging Instrument | Natural gas contracts | Other long-term liabilities
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Liability	$ 0	$ 1

Fair Value of Financial Instr_3

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jul. 02, 2020	Dec. 31, 2019
Carrying Value Reported Amount Fair Value Disclosure
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 572.2		$ 490.8
Derivative financial instruments, Assets	4.1		5.6
Derivative Financial Instruments, liabilities	4.3		7.5
Debt	1,605.4		1,411.2
Estimate Of Fair Value Fair Value Disclosure
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	572.2		490.8
Derivative financial instruments, Assets	4.1		5.6
Derivative Financial Instruments, liabilities	4.3		7.5
Debt	1,597.7		1,676.5
Estimate Of Fair Value Fair Value Disclosure | Fair Value Inputs Level 1
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	572.2		490.8
Derivative financial instruments, Assets	0		0
Derivative Financial Instruments, liabilities	0		0
Debt	1,367.9		1,552.8
Estimate Of Fair Value Fair Value Disclosure | Fair Value Inputs Level 2
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	0		0
Derivative financial instruments, Assets	4.1		5.6
Derivative Financial Instruments, liabilities	4.3		7.5
Debt	229.8		123.7
Convertible Debt | Allegheny Technologies 3.5% Convertible Senior Notes due 2025
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Equity component of convertible debt	$ 49	$ 51.4	$ 0

Retirement Benefits (Details1)

Retirement Benefits (Details1) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension Plans Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the year	$ 3.2	$ 3.2	$ 9.5	$ 9.5
Interest cost on benefits earned in prior years	21.6	26.4	64.7	79.1
Expected return on plan assets	(33.6)	(32.8)	(100.8)	(98.5)
Amortization of prior service cost (credit)	0.1	0	0.5	0.2
Amortization of net actuarial loss	18.6	18.5	55.8	55.3
Total retirement benefit expense	9.9	15.3	29.7	45.6
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the year	0.6	0.5	1.7	1.4
Interest cost on benefits earned in prior years	2.6	3.7	8	11.1
Expected return on plan assets	0	0	0	0
Amortization of prior service cost (credit)	(0.9)	(0.7)	(2.8)	(2.1)
Amortization of net actuarial loss	2.7	3.4	8.1	10.1
Total retirement benefit expense	$ 5	$ 6.9	$ 15	$ 20.5

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Investments, Owned, Federal Income Tax Note [Line Items]
Income tax (provision) benefit	$ (0.8)		$ (3.7)	$ (104.2)	$ (10.3)
Effective income tax rate	(1.70%)		3.10%	(30.70%)	4.70%
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount		$ 99

Business Segments (Details)

Business Segments (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Sales	$ 598		$ 1,018.7	$ 2,323.8	$ 3,103.9
Operating profit (loss)	(6.8)		99.2	101.7	275.4
LIFO and net realizable value reserves	0		0	0	(0.1)
Corporate expenses	(10.9)		(16)	(31.4)	(50.6)
Closed operations and other expense	(1.1)		(8.2)	(5.3)	(19.2)
Restructuring and other charges	(2.3)		0	(27)	0
Impairment of goodwill	0		0	(287)	0
Income (loss) from equity investments	(1)		(2.4)	(6.6)	(8.7)
Gain on asset sales, net	0		68.2	2.5	89.8
Debt extinguishment charge	0		0	(21.5)	0
Interest expense, net	(25.1)		(24.2)	(68.7)	(74.9)
Income (loss) before income taxes	(46.2)		119	(339.1)	220.4
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal				0	(1.8)
Allegheny & Tsingshan Stainless | Employee Severance [Member]
Segment Reporting Information [Line Items]
Income (loss) from equity investments	0	$ (2.4)	0	(2.4)	0
Cast Products [Member]
Segment Reporting Information [Line Items]
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal			6.2		6.2
Forged Products
Segment Reporting Information [Line Items]
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal			(0.4)		(8.1)
Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless
Segment Reporting Information [Line Items]
Income (loss) from equity investments	(1.6)		(2.6)	(10.1)	(9.9)
Operating Segments
Segment Reporting Information [Line Items]
Sales	623.4		1,097.8	2,482.4	3,351.9
Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	230.1		471.6	995.3	1,540.7
Operating profit (loss)	(2.7)		68.4	63.6	198.5
Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Sales	393.3		626.2	1,487.1	1,811.2
Operating profit (loss)	(4.1)		30.8	38.1	76.9
Internal Customers | Operating Segments
Segment Reporting Information [Line Items]
Sales	25.4		79.1	158.6	248
Internal Customers | Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	8.8		19.8	53	59.2
Internal Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Sales	16.6		59.3	105.6	188.8
External Customers | Operating Segments
Segment Reporting Information [Line Items]
Sales	598		1,018.7	2,323.8	3,103.9
External Customers | Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	221.3		451.8	942.3	1,481.5
External Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Sales	$ 376.7		$ 566.9	$ 1,381.5	$ 1,622.4

