ATI (ATI) 10-Q 2 Nov 21 2021 Q3 Quarterly report Financial data

Cover

Cover - shares	9 Months Ended
	Sep. 30, 2021	Oct. 15, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2021
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)		127,248,348
Amendment Flag	false
Entity Central Index Key	0001018963
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Current Assets:
Cash and cash equivalents	$ 1,006.8	$ 645.9
Accounts receivable, net	502	345.8
Short-term contract assets	55.7	38.9
Inventories, net	1,055.8	997.1
Prepaid expenses and other current assets	68.8	38.3
Total current assets	2,689.1	2,066
Property, plant and equipment, net	1,487.6	1,469.2
Goodwill	229	240.7
Other assets	220.8	259
Total assets	4,626.5	4,034.9
Current Liabilities:
Accounts payable	290.7	290.6
Short-term contract liabilities	86.7	111.8
Short-term debt and current portion of long-term debt	606.9	17.8
Other current liabilities	223	233.1
Total current liabilities	1,207.3	653.3
Long-term debt	1,683.6	1,550
Accrued postretirement benefits	269.4	326.7
Pension liabilities	565.9	673.6
Other long-term liabilities	220.1	189.9
Total liabilities	3,946.3	3,393.5
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-127,477,621 shares at September 30, 2021 and 126,820,440 shares at December 31, 2020; outstanding-127,248,348 shares at September 30, 2021 and 126,817,768 shares at December 31, 2020	12.7	12.7
Additional paid-in capital	1,592.1	1,625.5
Retained earnings	102.5	106.5
Treasury stock: 229,273 shares at September 30, 2021 and 2,672 shares at December 31, 2020	(4.8)	0
Accumulated other comprehensive loss, net of tax	(1,161.1)	(1,223.6)
Total ATI stockholders’ equity	541.4	521.1
Noncontrolling interests
Noncontrolling interests	138.8	120.3
Total Equity	680.2	641.4
Total Liabilities and Equity	$ 4,626.5	$ 4,034.9

CONSOLIDATED BALANCE SHEETS (PA

CONSOLIDATED BALANCE SHEETS (PARENTHETICAL) - $ / shares	Sep. 30, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Preferred stock, par value (USD per share)	$ 0.10	$ 0.10
Preferred stock, authorized (in shares)	50,000,000	50,000,000
Preferred stock, issued (in shares)	0	0
Common stock, par value (USD per share)	$ 0.10	$ 0.10
Common stock, authorized (in shares)	500,000,000	500,000,000
Common stock, issued (in shares)	127,477,621	126,820,440
Common stock, outstanding (in shares)	127,248,348	126,817,768
Treasury stock (in shares)	229,273	2,672

CONSOLIDATED STATEMENTS OF INCO

CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Sales	$ 725.7	$ 598	$ 2,034.4	$ 2,323.8
Cost of sales	643.2	559.9	1,823.4	2,076.2
Gross profit	82.5	38.1	211	247.6
Selling and administrative expenses	54.9	45.4	169.1	148.2
Impairment of goodwill	0	0	0	287
Restructuring charges (credits)	(2.3)	2.3	(8.5)	27
Operating income (loss)	29.9	(9.6)	50.4	(214.6)
Nonoperating retirement benefit income (expense)	57.9	(11.1)	44.3	(33.5)
Interest expense, net	(25.1)	(25.1)	(72.2)	(68.7)
Debt extinguishment charge	0	0	0	(21.5)
Other income (expense), net	14.5	(0.4)	17.4	(0.8)
Income (loss) before income taxes	77.2	(46.2)	39.9	(339.1)
Income tax provision	22	0.8	31.5	104.2
Net income (loss)	55.2	(47)	8.4	(443.3)
Less: Net income attributable to noncontrolling interests	6.5	3.1	16.8	8.3
Net income (loss) attributable to ATI	$ 48.7	$ (50.1)	$ (8.4)	$ (451.6)
Basic net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.38	$ (0.40)	$ (0.07)	$ (3.57)
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.35	$ (0.40)	$ (0.07)	$ (3.57)

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 55.2	$ (47)	$ 8.4	$ (443.3)
Currency translation adjustment
Unrealized net change arising during the period	(5)	23.2	(2.8)	14
Derivatives
Net derivatives gain (loss) on hedge transactions	6.7	6.1	14.1	(4.2)
Reclassification to net income (loss) of net realized (gain) loss	(3)	0.4	(6.2)	6.4
Income taxes on derivative transactions	0	0	0	0
Total	3.7	6.5	7.9	2.2
Postretirement benefit plans
Amortization of net actuarial loss	22.5	21.3	66.9	63.9
Amortization to net income (loss) of net prior service credits	(0.4)	(0.8)	(1.4)	(2.3)
Settlement gain	(21.9)	0	(21.9)	0
Income taxes on postretirement benefit plans	(15.5)	0	(15.5)	0
Total	15.7	20.5	59.1	61.6
Other comprehensive income, net of tax	14.4	50.2	64.2	77.8
Comprehensive income (loss)	69.6	3.2	72.6	(365.5)
Less: Comprehensive income attributable to noncontrolling interests	5.7	9.1	18.5	13
Comprehensive income (loss) attributable to ATI	$ 63.9	$ (5.9)	$ 54.1	$ (378.5)

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Operating Activities:
Net income (loss)	$ 8.4	$ (443.3)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization	108	108.3
Deferred taxes	1.9	98.5
Impairment of goodwill	0	287
Debt extinguishment charge	0	21.5
Net gains from disposal of property, plant and equipment	(2.5)	(3.3)
Gain from sale of business	(13.7)	0
Changes in operating assets and liabilities:
Inventories	(63.4)	119.3
Accounts receivable	(158)	166.2
Accounts payable	0.6	(280.5)
Retirement benefits	(112.9)	(78.3)
Accrued liabilities and other	(13.2)	49.6
Cash provided by (used in) operating activities	(244.8)	45
Investing Activities:
Purchases of property, plant and equipment	(104.2)	(94.6)
Proceeds from sale of business, net of transaction costs	53	0
Proceeds from disposal of property, plant and equipment	2.9	4
Other	(0.2)	1.4
Cash used in investing activities	(48.5)	(89.2)
Financing Activities:
Borrowings on long-term debt	675.7	391.4
Payments on long-term debt and finance leases	(11)	(209.5)
Net borrowings (payments) under credit facilities	3.6	(0.9)
Purchase of convertible note capped call	0	(19.4)
Debt issuance costs	(9.3)	(9.1)
Debt extinguishment charge	0	(19.1)
Shares repurchased for income tax withholding on share-based compensation and other	(4.8)	(7.8)
Cash provided by financing activities	654.2	125.6
Increase (decrease) in cash and cash equivalents
Increase in cash and cash equivalents	360.9	81.4
Cash and cash equivalents at beginning of period	645.9	490.8
Cash and cash equivalents at end of period	$ 1,006.8	$ 572.2

STATEMENTS OF CHANGES IN CONSOL

STATEMENTS OF CHANGES IN CONSOLIDATED EQUITY - USD ($) $ in Millions	Total	[CommonStockMember]	Additional Paid-in Capital [Member]	[RetainedEarningsMember]	[TreasuryStockMember]	[AccumulatedOtherComprehensiveIncomeMember]	[NoncontrollingInterestMember]
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
Total Stockholders' Equity at Dec. 31, 2020	641.4	12.7	1,625.5	106.5	0	(1,223.6)	120.3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	8.4			(8.4)			16.8
Other comprehensive income (loss)	64.2					62.5	1.7
Employee stock plans	11.6		16.4	0	(4.8)
Total Stockholders' Equity at Sep. 30, 2021	680.2	12.7	1,592.1	102.5	(4.8)	(1,161.1)	138.8
Total Stockholders' Equity at Jun. 30, 2021	606.1	12.7	1,587.5	53.8	(4.7)	(1,176.3)	133.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	55.2			48.7			6.5
Other comprehensive income (loss)	14.4					15.2	(0.8)
Employee stock plans	4.5		4.6	0	(0.1)
Total Stockholders' Equity at Sep. 30, 2021	$ 680.2	$ 12.7	$ 1,592.1	$ 102.5	$ (4.8)	$ (1,161.1)	$ 138.8

Accounting Policies

Accounting Policies	9 Months Ended
	Sep. 30, 2021
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. These unaudited consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto included in the Company’s 2020 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, 2020 financial information has been derived from the Company’s audited consolidated financial statements. New Accounting Pronouncements Adopted In August 2020, the Financial Accounting Standards Board 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. The Company adopted this new accounting guidance related to accounting for convertible instruments effective January 1, 2021 using the modified transition approach with the cumulative effect recognized as an adjustment to the opening balance of retained earnings. This new guidance is applicable to the Company’s 3.5% Convertible Senior Notes due 2025 (the 2025 Convertible Notes) that were issued in June 2020, for which the embedded conversion option was required to be separately accounted for as a component of stockholders’ equity. Upon adoption on January 1, 2021, long-term debt increased by $45.4 million and stockholders’ equity decreased by the same amount, representing the net impact of two adjustments: (1) the $49.8 million value of the embedded conversion, which is net of allocated offering costs, previously classified in additional paid-in capital in stockholders’ equity, and (2) a $4.4 million increase to retained earnings for the cumulative effect of adoption primarily related to the non-cash interest expense recorded in fiscal year 2020 for the amortization of the portion of the 2025 Convertible Notes allocated to stockholders’ equity. Prospectively, the reported interest expense for the 2025 Convertible Notes will no longer include the non-cash interest expense of the equity component as required under prior accounting standards and will be closer to the 3.5% cash coupon rate. There was no impact to the Company’s earnings per share calculation as it previously applied the if-converted method to the 2025 Convertible Notes given ATI’s flexibility to settle conversions of the 2025 Convertible Notes in cash, shares of ATI’s common stock or a combination thereof, at ATI’s election.