Business Segments - Additional

Business Segments - Additional Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2020USD ($)segment	Sep. 30, 2019USD ($)	Dec. 31, 2019USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring reserve	$ 20.1		$ 20.1		$ 4.5
Gain on asset sales, net	0	$ 68.2	$ 2.5	$ 89.8
Number of business segments | segment			2
Goodwill	238.4		$ 238.4		$ 525.8
Adjustment to indemnification for conditional ARO costs	0	0	4.3	0
Gain on the sale of oil and gas rights		62.4	2.5	91.7
Loss from sale of businesses	$ 0	5.8	0	(1.9)
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal			$ 0	(1.8)
Cast Products [Member]
Restructuring Cost and Reserve [Line Items]
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal		6.2		6.2
Forged Products
Restructuring Cost and Reserve [Line Items]
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal		$ (0.4)		$ (8.1)
Aerospace and Defense Market Concentration [Member] | High Performance Materials & Components | Revenue from Contract with Customer, Segment Benchmark [Member]
Restructuring Cost and Reserve [Line Items]
Concentration Risk, Percentage			80.00%
Energy and Aerospace & Defense Market Concentration [Member] | Advanced Alloys & Solutions [Member] | Revenue from Contract with Customer, Segment Benchmark [Member]
Restructuring Cost and Reserve [Line Items]
Concentration Risk, Percentage			50.00%
Allegheny & Tsingshan Stainless
Restructuring Cost and Reserve [Line Items]
Equity method investment ownership percentage	50.00%		50.00%
Uniti
Restructuring Cost and Reserve [Line Items]
Equity method investment ownership percentage	50.00%		50.00%
Allegheny Technologies Inc | Shanghai STAL Precision Stainless Steel Co Ltd | Advanced Alloys & Solutions [Member]
Restructuring Cost and Reserve [Line Items]
Joint venture ownership percentage	60.00%		60.00%

Restructuring - Narrative (Deta

Restructuring - Narrative (Details) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2020USD ($)employee	Jun. 30, 2020employee	Mar. 31, 2020employee	Sep. 30, 2019USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2019USD ($)	Dec. 31, 2019USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 2.3			$ 0	$ 27	$ 0
Number of positions eliminated | employee	100	550	90
Restructuring reserve	$ 20.1				20.1		$ 4.5
Other Current Liabilities
Restructuring Cost and Reserve [Line Items]
Restructuring reserve	18.9				18.9
Other Noncurrent Liabilities
Restructuring Cost and Reserve [Line Items]
Restructuring reserve	$ 1.2				$ 1.2

Restructuring - Restructuring R

Restructuring - Restructuring Reserve Roll Forward (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Restructuring Reserve [Roll Forward]
Balance at December 31, 2019			$ 4.5
Additions	$ 2.3	$ 0	27	$ 0
Payments			(11.4)
Balance at September 30, 2020	$ 20.1		$ 20.1

Per Share Information (Details)

Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income (loss) attributable to ATI	$ (50.1)	$ 111	$ (451.6)	$ 201.1
Net income (loss) attributable to ATI after assumed conversions	$ (50.1)	$ 114.2	$ (451.6)	$ 210.8
Denominator for basic net income (loss) per common share – weighted average shares	126.6	125.9	126.5	125.7
Share-based compensation	0	0.9	0	0.8
Denominator for diluted net income (loss) per common share – adjusted weighted average shares and assumed conversions	126.6	146.7	126.5	146.4
Basic net (loss) income attributable to ATI per common share (in dollars per share)	$ (0.40)	$ 0.88	$ (3.57)	$ 1.60
Diluted net (loss) income attributable to ATI per common share (in dollars per share)	$ (0.40)	$ 0.78	$ (3.57)	$ 1.44
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	25	0	21.9	0
Allegheny Technologies 4.75% Convertible Senior Notes due 2022
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Convertible Senior Notes- Effect on numerator	$ 0	$ 3.2	$ 0	$ 9.7
Convertible Senior Notes- Effect on denominator	0	19.9	0	19.9
Interest rate	4.75%		4.75%
Allegheny Technologies 3.5% Convertible Senior Notes due 2025
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Convertible Senior Notes- Effect on numerator	$ 0	$ 0	$ 0	$ 0
Convertible Senior Notes- Effect on denominator	0	0	0	0

Accumulated Other Comprehensi_3

Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		$ (1,172.8)	$ (1,084.4)	$ (1,201.7)	$ (1,133.8)
OCI before reclassifications		21.9	(1.1)	6.2	3.5
Amounts reclassified from AOCI		22.3	24.1	66.9	68.9
Net current-period OCI		44.2	23	73.1	72.4
Ending balance		(1,128.6)	(1,061.4)	(1,128.6)	(1,061.4)
Increase (Decrease) in Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Beginning balance		8.5	12.7	9.8	11.1
OCI before reclassifications		6	(4.6)	4.7	(3)
Amounts reclassified from AOCI		0	0	0	0
Net current-period OCI		6	(4.6)	4.7	(3)
Ending balance		14.5	8.1	14.5	8.1
Post- retirement benefit plans [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		(1,051.8)	(973.4)	(1,083.1)	(1,005.8)
OCI before reclassifications		0	0	0	0
Amounts reclassified from AOCI	[1]	15.6	16.1	46.9	48.5
Net current-period OCI		15.6	16.1	46.9	48.5
Ending balance		(1,036.2)	(957.3)	(1,036.2)	(957.3)
Increase (Decrease) in Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Beginning balance		0	0	0	0
OCI before reclassifications		0	0	0	0
Amounts reclassified from AOCI		0	0	0	0
Net current-period OCI		0	0	0	0
Ending balance		0	0	0	0
Currency translation adjustment [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		(84.5)	(73)	(76.6)	(73.9)
OCI before reclassifications		17.2	(11.7)	9.3	(10.8)
Amounts reclassified from AOCI	[2]	0	0	0	0
Net current-period OCI		17.2	(11.7)	9.3	(10.8)
Ending balance		(67.3)	(84.7)	(67.3)	(84.7)
Increase (Decrease) in Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Beginning balance		8.5	12.7	9.8	11.1
OCI before reclassifications		6	(4.6)	4.7	(3)
Amounts reclassified from AOCI	[2]	0	0	0	0
Net current-period OCI		6	(4.6)	4.7	(3)
Ending balance		14.5	8.1	14.5	8.1
Derivatives [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		(3.7)	0	(0.5)	(4.8)
OCI before reclassifications		4.7	10.6	(3.1)	14.3
Amounts reclassified from AOCI	[3]	0.3	(0.7)	4.9	0.4
Net current-period OCI		5	9.9	1.8	14.7
Ending balance		1.3	9.9	1.3	9.9
Increase (Decrease) in Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Beginning balance		0	0	0	0
OCI before reclassifications		0	0	0	0
Amounts reclassified from AOCI		0	0	0	0
Net current-period OCI		0	0	0	0
Ending balance		0	0	0	0
Accumulated Deferred Tax Asset Valuation Allowance [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		(32.8)	(38)	(41.5)	(49.3)
OCI before reclassifications		0	0	0	0
Amounts reclassified from AOCI	[4]	6.4	8.7	15.1	20
Net current-period OCI		6.4	8.7	15.1	20
Ending balance		(26.4)	(29.3)	(26.4)	(29.3)
Increase (Decrease) in Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Beginning balance		0	0	0	0
OCI before reclassifications		0	0	0	0
Amounts reclassified from AOCI		0	0	0	0
Net current-period OCI		0	0	0	0
Ending balance		$ 0	$ 0	$ 0	$ 0
[1]	Amounts were included in net periodic benefit cost for pension and other postretirement benefit plans (see Note 11).
[2]	No amounts were reclassified to earnings.
[3]	Amounts related to derivatives are included in cost of goods sold or interest expense in the period or periods the hedged item affects earnings (see Note 9).
[4]	Represents the net change in deferred tax asset valuation allowances on changes in AOCI balances between the balance sheet dates.