Revenue from Contracts with Cus

Revenue from Contracts with Customers	9 Months Ended
	Sep. 30, 2021
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, 2021 and 2020 were as follows: (in millions) Third quarter ended September 30, 2021 September 30, 2020 HPMC AA&S Total HPMC AA&S Total Diversified Global Markets: Aerospace & Defense $ 207.5 $ 75.4 $ 282.9 $ 171.8 $ 74.3 $ 246.1 Energy* 57.5 108.7 166.2 24.4 106.0 130.4 Automotive 2.1 89.7 91.8 1.5 57.9 59.4 Food Equipment & Appliances — 51.6 51.6 — 33.8 33.8 Electronics 0.4 34.4 34.8 0.2 45.7 45.9 Medical 16.8 17.5 34.3 10.6 16.7 27.3 Construction/Mining 5.6 19.7 25.3 4.0 24.3 28.3 Other 10.1 28.7 38.8 8.8 18.0 26.8 Total $ 300.0 $ 425.7 $ 725.7 $ 221.3 $ 376.7 $ 598.0 (in millions) Nine Months Ended September 30, 2021 September 30, 2020 HPMC AA&S Total HPMC AA&S Total Diversified Global Markets: Aerospace & Defense $ 605.9 $ 212.4 $ 818.3 $ 782.9 $ 321.3 $ 1,104.2 Energy* 136.6 297.0 433.6 71.3 397.2 468.5 Automotive 5.5 245.8 251.3 4.5 182.2 186.7 Electronics 0.9 132.8 133.7 0.7 117.9 118.6 Food Equipment & Appliances 0.1 107.6 107.7 0.1 131.3 131.4 Medical 42.6 52.7 95.3 37.9 57.9 95.8 Construction/Mining 16.8 72.9 89.7 14.3 92.4 106.7 Other 33.1 71.7 104.8 30.6 81.3 111.9 Total $ 841.5 $ 1,192.9 $ 2,034.4 $ 942.3 $ 1,381.5 $ 2,323.8 *Includes the oil & gas, downstream processing, and specialty energy markets. (in millions) Third quarter ended September 30, 2021 September 30, 2020 HPMC AA&S Total HPMC AA&S Total Primary Geographical Market: United States $ 140.8 $ 256.5 $ 397.3 $ 129.3 $ 219.3 $ 348.6 Asia 58.3 96.2 154.5 19.9 103.0 122.9 Europe 85.3 26.0 111.3 60.7 34.1 94.8 Canada 9.9 8.5 18.4 4.4 8.5 12.9 South America, Middle East and other 5.7 38.5 44.2 7.0 11.8 18.8 Total $ 300.0 $ 425.7 $ 725.7 $ 221.3 $ 376.7 $ 598.0 (in millions) Nine Months Ended September 30, 2021 September 30, 2020 HPMC AA&S Total HPMC AA&S Total Primary Geographical Market: United States $ 426.7 $ 700.2 $ 1,126.9 $ 522.8 $ 909.1 $ 1,431.9 Asia 119.0 313.5 432.5 72.6 288.3 360.9 Europe 250.7 87.1 337.8 286.2 104.1 390.3 Canada 27.4 28.3 55.7 21.1 30.0 51.1 South America, Middle East and other 17.7 63.8 81.5 39.6 50.0 89.6 Total $ 841.5 $ 1,192.9 $ 2,034.4 $ 942.3 $ 1,381.5 $ 2,323.8 Comparative information of the Company’s major high-value and standard products based on their percentages of sales is included in the following table. In conjunction with the Company’s announced ongoing exit of lower-margin standard stainless sheet products, in the fourth quarter of 2020 ATI reclassified certain items as High-Value Products within AA&S segment results. Prior period information reflects these reclassifications. Hot-Rolling and Processing Facility (HRPF) conversion service sales in the AA&S segment are excluded from this presentation. Third quarter ended September 30, 2021 September 30, 2020 HPMC AA&S Total HPMC AA&S Total Diversified Products and Services: High-Value Products Nickel-based alloys and specialty alloys 46 % 31 % 37 % 38 % 29 % 32 % Precision rolled strip products — % 33 % 19 % — % 28 % 18 % Precision forgings, castings and components 35 % — % 15 % 30 % — % 11 % Titanium and titanium-based alloys 19 % 7 % 12 % 32 % 8 % 17 % Zirconium and related alloys — % 15 % 9 % — % 16 % 10 % Total High-Value Products 100 % 86 % 92 % 100 % 81 % 88 % Standard Products Standard stainless products — % 14 % 8 % — % 19 % 12 % Total 100 % 100 % 100 % 100 % 100 % 100 % Nine Months Ended September 30, 2021 September 30, 2020 HPMC AA&S Total HPMC AA&S Total Diversified Products and Services: High-Value Products Nickel-based alloys and specialty alloys 43 % 27 % 34 % 38 % 29 % 34 % Precision rolled strip products — % 34 % 19 % — % 22 % 13 % Precision forgings, castings and components 37 % — % 16 % 34 % — % 14 % Titanium and titanium-based alloys 20 % 7 % 12 % 28 % 10 % 18 % Zirconium and related alloys — % 17 % 10 % — % 14 % 9 % Total High-Value Products 100 % 85 % 91 % 100 % 75 % 88 % Standard Products Standard stainless products — % 15 % 9 % — % 25 % 12 % Total 100 % 100 % 100 % 100 % 100 % 100 % The Company maintained a backlog of confirmed orders totaling $1.7 billion and $1.4 billion at September 30, 2021 and 2020, respectively. Due to the structure of the Company’s long-term agreements, approximately 80% of this backlog at September 30, 2021 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, 2021 and December 31, 2020, accounts receivable with customers were $506.1 million and $350.1 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, 2021 and 2020: (in millions) Accounts Receivable - Reserve for Doubtful Accounts September 30, September 30, Balance as of beginning of fiscal year $ 4.3 $ 4.6 Expense to increase the reserve 0.4 0.1 Write-off of uncollectible accounts (0.6) (0.4) Balance as of period end $ 4.1 $ 4.3 (in millions) Contract Assets Short-term September 30, September 30, Balance as of beginning of fiscal year $ 38.9 $ 38.5 Recognized in current year 74.3 55.8 Reclassified to accounts receivable (49.0) (52.7) Impairment — — Reclassification to/from long-term and contract liability (8.5) 0.1 Balance as of period end $ 55.7 $ 41.7 Long-term September 30, September 30, Balance as of beginning of fiscal year $ — $ 0.1 Recognized in current year — — Reclassified to accounts receivable — — Impairment — — Reclassification to/from short-term — (0.1) Balance as of period end $ — $ — (in millions) Contract Liabilities Short-term September 30, September 30, Balance as of beginning of fiscal year $ 111.8 $ 78.7 Recognized in current year 102.0 121.5 Amounts in beginning balance reclassified to revenue (74.4) (42.9) Current year amounts reclassified to revenue (46.2) (65.6) Divestiture (0.8) — Other 0.2 — Reclassification to/from long-term and contract asset (5.9) 6.1 Balance as of period end $ 86.7 $ 97.8 Long-term September 30, September 30, Balance as of beginning of fiscal year $ 32.0 $ 25.9 Recognized in current year 45.4 12.9 Amounts in beginning balance reclassified to revenue (0.8) (0.8) Current year amounts reclassified to revenue — — Other — — Reclassification to/from short-term (2.6) (6.1) Balance as of period end $ 74.0 $ 31.9 Contract costs for obtaining and fulfilling a contract were $5.3 million and $5.4 million as of September 30, 2021 and December 31, 2020, 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, 2021 was $0.2 million and $0.7 million, respectively.

Inventories

Inventories	9 Months Ended
	Sep. 30, 2021
Inventory Disclosure [Abstract]
Inventories	Inventories Inventories at September 30, 2021 and December 31, 2020 were as follows (in millions): September 30, December 31, Raw materials and supplies $ 195.5 $ 207.6 Work-in-process 804.0 690.7 Finished goods 125.1 181.6 Total inventories at current cost 1,124.6 1,079.9 Adjustment from current cost to LIFO cost basis 0.3 44.1 Inventory valuation reserves (69.1) (126.9) Total inventories, net $ 1,055.8 $ 997.1 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 the carrying value of LIFO inventory to current replacement cost. These NRV reserves were $0.3 million at September 30, 2021 and $44.1 million at December 31, 2020. 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, 2021 2020 LIFO benefit (charge) $ (43.8) $ 7.5 NRV benefit (charge) 43.8 (7.5) Net cost of sales impact $ — $ —

Property Plant And Equipment

Property Plant And Equipment	9 Months Ended
	Sep. 30, 2021
Property, Plant and Equipment [Abstract]
Property Plant and Equipment	Property, Plant and Equipment Property, plant and equipment at September 30, 2021 and December 31, 2020 was as follows (in millions): September 30, December 31, Land $ 34.9 $ 34.8 Buildings 572.1 564.7 Equipment and leasehold improvements 2,804.9 2,736.9 3,411.9 3,336.4 Accumulated depreciation and amortization (1,924.3) (1,867.2) Total property, plant and equipment, net $ 1,487.6 $ 1,469.2

Divestitures

Divestitures	9 Months Ended
	Sep. 30, 2021
Discontinued Operations and Disposal Groups [Abstract]
Divestitures	DivestituresOn August 13, 2021, the Company completed the sale of its Flowform Products business for $55 million. Located in Billerica, MA, this operation uses flowforming process technologies to produce thin-walled components in net or near-net shapes across multiple alloy systems for use in the aerospace & defense and energy markets. The Company received cash proceeds, net of transaction costs and net working capital adjustments, of $53 million on the sale of this business during the quarter ended September 30, 2021, which is reported as an investing activity on the consolidated statement of cash flows. With $12.2 million of goodwill allocated to this operation from ATI’s Forged Products reporting unit, the Company recognized a $13.7 million pre-tax gain in the quarter ended September 30, 2021, which is recorded in other income/expense, net, on the consolidated statement of operations and is excluded from HPMC segment results. This business is reported as part of the HPMC segment through the date of sale. Flowform Products’ sales were $26 million in fiscal year 2020.

Joint Ventures

Joint Ventures	9 Months Ended
	Sep. 30, 2021
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 and automotive markets located in Asia. Cash and cash equivalents held by STAL as of September 30, 2021 were $54.9 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, 2021 were $1.4 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 included the Company’s previously-idled direct roll and pickle (DRAP) facility in Midland, PA. ATI provided hot-rolling conversion services to A&T Stainless using the AA&S segment’s HRPF. ATI accounts for the A&T Stainless joint venture under the equity method of accounting. In 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. Due to repeated tariff exclusion denials, the DRAP facility was idled in an orderly shut down process that was completed in the third quarter of 2020. ATI’s share of A&T Stainless results was income of $0.3 million for the three months ended September 30, 2021 and a net loss of $0.6 million for the nine months ended September 30, 2021, and losses of $1.6 million and $10.1 million for the three and nine months ended September 30, 2020, 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 a charge recorded by A&T Stainless for termination benefits during the quarter ended June 30, 2020. As of September 30, 2021 and December 31, 2020, ATI had net receivables for working capital advances and administrative services from A&T Stainless of $4.8 million and $14.0 million, respectively. For the September 30, 2021 balance of net receivables, $0.8 million was reported in prepaid expenses and other current assets and $4.0 million in other long-term assets on the consolidated balance sheet, while for December 31, 2020, $0.5 million was reported in prepaid expenses and other current assets and $13.5 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 $0.1 million and $1.0 million for the three and nine months ended September 30, 2021, respectively, and $0.6 million and $1.1 million for the three and nine months ended September 30, 2020, 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, 2021
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, 2021 and 2020 was as follows: (in millions) Three months ended September 30, Nine months ended September 30, 2021 2020 2021 2020 Rent and royalty income $ 0.2 $ 0.1 $ 0.7 $ 0.6 Gains from disposal of property, plant and equipment, net 0.1 0.8 2.5 3.3 Net equity income (loss) on joint ventures (See Note 6) 0.4 (1.0) 0.4 (6.6) Joint venture restructuring charges (See Note 6) — — — (2.4) Gain from sale of business (See Note 5) 13.7 — 13.7 — Adjustment to indemnification for conditional ARO costs — — — 4.3 Other 0.1 (0.3) 0.1 — Total other income (expense), net $ 14.5 $ (0.4) $ 17.4 $ (0.8) Gains from disposal of property, plant and equipment, net for the nine months ended September 30, 2020 included a $2.5 million gain on the sale of certain oil and gas rights. This cash gain was reported as an investing activity on the consolidated statement of cash flows for the nine months ended September 30, 2020 and is excluded from segment operating results. In the second quarter of 2020, the Company finalized a settlement agreement for an indemnity claim concerning a conditional asset retirement obligation (ARO) with the buyer of a formerly-owned business and as a result, the Company reduced ARO reserves by $4.3 million. Restructuring Restructuring charges for the third quarter ended September 30, 2021 were a net credit of $2.3 million for a reduction in severance-related reserves related to approximately 50 employees based on changes in planned operating rates and revised workforce reduction estimates. Restructuring charges for the nine months ended September 30, 2021 were a net credit of $8.5 million, reflecting a $9.2 million reduction in severance-related reserves related to approximately 250 employees based on changes in planned operating rates and revised workforce reduction estimates, partially offset by $0.7 million of other costs related to facility idlings. These amounts were presented as restructuring charges/credits in the consolidated statements of operations and are excluded from segment EBITDA. For the three 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/credits in the consolidated statements of operations and are excluded from segment EBITDA. 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 reduction 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. Restructuring reserves for severance cost activity is as follows: Severance and Employee Benefit Costs Balance at December 31, 2020 $ 43.4 Adjustments (9.2) Payments (12.4) Balance at September 30, 2021 $ 21.8

Debt

Debt	9 Months Ended
	Sep. 30, 2021
Debt Disclosure [Abstract]
Debt	Debt Debt at September 30, 2021 and December 31, 2020 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 5.125% Notes due 2031 350.0 — Allegheny Technologies 4.875% Notes due 2029 325.0 — Allegheny Technologies 3.5% Convertible Senior Notes due 2025 291.4 291.4 Allegheny Technologies 4.75% Convertible Senior Notes due 2022 84.2 84.2 Allegheny Ludlum 6.95% Debentures due 2025 (b) 150.0 150.0 Term Loan due 2024 200.0 200.0 U.S. revolving credit facility — — Foreign credit facilities 9.2 5.5 Finance leases and other 53.5 48.0 Debt issuance costs (22.8) (14.5) Equity component of convertible debt — (46.8) Total debt 2,290.5 1,567.8 Short-term debt and current portion of long-term debt 606.9 17.8 Total long-term debt $ 1,683.6 $ 1,550.0 (a) Bearing interest at 7.875% effective February 15, 2016. (b) The payment obligations of these debentures issued by Allegheny Ludlum, LLC are fully and unconditionally guaranteed by ATI. 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 a $200 million term loan (Term Loan). 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 fixed charge coverage ratio at September 30, 2021. As a result, the Company is unable to access 12.5%, or $87.5 million, of the ABL facility until it meets the 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, 2021, there were no outstanding borrowings under the revolving portion of the ABL facility, and $42.8 million was utilized to support the issuance of letters of credit. There were no average revolving credit borrowings under the ABL facility for the first nine months of 2021. 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. 2029 and 2031 Notes On September 14, 2021, ATI issued $325 million aggregate principal amount of 4.875% Senior Notes due 2029 (2029 Notes) and $350 million aggregate principal amount of 5.125% Senior Notes due 2031 (2031 Notes). Interest on the 2029 Notes is payable semi-annually in arrears at a rate of 4.875% per year, and the 2020 Notes will mature on October 1, 2029. Interest on the 2031 Notes is payable semi-annually in arrears at a rate of 5.125% per year, and the 2031 Notes will mature on October 1, 2031. Total combined net proceeds of $665.7 million from both of these issuances were primarily used to fund the redemption of all of the $500 million aggregate principal amount outstanding of the 5.875% Senior Notes due 2023 (2023 Notes) on October 14, 2021, including a make-whole payment and accrued interest, following a 30 day notice of redemption as required by the 2023 Notes indenture. As such, the 2023 Notes are classified as short-term debt on the consolidated balance sheet as of September 30, 2021. In the fourth quarter 2021, the Company will recognize a $65.5 million debt extinguishment charge, which includes a $64.5 million cash make-whole payment related to the early extinguishment of the 2023 Notes and a $1.0 million charge for the remaining unrecognized portion of the 2023 Notes deferred debt issue costs. Underwriting fees and other third-party expenses for the issuance of the 2029 and 2031 Notes were $4.7 million each, and are being amortized to interest expense over the 8-year and 10-year terms of the 2029 and 2031 Notes, respectively. The 2029 and 2031 Notes are unsecured and unsubordinated obligations of the Company and equally ranked with all of its existing and future senior unsecured debt. The 2029 and 2031 Notes restrict the Company’s ability to create certain liens, to enter into sale leaseback transactions, guarantee indebtedness and to consolidate or merge all, or substantially all, of its assets. The Company has the option to redeem the 2029 and 2031 Notes, as a whole or in part, at any time or from time to time, on at least 15 days, but not more than 60 days, prior notice to the holders of the Notes at redemption prices specified in the 2029 and 2031 Notes. The 2029 and 2031 Notes are subject to repurchase upon the occurrence of a change in control repurchase event (as defined in the 2029 and 2031 Notes) at a repurchase price in cash equal to 101% of the aggregate principal amount of the Notes repurchased, plus any accrued and unpaid interest on the 2029 or 2031 Notes repurchased, as applicable. 2025 Convertible Notes In 2020, the Company issued $291.4 million aggregate principal amount of 2025 Convertible Notes. 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 2022 Convertible Notes, resulting in a $21.5 million debt extinguishment charge. The Company also used 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. As of September 30, 2021, the fair value of the 2025 Convertible Notes is $402 million based on the quoted market price, which is classified in Level 1 of the fair value hierarchy. The 2025 Convertible Notes have a 3.5% cash coupon rate that is payable semi-annually in arrears on each June 15 and December 15. Including amortization of deferred issuance costs, the effective interest rate is 4.2% for the nine months ended September 30, 2021 and 8.4% for the quarter ended September 30, 2020. The effective interest rate is lower in 2021 due to the early-adoption of new accounting guidance on January 1, 2021, as described below. Remaining deferred issuance costs were $7.0 million and $8.2 million at September 30, 2021 and December 31, 2020, respectively. Interest expense on the 2025 Convertible Notes was as follows: Three months ended September 30, Nine months ended September 30, (in millions) 2021 2020 2021 2020 Contractual coupon rate $ 2.5 $ 2.4 $ 7.6 $ 2.4 Amortization of debt issuance costs 0.5 0.4 1.3 0.4 Total interest expense $ 3.0 $ 2.8 $ 8.9 $ 2.8 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 (18.8 million shares). 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 was 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. During the 2020 fiscal year, the equity component was amortized as additional non-cash interest expense, commonly referred to as phantom yield, over the term of the 2025 Convertible Notes using the effective interest method. As a result, as of December 31, 2020, $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). Due to the non-cash phantom yield and including debt issue cost amortization, the 2025 Convertible Notes had reported interest expense in 2020 at an 8.4% rate, higher than the 3.5% cash coupon rate. Effective January 1, 2021, ATI early-adopted new accounting guidance as discussed in Note 1 that eliminated the equity component classification of the embedded conversion option, as well as the phantom yield portion of interest expense on a prospective basis. Upon adoption on January 1, 2021, long-term debt increased by $45.4 million, representing the $46.8 million equity component of convertible debt as of December 31, 2020 in the above table, net of reclassified debt issue costs. ATI entered into privately negotiated capped call transactions with certain of the initial purchasers of the 2025 Convertible Notes 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 and/or offset any cash payments ATI is required to make in excess of the principal amount of converted 2025 Convertible Notes, as the case may be, with such reduction and/or offset subject to a cap based on the cap price. The cap price of the capped call transactions initially is approximately $19.76 per share, and is subject to adjustments under the terms of the capped call transactions. 2022 Convertible Notes As of June 30, 2021, the Company had outstanding $84.2 million aggregate principal amount of the 2022 Convertible Notes, which mature on July 1, 2022. As of September 30, 2021, the fair value of the 2022 Convertible Notes is $115 million based on the quoted market price, which is classified in Level 1 of the fair value hierarchy. Interest on the 2022 Convertible Notes at the 4.75% cash coupon rate is payable semi-annually in arrears on each January 1 and July 1. Including amortization of deferred issuance costs, the effective interest rate is 5.4% for the nine months ended September 30, 2021 and 2020. Remaining deferred issuance costs were $0.4 million and $0.7 million at September 30, 2021 and December 31, 2020, respectively. Interest expense on the 2022 Convertible Notes was as follows: Three months ended September 30, Nine months ended September 30, (in millions) 2021 2020 2021 2020 Contractual coupon rate $ 1.0 $ 1.0 $ 3.0 $ 7.6 Amortization of debt issuance costs 0.2 0.1 0.4 0.9 Total interest expense $ 1.2 $ 1.1 $ 3.4 $ 8.5 The Company does not have the right to redeem the 2022 Convertible Notes prior to their stated maturity date. Holders of the 2022 Convertible Notes have the option to convert their notes into shares of the Company’s common stock, at any time prior to the close of business on the business day immediately preceding the stated maturity date (July 1, 2022). The initial conversion rate for the remaining $84.2 million of 2022 Convertible Notes is 69.2042 shares of ATI common stock per $1,000 (in whole dollars) principal amount of Notes (5.8 million shares), equivalent to a conversion price of $14.45 per share, subject to adjustment in certain events. Other than receiving cash in lieu of fractional shares, holders do not have the option to receive cash instead of shares of common stock upon conversion. Accrued and unpaid interest that exists upon conversion of a note will be deemed paid by the delivery of shares of ATI common stock and no cash payment or additional shares will be given to the holders. If the Company undergoes a fundamental change as defined in the agreement, holders of the 2022 Convertible Notes may require the Company to repurchase the notes in whole or in part for cash at a price equal to 100% of the principal amount of the notes to be purchased plus any accrued and unpaid interest to, but excluding, the repurchase date.