Accumulated Other Comprehensi_4

Accumulated Other Comprehensive Income (Loss) (Details 2) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization to net income (loss) of net prior service credits		$ (0.8)	$ (0.7)	$ (2.3)	$ (1.9)
Amount reclassified from AOCI, Postretirement benefit plans, Actuarial losses		21.3	21.9	63.9	65.4
Income (loss) before income taxes		(46.2)	119	(339.1)	220.4
Income tax provision		0.8	3.7	104.2	10.3
Cost of sales		(559.9)	(859)	(2,076.2)	(2,635.4)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Post- retirement benefit plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization to net income (loss) of net prior service credits	[1]	0.8	0.7	2.3	1.9
Amount reclassified from AOCI, Postretirement benefit plans, Actuarial losses	[1]	(21.3)	(21.9)	(63.9)	(65.4)
Income (loss) before income taxes	[2]	(20.5)	(21.2)	(61.6)	(63.5)
Income tax provision	[3]	(4.9)	(5.1)	(14.7)	(15)
Net income (loss)		(15.6)	(16.1)	(46.9)	(48.5)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) before income taxes	[2]	(0.4)	0.8	(6.4)	(0.6)
Income tax provision	[3]	(0.1)	0.1	(1.5)	(0.2)
Net income (loss)		(0.3)	0.7	(4.9)	(0.4)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Nickel and other raw material contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of sales	[4]	0.9	1.5	(1.7)	0.7
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Natural gas contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of sales	[4]	(1.1)	(0.8)	(3.7)	(0.8)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Foreign exchange contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of sales	[4]	0	0.4	(0.1)	0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Interest Rate Swap
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest Expense	[4]	$ (0.2)	$ (0.3)	$ (0.9)	$ (0.5)
[1]	Amounts are reported in nonoperating retirement benefit expense (see Note 11).
[2]	For pretax items, positive amounts are income and negative amounts are expense in terms of the impact to net income. Tax effects are presented in conformity with ATI’s presentation in the consolidated statements of operations.
[3]	These amounts exclude the impact of any deferred tax asset valuation allowances, when applicable.
[4]	Amounts related to derivatives, with the exception of the interest rate swap, are included in cost of goods sold in the period or periods the hedged item affects earnings. Amounts related to the interest rate swap are included in interest expense in the same period as the interest expense on the Term Loan is recognized in earnings (see Note 8).

Commitments and Contingencies (

Commitments and Contingencies (Details) $ in Millions	Sep. 30, 2020USD ($)
Components of Environmental Loss Accrual [Abstract]
Accrual For Environmental Loss Contingencies	$ 14
Accrued Environmental Loss Contingencies Current	4
Federal Superfund and comparable state-managed sites	3
Formerly owned or operated sites	9
Owned or controlled sites at which Company operations have been discontinued	1
Environmental Remediation Obligation For Ongoing Operations	1
Maximum
Loss Contingency, Estimate [Abstract]
Loss contingency maximum possible loss	$ 15