Derivative Financial Instrument

Derivative Financial Instruments and Hedging	9 Months Ended
	Sep. 30, 2021
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, 2021, 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 4 million pounds of nickel with hedge dates through 2024. The aggregate notional amount hedged is approximately 10% of a single year’s estimated nickel raw material purchase requirements. At September 30, 2021, the outstanding financial derivatives used to hedge the Company’s exposure to energy cost volatility included natural gas cost hedges. At September 30, 2021, the Company hedged approximately 80% of its forecasted domestic requirements for natural gas for the remainder of 2021, approximately 75% for 2022, approximately 35% for 2023 and approximately 5% for 2024. 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, 2021, 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, was 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 $ 2.5 $ 3.7 Natural gas contracts Prepaid expenses and other current assets 8.0 0.2 Nickel and other raw material contracts Other assets 0.2 0.7 Natural gas contracts Other assets 1.5 0.2 Total derivatives designated as hedging instruments $ 12.2 $ 4.8 Liability derivatives Balance sheet location Derivatives designated as hedging instruments: Interest rate swap Other current liabilities $ 1.0 $ 1.0 Natural gas contracts Other current liabilities 0.3 0.3 Nickel and other raw material contracts Other current liabilities 0.6 0.1 Interest rate swap Other long-term liabilities 1.4 2.5 Natural gas contracts Other long-term liabilities 0.2 0.1 Nickel and other raw material contracts Other long-term liabilities 0.1 — Total derivatives designated as hedging instruments $ 3.6 $ 4.0 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, 2021. 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 15 for further explanation). Assuming market prices remain constant with those at September 30, 2021, a pre-tax gain of $8.6 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, 2021 and 2020 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 2021 2020 2021 2020 Nickel and other raw material contracts $ (0.2) $ 2.1 $ 1.5 $ 0.7 Natural gas contracts 5.6 2.5 1.0 (0.8) Foreign exchange contracts — — — — Interest rate swap (0.2) 0.1 (0.2) (0.2) Total $ 5.2 $ 4.7 $ 2.3 $ (0.3) Amount of Gain (Loss) Amount of Gain (Loss) Nine months ended September 30, Nine months ended September 30, Derivatives in Cash Flow Hedging Relationships 2021 2020 2021 2020 Nickel and other raw material contracts $ 2.6 $ (2.1) $ 4.1 $ (1.3) Natural gas contracts 8.0 1.2 1.2 (2.8) Foreign exchange contracts 0.1 (0.1) — (0.1) Interest rate swap (0.1) (2.1) (0.6) (0.7) Total $ 10.6 $ (3.1) $ 4.7 $ (4.9) (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, 2021
Fair Value Disclosures [Abstract]
Fair Value Of Financial Instruments	Fair Value of Financial Instruments The estimated fair value of financial instruments at September 30, 2021 was as follows: Fair Value Measurements at Reporting Date Using (In millions) Total Total Quoted Prices in Significant Cash and cash equivalents $ 1,006.8 $ 1,006.8 $ 1,006.8 $ — Derivative financial instruments: Assets 12.2 12.2 — 12.2 Liabilities 3.6 3.6 — 3.6 Debt (a) 2,313.3 2,558.7 2,296.0 262.7 The estimated fair value of financial instruments at December 31, 2020 was as follows: Fair Value Measurements at Reporting Date Using (In millions) Total Total Quoted Prices in Significant Cash and cash equivalents $ 645.9 $ 645.9 $ 645.9 $ — Derivative financial instruments: Assets 4.8 4.8 — 4.8 Liabilities 4.0 4.0 — 4.0 Debt (a) 1,629.1 1,847.7 1,594.2 253.5 (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 December 31, 2020 debt carrying value includes $46.8 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. Effective January 1, 2021, ATI early-adopted new accounting guidance that eliminated the equity component classification of the embedded conversion option of the 2025 Convertible Notes (see Note 1). 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, 2021
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 of 1986, as amended (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. As a result of the American Rescue Plan Act (ARPA) enacted in March 2021, the rules governing pension funding calculations have changed, and minimum funding requirements were reduced. Prior to the ARPA changes, ATI’s fiscal year 2021 funding requirements and expected cash contributions were approximately $87 million. As a result of these ARPA changes, ATI’s prior contributions have generated a credit balance that may be utilized to offset future minimum required contributions. ATI made a $50 million voluntary cash contribution to ATI Pension Plan in the third quarter 2021 to improve the plan’s funded position, bringing the total U.S. qualified defined benefit pension plan contributions to $67.5 million for fiscal year 2021. For the three month periods ended September 30, 2021 and 2020, the components of pension and other postretirement benefit expense (income) 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, 2021 2020 2021 2020 Service cost - benefits earned during the year $ 3.8 $ 3.2 $ 0.4 $ 0.6 Interest cost on benefits earned in prior years 17.1 21.6 1.9 2.6 Expected return on plan assets (34.1) (33.6) — — Amortization of prior service cost (credit) 0.2 0.1 (0.6) (0.9) Amortization of net actuarial loss 18.9 18.6 3.6 2.7 Settlement gain — — (64.9) — Total retirement benefit expense (income) $ 5.9 $ 9.9 $ (59.6) $ 5.0 For the nine month periods ended September 30, 2021 and 2020, 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, 2021 2020 2021 2020 Service cost - benefits earned during the year $ 11.3 $ 9.5 $ 1.1 $ 1.7 Interest cost on benefits earned in prior years 51.3 64.7 6.1 8.0 Expected return on plan assets (102.3) (100.8) — — Amortization of prior service cost (credit) 0.5 0.5 (1.9) (2.8) Amortization of net actuarial loss 56.7 55.8 10.2 8.1 Settlement gain — — (64.9) — Total retirement benefit expense (income) $ 17.5 $ 29.7 $ (49.4) $ 15.0 On July 14, 2021, ATI announced that a new four-year labor agreement with the USW was ratified (see Note 11 for further discussion). As a result of this new CBA, ATI recognized a $64.9 million pretax gain in the third quarter of 2021, which is recorded in nonoperating retirement benefit income/expense on the consolidated statement of operations, related to a plan termination that eliminated certain postretirement medical benefit liabilities, comprised of $43.0 million of long-term postretirement benefit liabilities as of July 2021 and $21.9 million of amounts recorded in accumulated other comprehensive income at that date. Discrete tax effects related to this event were $15.5 million of income tax expense (see Note 13 for further discussion).

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2021
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The provision for income taxes for the quarter and nine months ended September 30, 2021 was $22.0 million and $31.5 million, respectively. The current year tax expense is mainly attributable to the $15.5 million in discrete tax effects related to the postretirement medical benefits gain discussed in Note 12, in accordance with ATI’s accounting policy for recognizing deferred tax amounts stranded in accumulated other comprehensive income. The tax provision for the three 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, based on the actual year to date effective rate for this period. In the second quarter 2020, the Company entered into a three-year cumulative loss within the United States, 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. As a result, ATI recorded a $99.0 million deferred tax asset valuation allowance on its U.S. federal and state tax attributes in the second quarter of 2020. ATI continues to maintain valuation allowances on its U.S. federal and state deferred tax assets, as well as for certain foreign jurisdictions. Throughout 2020, the Company calculated the provision for income taxes based upon actual year to date results due to the uncertainty of future projections as a result of the global pandemic. For 2021, 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 U.S. operations could produce a significant variation in its annual effective tax rate. As a result, the Company recorded income tax for the nine months ended September 30, 2021 based upon an annual effective tax rate for its foreign, non-valuation allowance operations, combined with actual year-to-date tax expense related to its U.S. jurisdictions.

Business Segments

Business Segments	9 Months Ended
	Sep. 30, 2021
Segment Reporting [Abstract]
Business Segments	Business SegmentsThe Company operates in two business segments: High Performance Materials & Components (HPMC) and Advanced Alloys & Solutions (AA&S). In the fourth quarter 2020, the Company changed its segment performance measure from segment operating profit to segment EBITDA, based on internal reporting changes. Prior period results are presented using the new performance measure. The measure of segment EBITDA 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, depreciation and amortization, corporate expenses, net interest expense, closed operations and other income (expense), charges for goodwill and asset impairments, restructuring and other credits/charges, strike related costs, debt extinguishment charges and non-operating gains or losses. Management believes segment EBITDA, 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, 2021 2020 2021 2020 Total sales: High Performance Materials & Components $ 325.0 $ 230.1 $ 908.9 $ 995.3 Advanced Alloys & Solutions 464.7 393.3 1,280.1 1,487.1 789.7 623.4 2,189.0 2,482.4 Intersegment sales: High Performance Materials & Components 25.0 8.8 67.4 53.0 Advanced Alloys & Solutions 39.0 16.6 87.2 105.6 64.0 25.4 154.6 158.6 Sales to external customers: High Performance Materials & Components 300.0 221.3 841.5 942.3 Advanced Alloys & Solutions 425.7 376.7 1,192.9 1,381.5 $ 725.7 $ 598.0 $ 2,034.4 $ 2,323.8 Three months ended September 30, Nine months ended September 30, 2021 2020 2021 2020 EBITDA: High Performance Materials & Components $ 37.4 $ 16.8 $ 99.2 $ 122.1 Advanced Alloys & Solutions 56.8 11.0 142.5 85.5 Total segment EBITDA 94.2 27.8 241.7 207.6 LIFO and net realizable value reserves — — — — Corporate expenses (12.9) (10.2) (41.0) (29.7) Closed operations and other expense (1.4) (1.0) (4.5) (4.6) Total ATI Adjusted EBITDA 79.9 16.6 196.2 173.3 Depreciation & amortization (a) (35.6) (35.4) (108.0) (108.3) Interest expense, net (25.1) (25.1) (72.2) (68.7) Restructuring and other credits (charges) (See Note 7) 2.3 (2.3) 8.5 (27.0) Strike related costs (22.9) — (63.2) — Retirement benefit settlement gain (See Note 12) 64.9 — 64.9 — Impairment of goodwill — — — (287.0) Joint venture restructuring charge (See Note 6) — — — (2.4) Debt extinguishment charge (See Note 8) — — — (21.5) Gain on asset sales and sale of business (See Notes 5 and 6) 13.7 — 13.7 2.5 Income (loss) before income taxes $ 77.2 $ (46.2) $ 39.9 $ (339.1) (a) The following is depreciation & amortization by each business segment: Three months ended September 30, Nine months ended September 30, 2021 2020 2021 2020 High Performance Materials & Components $ 18.2 $ 19.5 $ 57.0 $ 58.5 Advanced Alloys & Solutions 16.3 15.1 47.9 47.4 Other 1.1 0.8 3.1 2.4 $ 35.6 $ 35.4 108.0 108.3 Corporate expenses in the third quarter and first nine months of 2021 reflect higher incentive compensation costs compared to the prior year periods. The first nine months of 2020 included reversals of previously-recognized incentive compensation expense due to COVID-19 pandemic impacts. The Company’s collective bargaining agreement (CBA) with the United Steel, Paper and Forestry, Rubber, Manufacturing, Energy, Allied & Industrial Service Workers International Union, AFL-CIO, CLC (USW) involving approximately 1,100 active full-time represented employees located primarily within the AA&S segment operations, as well as a number of inactive employees, expired on February 28, 2021. USW-represented employees continued to work under the terms of the expired CBA until March 30, 2021 when they engaged in a strike. On July 14, 2021, ATI announced that a new four-year labor agreement with the USW was ratified, ending the strike. During the third quarter of 2021, the Company recorded $22.9 million in strike related costs, of which $21.5 million were excluded from AA&S segment EBITDA and $1.4 million were excluded from HPMC segment EBITDA. During the first nine months of 2021, the Company recorded $63.2 million in strike related costs, of which $59.7 million were excluded from AA&S segment EBITDA and $3.5 million were excluded from HPMC segment EBITDA. These items primarily consisted of overhead costs recognized in the period due to below-normal operating rates and higher costs for outside conversion activities. During the second quarter of 2020, the Company recorded a $287.0 million goodwill impairment charge for the partial impairment of the Forged Products reporting unit goodwill in the HPMC segment. This goodwill impairment charge was excluded from 2020 HPMC business segment EBITDA.

Per Share Information

Per Share Information	9 Months Ended
	Sep. 30, 2021
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: (In millions, except per share amounts) Three months ended September 30, Nine months ended September 30, 2021 2020 2021 2020 Numerator: Numerator for basic income (loss) per common share – Net income (loss) attributable to ATI $ 48.7 $ (50.1) $ (8.4) $ (451.6) Effect of dilutive securities: 4.75% Convertible Senior Notes due 2022 1.1 — — — 3.5% Convertible Senior Notes due 2025 2.9 — — — Numerator for diluted income (loss) per common share – Net income (loss) attributable to ATI after assumed conversions $ 52.7 $ (50.1) $ (8.4) $ (451.6) Denominator: Denominator for basic net income (loss) per common share – weighted average shares 127.2 126.6 127.0 126.5 Effect of dilutive securities: Share-based compensation 0.8 — — — 4.75% Convertible Senior Notes due 2022 5.8 — — — 3.5% Convertible Senior Notes due 2025 18.8 — — — Denominator for diluted net income (loss) per common share – adjusted weighted average shares and assumed conversions 152.6 126.6 127.0 126.5 Basic net income (loss) attributable to ATI per common share $ 0.38 $ (0.40) $ (0.07) $ (3.57) Diluted net income (loss) attributable to ATI per common share $ 0.35 $ (0.40) $ (0.07) $ (3.57) 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 no anti-dilutive shares for the three months ended September 30, 2021. There were 25.6 million anti-dilutive shares for the nine months ended September 30, 2021, and 25.0 million and 21.9 million anti-dilutive shares for the three and nine months ended September 30, 2020, respectively.

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income (Loss)	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) The changes in AOCI by component, net of tax, for the three month period ended September 30, 2021 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, June 30, 2021 $ (1,086.9) $ (55.8) $ 5.1 $ (38.7) $ (1,176.3) OCI before reclassifications — (4.2) 5.2 — 1.0 Amounts reclassified from AOCI (a) 4.0 (b) — (c) (2.3) (d) 12.5 14.2 Net current-period OCI 4.0 (4.2) 2.9 12.5 15.2 Balance, September 30, 2021 $ (1,082.9) $ (60.0) $ 8.0 $ (26.2) $ (1,161.1) Attributable to noncontrolling interests: Balance, June 30, 2021 $ — $ 23.7 $ — $ — $ 23.7 OCI before reclassifications — (0.8) — — (0.8) Amounts reclassified from AOCI — (b) — — — — Net current-period OCI — (0.8) — — (0.8) Balance, September 30, 2021 $ — $ 22.9 $ — $ — $ 22.9 The changes in AOCI by component, net of tax, for the nine month period ended September 30, 2021 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, December 31, 2020 $ (1,119.9) $ (55.5) $ 2.1 $ (50.3) $ (1,223.6) OCI before reclassifications — (4.5) 10.6 — 6.1 Amounts reclassified from AOCI (a) 37.0 (b) — (c) (4.7) (d) 24.1 56.4 Net current-period OCI 37.0 (4.5) 5.9 24.1 62.5 Balance, September 30, 2021 $ (1,082.9) $ (60.0) $ 8.0 $ (26.2) $ (1,161.1) Attributable to noncontrolling interests: Balance, December 31, 2020 $ — $ 21.2 $ — $ — $ 21.2 OCI before reclassifications — 1.7 — — 1.7 Amounts reclassified from AOCI — (b) — — — — Net current-period OCI — 1.7 — — 1.7 Balance, September 30, 2021 $ — $ 22.9 $ — $ — $ 22.9 (a) Amounts were included in net periodic benefit cost for pension and other postretirement benefit plans (see Note 12). (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, 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 12). (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, 2021 and 2020 were as follows: Amounts reclassified to AOCI Details about AOCI Components (In millions) Three months ended September 30, 2021 Three months ended September 30, 2020 Nine months ended September 30, 2021 Nine months ended September 30, 2020 Affected line item in the statements Postretirement benefit plans Prior service credit $ 0.4 $ 0.8 $ 1.4 $ 2.3 (a) Actuarial losses (22.5) (21.3) (66.9) (63.9) (a) Settlement gain 21.9 — 21.9 — (a) (0.2) (20.5) (43.6) (61.6) (c) Total before tax 3.8 (4.9) (6.6) (14.7) Tax expense (benefit) (d) $ (4.0) $ (15.6) $ (37.0) $ (46.9) Net of tax Derivatives Nickel and other raw material contracts $ 2.0 $ 0.9 $ 5.4 $ (1.7) (b) Natural gas contracts 1.3 (1.1) 1.6 (3.7) (b) Foreign exchange contracts — — — (0.1) (b) Interest rate swap (0.3) (0.2) (0.8) (0.9) (b) 3.0 (0.4) 6.2 (6.4) (c) Total before tax 0.7 (0.1) 1.5 (1.5) Tax expense (benefit) (d) $ 2.3 $ (0.3) $ 4.7 $ (4.9) Net of tax (a) Amounts are reported in nonoperating retirement benefit expense (see Note 12). (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 9). (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, 2021
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, 2021, the Company’s reserves for environmental remediation obligations totaled approximately $13 million, of which $5 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; $8 million for formerly owned or operated sites for which the Company has remediation or indemnification obligations; and $2 million for owned or controlled sites at which Company operations have been or plan to be discontinued. 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 $16 million. Future investigation or remediation activities may result in the discovery of additional hazardous materials or potentially higher levels of contamination than discovered during prior investigation, and may impact costs associated with 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.

Accounting Policies (Policies)

Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2021
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. These unaudited consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto included in the Company’s 2020 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, 2020 financial information has been derived from the Company’s audited consolidated financial statements.
New Accounting Pronouncements Adopted	New Accounting Pronouncements Adopted In August 2020, the Financial Accounting Standards Board 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. The Company adopted this new accounting guidance related to accounting for convertible instruments effective January 1, 2021 using the modified transition approach with the cumulative effect recognized as an adjustment to the opening balance of retained earnings. This new guidance is applicable to the Company’s 3.5% Convertible Senior Notes due 2025 (the 2025 Convertible Notes) that were issued in June 2020, for which the embedded conversion option was required to be separately accounted for as a component of stockholders’ equity. Upon adoption on January 1, 2021, long-term debt increased by $45.4 million and stockholders’ equity decreased by the same amount, representing the net impact of two adjustments: (1) the $49.8 million value of the embedded conversion, which is net of allocated offering costs, previously classified in additional paid-in capital in stockholders’ equity, and (2) a $4.4 million increase to retained earnings for the cumulative effect of adoption primarily related to the non-cash interest expense recorded in fiscal year 2020 for the amortization of the portion of the 2025 Convertible Notes allocated to stockholders’ equity. Prospectively, the reported interest expense for the 2025 Convertible Notes will no longer include the non-cash interest expense of the equity component as required under prior accounting standards and will be closer to the 3.5% cash coupon rate. There was no impact to the Company’s earnings per share calculation as it previously applied the if-converted method to the 2025 Convertible Notes given ATI’s flexibility to settle conversions of the 2025 Convertible Notes in cash, shares of ATI’s common stock or a combination thereof, at ATI’s election.
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 the 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, 2021, 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 4 million pounds of nickel with hedge dates through 2024. The aggregate notional amount hedged is approximately 10% of a single year’s estimated nickel raw material purchase requirements. At September 30, 2021, the outstanding financial derivatives used to hedge the Company’s exposure to energy cost volatility included natural gas cost hedges. At September 30, 2021, the Company hedged approximately 80% of its forecasted domestic requirements for natural gas for the remainder of 2021, approximately 75% for 2022, approximately 35% for 2023 and approximately 5% for 2024. 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, 2021, 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, was 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 of 1986, as amended (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, 2021
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 the carrying value of LIFO inventory to current replacement cost.

Derivative Instruments and Hedg

Derivative Instruments and Hedging Activities (Policies)	9 Months Ended
	Sep. 30, 2021
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, 2021, 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 4 million pounds of nickel with hedge dates through 2024. The aggregate notional amount hedged is approximately 10% of a single year’s estimated nickel raw material purchase requirements. At September 30, 2021, the outstanding financial derivatives used to hedge the Company’s exposure to energy cost volatility included natural gas cost hedges. At September 30, 2021, the Company hedged approximately 80% of its forecasted domestic requirements for natural gas for the remainder of 2021, approximately 75% for 2022, approximately 35% for 2023 and approximately 5% for 2024. 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, 2021, 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, was 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, 2021
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, 2021
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, 2021 and 2020 were as follows: (in millions) Third quarter ended September 30, 2021 September 30, 2020 HPMC AA&S Total HPMC AA&S Total Diversified Global Markets: Aerospace & Defense $ 207.5 $ 75.4 $ 282.9 $ 171.8 $ 74.3 $ 246.1 Energy* 57.5 108.7 166.2 24.4 106.0 130.4 Automotive 2.1 89.7 91.8 1.5 57.9 59.4 Food Equipment & Appliances — 51.6 51.6 — 33.8 33.8 Electronics 0.4 34.4 34.8 0.2 45.7 45.9 Medical 16.8 17.5 34.3 10.6 16.7 27.3 Construction/Mining 5.6 19.7 25.3 4.0 24.3 28.3 Other 10.1 28.7 38.8 8.8 18.0 26.8 Total $ 300.0 $ 425.7 $ 725.7 $ 221.3 $ 376.7 $ 598.0 (in millions) Nine Months Ended September 30, 2021 September 30, 2020 HPMC AA&S Total HPMC AA&S Total Diversified Global Markets: Aerospace & Defense $ 605.9 $ 212.4 $ 818.3 $ 782.9 $ 321.3 $ 1,104.2 Energy* 136.6 297.0 433.6 71.3 397.2 468.5 Automotive 5.5 245.8 251.3 4.5 182.2 186.7 Electronics 0.9 132.8 133.7 0.7 117.9 118.6 Food Equipment & Appliances 0.1 107.6 107.7 0.1 131.3 131.4 Medical 42.6 52.7 95.3 37.9 57.9 95.8 Construction/Mining 16.8 72.9 89.7 14.3 92.4 106.7 Other 33.1 71.7 104.8 30.6 81.3 111.9 Total $ 841.5 $ 1,192.9 $ 2,034.4 $ 942.3 $ 1,381.5 $ 2,323.8 *Includes the oil & gas, downstream processing, and specialty energy markets. (in millions) Third quarter ended September 30, 2021 September 30, 2020 HPMC AA&S Total HPMC AA&S Total Primary Geographical Market: United States $ 140.8 $ 256.5 $ 397.3 $ 129.3 $ 219.3 $ 348.6 Asia 58.3 96.2 154.5 19.9 103.0 122.9 Europe 85.3 26.0 111.3 60.7 34.1 94.8 Canada 9.9 8.5 18.4 4.4 8.5 12.9 South America, Middle East and other 5.7 38.5 44.2 7.0 11.8 18.8 Total $ 300.0 $ 425.7 $ 725.7 $ 221.3 $ 376.7 $ 598.0 (in millions) Nine Months Ended September 30, 2021 September 30, 2020 HPMC AA&S Total HPMC AA&S Total Primary Geographical Market: United States $ 426.7 $ 700.2 $ 1,126.9 $ 522.8 $ 909.1 $ 1,431.9 Asia 119.0 313.5 432.5 72.6 288.3 360.9 Europe 250.7 87.1 337.8 286.2 104.1 390.3 Canada 27.4 28.3 55.7 21.1 30.0 51.1 South America, Middle East and other 17.7 63.8 81.5 39.6 50.0 89.6 Total $ 841.5 $ 1,192.9 $ 2,034.4 $ 942.3 $ 1,381.5 $ 2,323.8 Comparative information of the Company’s major high-value and standard products based on their percentages of sales is included in the following table. In conjunction with the Company’s announced ongoing exit of lower-margin standard stainless sheet products, in the fourth quarter of 2020 ATI reclassified certain items as High-Value Products within AA&S segment results. Prior period information reflects these reclassifications. Hot-Rolling and Processing Facility (HRPF) conversion service sales in the AA&S segment are excluded from this presentation. Third quarter ended September 30, 2021 September 30, 2020 HPMC AA&S Total HPMC AA&S Total Diversified Products and Services: High-Value Products Nickel-based alloys and specialty alloys 46 % 31 % 37 % 38 % 29 % 32 % Precision rolled strip products — % 33 % 19 % — % 28 % 18 % Precision forgings, castings and components 35 % — % 15 % 30 % — % 11 % Titanium and titanium-based alloys 19 % 7 % 12 % 32 % 8 % 17 % Zirconium and related alloys — % 15 % 9 % — % 16 % 10 % Total High-Value Products 100 % 86 % 92 % 100 % 81 % 88 % Standard Products Standard stainless products — % 14 % 8 % — % 19 % 12 % Total 100 % 100 % 100 % 100 % 100 % 100 % Nine Months Ended September 30, 2021 September 30, 2020 HPMC AA&S Total HPMC AA&S Total Diversified Products and Services: High-Value Products Nickel-based alloys and specialty alloys 43 % 27 % 34 % 38 % 29 % 34 % Precision rolled strip products — % 34 % 19 % — % 22 % 13 % Precision forgings, castings and components 37 % — % 16 % 34 % — % 14 % Titanium and titanium-based alloys 20 % 7 % 12 % 28 % 10 % 18 % Zirconium and related alloys — % 17 % 10 % — % 14 % 9 % Total High-Value Products 100 % 85 % 91 % 100 % 75 % 88 % Standard Products Standard stainless products — % 15 % 9 % — % 25 % 12 % 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 $ 4.3 $ 4.6 Expense to increase the reserve 0.4 0.1 Write-off of uncollectible accounts (0.6) (0.4) Balance as of period end $ 4.1 $ 4.3
Schedule of Contract Assets and Liabilities	(in millions) Contract Assets Short-term September 30, September 30, Balance as of beginning of fiscal year $ 38.9 $ 38.5 Recognized in current year 74.3 55.8 Reclassified to accounts receivable (49.0) (52.7) Impairment — — Reclassification to/from long-term and contract liability (8.5) 0.1 Balance as of period end $ 55.7 $ 41.7 Long-term September 30, September 30, Balance as of beginning of fiscal year $ — $ 0.1 Recognized in current year — — Reclassified to accounts receivable — — Impairment — — Reclassification to/from short-term — (0.1) Balance as of period end $ — $ — (in millions) Contract Liabilities Short-term September 30, September 30, Balance as of beginning of fiscal year $ 111.8 $ 78.7 Recognized in current year 102.0 121.5 Amounts in beginning balance reclassified to revenue (74.4) (42.9) Current year amounts reclassified to revenue (46.2) (65.6) Divestiture (0.8) — Other 0.2 — Reclassification to/from long-term and contract asset (5.9) 6.1 Balance as of period end $ 86.7 $ 97.8 Long-term September 30, September 30, Balance as of beginning of fiscal year $ 32.0 $ 25.9 Recognized in current year 45.4 12.9 Amounts in beginning balance reclassified to revenue (0.8) (0.8) Current year amounts reclassified to revenue — — Other — — Reclassification to/from short-term (2.6) (6.1) Balance as of period end $ 74.0 $ 31.9

Inventories (Tables)

Inventories (Tables)	9 Months Ended
	Sep. 30, 2021
Inventory Disclosure [Abstract]
Schedule of Inventories	Inventories at September 30, 2021 and December 31, 2020 were as follows (in millions): September 30, December 31, Raw materials and supplies $ 195.5 $ 207.6 Work-in-process 804.0 690.7 Finished goods 125.1 181.6 Total inventories at current cost 1,124.6 1,079.9 Adjustment from current cost to LIFO cost basis 0.3 44.1 Inventory valuation reserves (69.1) (126.9) Total inventories, net $ 1,055.8 $ 997.1
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, 2021 2020 LIFO benefit (charge) $ (43.8) $ 7.5 NRV benefit (charge) 43.8 (7.5) Net cost of sales impact $ — $ —

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	9 Months Ended
	Sep. 30, 2021
Property, Plant and Equipment [Abstract]
Property Plant And Equipment	Property, plant and equipment at September 30, 2021 and December 31, 2020 was as follows (in millions): September 30, December 31, Land $ 34.9 $ 34.8 Buildings 572.1 564.7 Equipment and leasehold improvements 2,804.9 2,736.9 3,411.9 3,336.4 Accumulated depreciation and amortization (1,924.3) (1,867.2) Total property, plant and equipment, net $ 1,487.6 $ 1,469.2

Supplemental Financial Statem_2

Supplemental Financial Statement Information - (Tables)	9 Months Ended
	Sep. 30, 2021
Other Income and Expenses [Abstract]
Schedule of Other Expense, Net	Other income (expense), net for the three and nine months ended September 30, 2021 and 2020 was as follows: (in millions) Three months ended September 30, Nine months ended September 30, 2021 2020 2021 2020 Rent and royalty income $ 0.2 $ 0.1 $ 0.7 $ 0.6 Gains from disposal of property, plant and equipment, net 0.1 0.8 2.5 3.3 Net equity income (loss) on joint ventures (See Note 6) 0.4 (1.0) 0.4 (6.6) Joint venture restructuring charges (See Note 6) — — — (2.4) Gain from sale of business (See Note 5) 13.7 — 13.7 — Adjustment to indemnification for conditional ARO costs — — — 4.3 Other 0.1 (0.3) 0.1 — Total other income (expense), net $ 14.5 $ (0.4) $ 17.4 $ (0.8)
Roll Forward of Restructuring Reserve Activity	Restructuring reserves for severance cost activity is as follows: Severance and Employee Benefit Costs Balance at December 31, 2020 $ 43.4 Adjustments (9.2) Payments (12.4) Balance at September 30, 2021 $ 21.8

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2021
Debt Disclosure [Abstract]
Schedule of Debt Instruments	Debt at September 30, 2021 and December 31, 2020 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 5.125% Notes due 2031 350.0 — Allegheny Technologies 4.875% Notes due 2029 325.0 — Allegheny Technologies 3.5% Convertible Senior Notes due 2025 291.4 291.4 Allegheny Technologies 4.75% Convertible Senior Notes due 2022 84.2 84.2 Allegheny Ludlum 6.95% Debentures due 2025 (b) 150.0 150.0 Term Loan due 2024 200.0 200.0 U.S. revolving credit facility — — Foreign credit facilities 9.2 5.5 Finance leases and other 53.5 48.0 Debt issuance costs (22.8) (14.5) Equity component of convertible debt — (46.8) Total debt 2,290.5 1,567.8 Short-term debt and current portion of long-term debt 606.9 17.8 Total long-term debt $ 1,683.6 $ 1,550.0 (a) Bearing interest at 7.875% effective February 15, 2016. (b) The payment obligations of these debentures issued by Allegheny Ludlum, LLC are fully and unconditionally guaranteed by ATI.
Interest Income and Interest Expense Disclosure	Interest expense on the 2025 Convertible Notes was as follows: Three months ended September 30, Nine months ended September 30, (in millions) 2021 2020 2021 2020 Contractual coupon rate $ 2.5 $ 2.4 $ 7.6 $ 2.4 Amortization of debt issuance costs 0.5 0.4 1.3 0.4 Total interest expense $ 3.0 $ 2.8 $ 8.9 $ 2.8 Three months ended September 30, Nine months ended September 30, (in millions) 2021 2020 2021 2020 Contractual coupon rate $ 1.0 $ 1.0 $ 3.0 $ 7.6 Amortization of debt issuance costs 0.2 0.1 0.4 0.9 Total interest expense $ 1.2 $ 1.1 $ 3.4 $ 8.5

Derivative Financial Instrume_2

Derivative Financial Instruments and Hedging (Tables)	9 Months Ended
	Sep. 30, 2021
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 $ 2.5 $ 3.7 Natural gas contracts Prepaid expenses and other current assets 8.0 0.2 Nickel and other raw material contracts Other assets 0.2 0.7 Natural gas contracts Other assets 1.5 0.2 Total derivatives designated as hedging instruments $ 12.2 $ 4.8 Liability derivatives Balance sheet location Derivatives designated as hedging instruments: Interest rate swap Other current liabilities $ 1.0 $ 1.0 Natural gas contracts Other current liabilities 0.3 0.3 Nickel and other raw material contracts Other current liabilities 0.6 0.1 Interest rate swap Other long-term liabilities 1.4 2.5 Natural gas contracts Other long-term liabilities 0.2 0.1 Nickel and other raw material contracts Other long-term liabilities 0.1 — Total derivatives designated as hedging instruments $ 3.6 $ 4.0
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, 2021 and 2020 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 2021 2020 2021 2020 Nickel and other raw material contracts $ (0.2) $ 2.1 $ 1.5 $ 0.7 Natural gas contracts 5.6 2.5 1.0 (0.8) Foreign exchange contracts — — — — Interest rate swap (0.2) 0.1 (0.2) (0.2) Total $ 5.2 $ 4.7 $ 2.3 $ (0.3) Amount of Gain (Loss) Amount of Gain (Loss) Nine months ended September 30, Nine months ended September 30, Derivatives in Cash Flow Hedging Relationships 2021 2020 2021 2020 Nickel and other raw material contracts $ 2.6 $ (2.1) $ 4.1 $ (1.3) Natural gas contracts 8.0 1.2 1.2 (2.8) Foreign exchange contracts 0.1 (0.1) — (0.1) Interest rate swap (0.1) (2.1) (0.6) (0.7) Total $ 10.6 $ (3.1) $ 4.7 $ (4.9)

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Fair Value By Balance Sheet Grouping	The estimated fair value of financial instruments at September 30, 2021 was as follows: Fair Value Measurements at Reporting Date Using (In millions) Total Total Quoted Prices in Significant Cash and cash equivalents $ 1,006.8 $ 1,006.8 $ 1,006.8 $ — Derivative financial instruments: Assets 12.2 12.2 — 12.2 Liabilities 3.6 3.6 — 3.6 Debt (a) 2,313.3 2,558.7 2,296.0 262.7 The estimated fair value of financial instruments at December 31, 2020 was as follows: Fair Value Measurements at Reporting Date Using (In millions) Total Total Quoted Prices in Significant Cash and cash equivalents $ 645.9 $ 645.9 $ 645.9 $ — Derivative financial instruments: Assets 4.8 4.8 — 4.8 Liabilities 4.0 4.0 — 4.0 Debt (a) 1,629.1 1,847.7 1,594.2 253.5 (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 December 31, 2020 debt carrying value includes $46.8 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. Effective January 1, 2021, ATI early-adopted new accounting guidance that eliminated the equity component classification of the embedded conversion option of the 2025 Convertible Notes (see Note 1).

Retirement Benefits (Tables)

Retirement Benefits (Tables)	9 Months Ended
	Sep. 30, 2021
Retirement Benefits [Abstract]
Schedule Of Defined Benefit Plans Disclosures [Table Text Block]	For the three month periods ended September 30, 2021 and 2020, the components of pension and other postretirement benefit expense (income) 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, 2021 2020 2021 2020 Service cost - benefits earned during the year $ 3.8 $ 3.2 $ 0.4 $ 0.6 Interest cost on benefits earned in prior years 17.1 21.6 1.9 2.6 Expected return on plan assets (34.1) (33.6) — — Amortization of prior service cost (credit) 0.2 0.1 (0.6) (0.9) Amortization of net actuarial loss 18.9 18.6 3.6 2.7 Settlement gain — — (64.9) — Total retirement benefit expense (income) $ 5.9 $ 9.9 $ (59.6) $ 5.0 For the nine month periods ended September 30, 2021 and 2020, 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, 2021 2020 2021 2020 Service cost - benefits earned during the year $ 11.3 $ 9.5 $ 1.1 $ 1.7 Interest cost on benefits earned in prior years 51.3 64.7 6.1 8.0 Expected return on plan assets (102.3) (100.8) — — Amortization of prior service cost (credit) 0.5 0.5 (1.9) (2.8) Amortization of net actuarial loss 56.7 55.8 10.2 8.1 Settlement gain — — (64.9) — Total retirement benefit expense (income) $ 17.5 $ 29.7 $ (49.4) $ 15.0

Business Segments (Tables)

Business Segments (Tables)	9 Months Ended
	Sep. 30, 2021
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, 2021 2020 2021 2020 Total sales: High Performance Materials & Components $ 325.0 $ 230.1 $ 908.9 $ 995.3 Advanced Alloys & Solutions 464.7 393.3 1,280.1 1,487.1 789.7 623.4 2,189.0 2,482.4 Intersegment sales: High Performance Materials & Components 25.0 8.8 67.4 53.0 Advanced Alloys & Solutions 39.0 16.6 87.2 105.6 64.0 25.4 154.6 158.6 Sales to external customers: High Performance Materials & Components 300.0 221.3 841.5 942.3 Advanced Alloys & Solutions 425.7 376.7 1,192.9 1,381.5 $ 725.7 $ 598.0 $ 2,034.4 $ 2,323.8 Three months ended September 30, Nine months ended September 30, 2021 2020 2021 2020 EBITDA: High Performance Materials & Components $ 37.4 $ 16.8 $ 99.2 $ 122.1 Advanced Alloys & Solutions 56.8 11.0 142.5 85.5 Total segment EBITDA 94.2 27.8 241.7 207.6 LIFO and net realizable value reserves — — — — Corporate expenses (12.9) (10.2) (41.0) (29.7) Closed operations and other expense (1.4) (1.0) (4.5) (4.6) Total ATI Adjusted EBITDA 79.9 16.6 196.2 173.3 Depreciation & amortization (a) (35.6) (35.4) (108.0) (108.3) Interest expense, net (25.1) (25.1) (72.2) (68.7) Restructuring and other credits (charges) (See Note 7) 2.3 (2.3) 8.5 (27.0) Strike related costs (22.9) — (63.2) — Retirement benefit settlement gain (See Note 12) 64.9 — 64.9 — Impairment of goodwill — — — (287.0) Joint venture restructuring charge (See Note 6) — — — (2.4) Debt extinguishment charge (See Note 8) — — — (21.5) Gain on asset sales and sale of business (See Notes 5 and 6) 13.7 — 13.7 2.5 Income (loss) before income taxes $ 77.2 $ (46.2) $ 39.9 $ (339.1) (a) The following is depreciation & amortization by each business segment: Three months ended September 30, Nine months ended September 30, 2021 2020 2021 2020 High Performance Materials & Components $ 18.2 $ 19.5 $ 57.0 $ 58.5 Advanced Alloys & Solutions 16.3 15.1 47.9 47.4 Other 1.1 0.8 3.1 2.4 $ 35.6 $ 35.4 108.0 108.3

Per Share Information (Tables)

Per Share Information (Tables)	9 Months Ended
	Sep. 30, 2021
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: (In millions, except per share amounts) Three months ended September 30, Nine months ended September 30, 2021 2020 2021 2020 Numerator: Numerator for basic income (loss) per common share – Net income (loss) attributable to ATI $ 48.7 $ (50.1) $ (8.4) $ (451.6) Effect of dilutive securities: 4.75% Convertible Senior Notes due 2022 1.1 — — — 3.5% Convertible Senior Notes due 2025 2.9 — — — Numerator for diluted income (loss) per common share – Net income (loss) attributable to ATI after assumed conversions $ 52.7 $ (50.1) $ (8.4) $ (451.6) Denominator: Denominator for basic net income (loss) per common share – weighted average shares 127.2 126.6 127.0 126.5 Effect of dilutive securities: Share-based compensation 0.8 — — — 4.75% Convertible Senior Notes due 2022 5.8 — — — 3.5% Convertible Senior Notes due 2025 18.8 — — — Denominator for diluted net income (loss) per common share – adjusted weighted average shares and assumed conversions 152.6 126.6 127.0 126.5 Basic net income (loss) attributable to ATI per common share $ 0.38 $ (0.40) $ (0.07) $ (3.57) Diluted net income (loss) attributable to ATI per common share $ 0.35 $ (0.40) $ (0.07) $ (3.57)

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Income (Loss) (Tables)	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Schedule Of Accumulated Other Comprehensive Income Loss	The changes in AOCI by component, net of tax, for the three month period ended September 30, 2021 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, June 30, 2021 $ (1,086.9) $ (55.8) $ 5.1 $ (38.7) $ (1,176.3) OCI before reclassifications — (4.2) 5.2 — 1.0 Amounts reclassified from AOCI (a) 4.0 (b) — (c) (2.3) (d) 12.5 14.2 Net current-period OCI 4.0 (4.2) 2.9 12.5 15.2 Balance, September 30, 2021 $ (1,082.9) $ (60.0) $ 8.0 $ (26.2) $ (1,161.1) Attributable to noncontrolling interests: Balance, June 30, 2021 $ — $ 23.7 $ — $ — $ 23.7 OCI before reclassifications — (0.8) — — (0.8) Amounts reclassified from AOCI — (b) — — — — Net current-period OCI — (0.8) — — (0.8) Balance, September 30, 2021 $ — $ 22.9 $ — $ — $ 22.9 The changes in AOCI by component, net of tax, for the nine month period ended September 30, 2021 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, December 31, 2020 $ (1,119.9) $ (55.5) $ 2.1 $ (50.3) $ (1,223.6) OCI before reclassifications — (4.5) 10.6 — 6.1 Amounts reclassified from AOCI (a) 37.0 (b) — (c) (4.7) (d) 24.1 56.4 Net current-period OCI 37.0 (4.5) 5.9 24.1 62.5 Balance, September 30, 2021 $ (1,082.9) $ (60.0) $ 8.0 $ (26.2) $ (1,161.1) Attributable to noncontrolling interests: Balance, December 31, 2020 $ — $ 21.2 $ — $ — $ 21.2 OCI before reclassifications — 1.7 — — 1.7 Amounts reclassified from AOCI — (b) — — — — Net current-period OCI — 1.7 — — 1.7 Balance, September 30, 2021 $ — $ 22.9 $ — $ — $ 22.9 (a) Amounts were included in net periodic benefit cost for pension and other postretirement benefit plans (see Note 12). (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, 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 12). (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.
Reclassification out of Accumulated Other Comprehensive Income	Reclassifications out of AOCI for the three and nine month periods ended September 30, 2021 and 2020 were as follows: Amounts reclassified to AOCI Details about AOCI Components (In millions) Three months ended September 30, 2021 Three months ended September 30, 2020 Nine months ended September 30, 2021 Nine months ended September 30, 2020 Affected line item in the statements Postretirement benefit plans Prior service credit $ 0.4 $ 0.8 $ 1.4 $ 2.3 (a) Actuarial losses (22.5) (21.3) (66.9) (63.9) (a) Settlement gain 21.9 — 21.9 — (a) (0.2) (20.5) (43.6) (61.6) (c) Total before tax 3.8 (4.9) (6.6) (14.7) Tax expense (benefit) (d) $ (4.0) $ (15.6) $ (37.0) $ (46.9) Net of tax Derivatives Nickel and other raw material contracts $ 2.0 $ 0.9 $ 5.4 $ (1.7) (b) Natural gas contracts 1.3 (1.1) 1.6 (3.7) (b) Foreign exchange contracts — — — (0.1) (b) Interest rate swap (0.3) (0.2) (0.8) (0.9) (b) 3.0 (0.4) 6.2 (6.4) (c) Total before tax 0.7 (0.1) 1.5 (1.5) Tax expense (benefit) (d) $ 2.3 $ (0.3) $ 4.7 $ (4.9) Net of tax (a) Amounts are reported in nonoperating retirement benefit expense (see Note 12). (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 9). (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, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)segment	Sep. 30, 2020USD ($)	Jun. 30, 2021USD ($)	Jan. 01, 2021USD ($)	Dec. 31, 2020USD ($)	Jun. 30, 2020USD ($)	Dec. 31, 2019USD ($)
Revenue recognition [Line Items]
Number of business segments | segment			2
Impact of new accounting principle on prior period operating income	$ 29.9	$ (9.6)	$ 50.4	$ (214.6)
Unfavorable impact on pre-tax operating results	(77.2)	46.2	(39.9)	339.1
Equity adjustment for adoption	680.2	1,849.8	680.2	1,849.8	$ 606.1		$ 641.4	$ 1,842.3	$ 2,193.2
Additional Paid-in Capital [Member]
Revenue recognition [Line Items]
Equity adjustment for adoption	1,592.1	1,622.2	1,592.1	1,622.2	1,587.5		1,625.5	1,617.8	1,618
[RetainedEarningsMember]
Revenue recognition [Line Items]
Equity adjustment for adoption	$ 102.5	$ 1,227.4	$ 102.5	$ 1,227.4	$ 53.8		$ 106.5	$ 1,277.6	$ 1,679.3
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%				3.50%
Accounting Standards Update 2020-06 | Additional Paid-in Capital [Member]
Revenue recognition [Line Items]
Equity adjustment for adoption						$ (49.8)
Accounting Standards Update 2020-06 | [RetainedEarningsMember]
Revenue recognition [Line Items]
Equity adjustment for adoption						4.4
Accounting Standards Update 2020-06 | Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt
Revenue recognition [Line Items]
Equity adjustment for adoption						$ (45.4)
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, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)segment	Sep. 30, 2020USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)
Revenue recognition [Line Items]
Inventories, net	$ 1,055,800,000		$ 1,055,800,000		$ 997,100,000
Adjustment from current cost to LIFO cost basis	(300,000)		(300,000)		(44,100,000)
Sales	725,700,000	$ 598,000,000	2,034,400,000	$ 2,323,800,000
Cost of sales	643,200,000	559,900,000	1,823,400,000	2,076,200,000
Short-term contract assets	55,700,000	41,700,000	55,700,000	41,700,000	38,900,000	$ 38,500,000
Long-term contract assets	0	0	0	0	0	100,000
Other current liabilities	223,000,000		223,000,000		233,100,000
Other long-term liabilities	220,100,000		220,100,000		189,900,000
Short-term contract liabilities	86,700,000	97,800,000	86,700,000	97,800,000	111,800,000	78,700,000
Long-term contract liabilities	74,000,000	31,900,000	$ 74,000,000	31,900,000	32,000,000	$ 25,900,000
Number of business segments | segment			2
Revenue, Performance Obligation [Abstract]
Confirmed order backlog	1,700,000,000	1,400,000,000	$ 1,700,000,000	1,400,000,000
Confirmed orders with current performance obligations	0.80		0.80
Accounts receivable with customers	506,100,000		506,100,000		350,100,000
Contract costs for obtaining and fulfilling contracts	5,300,000		5,300,000		$ 5,400,000
Amortization of contract costs	$ 200,000	$ 400,000	$ 700,000	$ 1,100,000

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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue		$ 725.7	$ 598	$ 2,034.4	$ 2,323.8
Percent of revenue		100.00%	100.00%	100.00%	100.00%
Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of revenue		92.00%	88.00%	91.00%	88.00%
Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		37.00%	32.00%	34.00%	34.00%
Precision rolled strip products
Disaggregation of Revenue [Line Items]
Percent of revenue		19.00%	18.00%	19.00%	13.00%
Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue		15.00%	11.00%	16.00%	14.00%
Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		12.00%	17.00%	12.00%	18.00%
Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		9.00%	10.00%	10.00%	9.00%
Standard stainless products
Disaggregation of Revenue [Line Items]
Percent of revenue		8.00%	12.00%	9.00%	12.00%
United States
Disaggregation of Revenue [Line Items]
Revenue		$ 397.3	$ 348.6	$ 1,126.9	$ 1,431.9
Asia
Disaggregation of Revenue [Line Items]
Revenue		154.5	122.9	432.5	360.9
Europe
Disaggregation of Revenue [Line Items]
Revenue		111.3	94.8	337.8	390.3
Canada
Disaggregation of Revenue [Line Items]
Revenue		18.4	12.9	55.7	51.1
South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		44.2	18.8	81.5	89.6
Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		282.9	246.1	818.3	1,104.2
Energy
Disaggregation of Revenue [Line Items]
Revenue	[1]	166.2	130.4	433.6	468.5
Automotive
Disaggregation of Revenue [Line Items]
Revenue		91.8	59.4	251.3	186.7
Electronics
Disaggregation of Revenue [Line Items]
Revenue		34.8	45.9	133.7	118.6
Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		25.3	28.3	89.7	106.7
Food Equipment and Appliances Market
Disaggregation of Revenue [Line Items]
Revenue		51.6	33.8	107.7	131.4
Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue		34.3	27.3	95.3	95.8
Other
Disaggregation of Revenue [Line Items]
Revenue		38.8	26.8	104.8	111.9
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		789.7	623.4	2,189	2,482.4
Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue		$ 325	$ 230.1	$ 908.9	$ 995.3
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		46.00%	38.00%	43.00%	38.00%
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 | Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue		35.00%	30.00%	37.00%	34.00%
Operating Segments | High Performance Materials & Components | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		19.00%	32.00%	20.00%	28.00%
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
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		$ 140.8	$ 129.3	$ 426.7	$ 522.8
Operating Segments | High Performance Materials & Components | Asia
Disaggregation of Revenue [Line Items]
Revenue		58.3	19.9	119	72.6
Operating Segments | High Performance Materials & Components | Europe
Disaggregation of Revenue [Line Items]
Revenue		85.3	60.7	250.7	286.2
Operating Segments | High Performance Materials & Components | Canada
Disaggregation of Revenue [Line Items]
Revenue		9.9	4.4	27.4	21.1
Operating Segments | High Performance Materials & Components | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		5.7	7	17.7	39.6
Operating Segments | High Performance Materials & Components | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		207.5	171.8	605.9	782.9
Operating Segments | High Performance Materials & Components | Energy
Disaggregation of Revenue [Line Items]
Revenue	[1]	57.5	24.4	136.6	71.3
Operating Segments | High Performance Materials & Components | Automotive
Disaggregation of Revenue [Line Items]
Revenue		2.1	1.5	5.5	4.5
Operating Segments | High Performance Materials & Components | Electronics
Disaggregation of Revenue [Line Items]
Revenue		0.4	0.2	0.9	0.7
Operating Segments | High Performance Materials & Components | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		5.6	4	16.8	14.3
Operating Segments | High Performance Materials & Components | Food Equipment and Appliances Market
Disaggregation of Revenue [Line Items]
Revenue		0	0	0.1	0.1
Operating Segments | High Performance Materials & Components | Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue		16.8	10.6	42.6	37.9
Operating Segments | High Performance Materials & Components | Other
Disaggregation of Revenue [Line Items]
Revenue		10.1	8.8	33.1	30.6
Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue		$ 464.7	$ 393.3	$ 1,280.1	$ 1,487.1
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		86.00%	81.00%	85.00%	75.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		31.00%	29.00%	27.00%	29.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Precision rolled strip products
Disaggregation of Revenue [Line Items]
Percent of revenue		33.00%	28.00%	34.00%	22.00%
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		7.00%	8.00%	7.00%	10.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		15.00%	16.00%	17.00%	14.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Standard stainless products
Disaggregation of Revenue [Line Items]
Percent of revenue		14.00%	19.00%	15.00%	25.00%
Operating Segments | Advanced Alloys & Solutions [Member] | United States
Disaggregation of Revenue [Line Items]
Revenue		$ 256.5	$ 219.3	$ 700.2	$ 909.1
Operating Segments | Advanced Alloys & Solutions [Member] | Asia
Disaggregation of Revenue [Line Items]
Revenue		96.2	103	313.5	288.3
Operating Segments | Advanced Alloys & Solutions [Member] | Europe
Disaggregation of Revenue [Line Items]
Revenue		26	34.1	87.1	104.1
Operating Segments | Advanced Alloys & Solutions [Member] | Canada
Disaggregation of Revenue [Line Items]
Revenue		8.5	8.5	28.3	30
Operating Segments | Advanced Alloys & Solutions [Member] | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		38.5	11.8	63.8	50
Operating Segments | Advanced Alloys & Solutions [Member] | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		75.4	74.3	212.4	321.3
Operating Segments | Advanced Alloys & Solutions [Member] | Energy
Disaggregation of Revenue [Line Items]
Revenue	[1]	108.7	106	297	397.2
Operating Segments | Advanced Alloys & Solutions [Member] | Automotive
Disaggregation of Revenue [Line Items]
Revenue		89.7	57.9	245.8	182.2
Operating Segments | Advanced Alloys & Solutions [Member] | Electronics
Disaggregation of Revenue [Line Items]
Revenue		34.4	45.7	132.8	117.9
Operating Segments | Advanced Alloys & Solutions [Member] | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		19.7	24.3	72.9	92.4
Operating Segments | Advanced Alloys & Solutions [Member] | Food Equipment and Appliances Market
Disaggregation of Revenue [Line Items]
Revenue		51.6	33.8	107.6	131.3
Operating Segments | Advanced Alloys & Solutions [Member] | Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue		17.5	16.7	52.7	57.9
Operating Segments | Advanced Alloys & Solutions [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue		28.7	18	71.7	81.3
External Customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		725.7	598	2,034.4	2,323.8
External Customers | Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue		300	221.3	841.5	942.3
External Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue		$ 425.7	$ 376.7	$ 1,192.9	$ 1,381.5
[1]	Includes the oil & gas, downstream processing, and specialty 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, 2021	Sep. 30, 2020
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of beginning of fiscal year	$ 4.3	$ 4.6
Expense to increase the reserve	0.4	0.1
Write-off of uncollectible accounts	(0.6)	(0.4)
Balance as of period end	$ 4.1	$ 4.3

Revenue from Contracts with C_6

Revenue from Contracts with Customers - Contract Assets and Liabilities (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Contract Assets, Current [Roll Forward]
Balance as of beginning of fiscal year	$ 38.9	$ 38.5
Recognized in current year	74.3	55.8
Reclassified to accounts receivable	(49)	(52.7)
Impairment	0	0
Reclassification to/from long-term and contract liability	(8.5)	0.1
Balance as of period end	55.7	41.7
Contract Assets, Noncurrent [Roll Forward]
Balance as of beginning of fiscal year	0	0.1
Recognized in current year	0	0
Reclassified to accounts receivable	0	0
Impairment	0	0
Reclassification to/from short-term	0	(0.1)
Balance as of period end	0	0
Contract Liabilities, Current [Roll Forward]
Balance as of beginning of fiscal year	111.8	78.7
Recognized in current year	102	121.5
Amounts in beginning balance reclassified to revenue	(74.4)	(42.9)
Current year amounts reclassified to revenue	(46.2)	(65.6)
Divestiture	(0.8)	0
Other	0.2	0
Reclassification to/from long-term and contract asset	(5.9)	6.1
Balance as of period end	86.7	97.8
Contract Liabilities, Noncurrent [Roll Forward]
Balance as of beginning of fiscal year	32	25.9
Recognized in current year	45.4	12.9
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	(2.6)	(6.1)
Balance as of period end	$ 74	$ 31.9

Inventories (Details)

Inventories (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Inventory [Line Items]
Raw materials and supplies	$ 195.5		$ 195.5		$ 207.6
Work-in-process	804		804		690.7
Finished goods	125.1		125.1		181.6
Total inventories at current cost	1,124.6		1,124.6		1,079.9
Adjustment from current cost to LIFO cost basis	0.3		0.3		44.1
Inventory valuation reserves	(69.1)		(69.1)		(126.9)
Total inventories, net	1,055.8		1,055.8		997.1
LIFO benefit (charge)			(43.8)	$ 7.5
NRV benefit (charge)			43.8	(7.5)
Net cost of sales impact	0	$ 0	0	$ 0
Net Realizable Value Reserve [Member]
Inventory [Line Items]
Inventory valuation reserves	$ (0.3)		$ (0.3)		$ (44.1)

Property, Plant and Equipment_2

Property, Plant and Equipment (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Land	$ 34.9	$ 34.8
Buildings	572.1	564.7
Equipment and leasehold improvements	2,804.9	2,736.9
Property Plant And Equipment, gross	3,411.9	3,336.4
Accumulated depreciation and amortization	(1,924.3)	(1,867.2)
Total property, plant and equipment, net	1,487.6	$ 1,469.2
Construction in progress	$ 214.8

Divestitures (Details)

Divestitures (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Aug. 13, 2021
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of business, net of transaction costs		$ 53	$ 0
Goodwill	$ 229	229		$ 240.7
Flowform Products
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total purchase consideration					$ 55
Proceeds from sale of business, net of transaction costs	53
Goodwill	12.2	$ 12.2
Gain on sale of business	$ 13.7
Product sales, disposal group				$ 26

Joint Ventures (Details)

Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		10 Months Ended	45 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2018	Dec. 31, 2021	Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 1,006.8		$ 1,006.8				$ 645.9
Income (loss) from equity investments	0.4	$ (1)	0.4	$ (6.6)
Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 1.4		$ 1.4
Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%		50.00%
Sales to noncontrolling interests					$ 12
Allegheny & Tsingshan Stainless | Employee Severance [Member]
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	$ 0	0	$ 0	(2.4)
Allegheny & Tsingshan Stainless | Forecast
Schedule of Equity Method Investments [Line Items]
Sales to noncontrolling interests						$ 17.5
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	54.9		54.9
Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	0.3	(1.6)	(0.6)	(10.1)
Due from Affiliates	4.8		4.8				14
Advanced Alloys & Solutions [Member] | Uniti
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	0.1	$ 0.6	1	$ 1.1
Prepaid expenses and other current assets | Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from Affiliates	0.8		0.8				0.5
Other Noncurrent Assets [Member] | Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from Affiliates	$ 4		$ 4				$ 13.5
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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Other Income and Expenses [Abstract]
Rent and royalty income	$ 0.2	$ 0.1	$ 0.7	$ 0.6
Gains from disposal of property, plant and equipment, net	0.1	0.8	2.5	3.3
Net equity (loss) gain on joint ventures	0.4	(1)	0.4	(6.6)
Gain from sale of business	13.7	0	13.7	0
Adjustment to indemnification for conditional ARO costs	0	0	0	4.3
Other	0.1	(0.3)	0.1	0
Other income (expense), net	14.5	(0.4)	17.4	(0.8)
Gain on the sale of oil and gas rights				2.5
Allegheny & Tsingshan Stainless | Employee Severance [Member]
Other Income and Expenses [Abstract]
Net equity (loss) gain on joint ventures	$ 0	$ 0	$ 0	$ (2.4)

Supplemental Financial Statem_4

Supplemental Financial Statement Information - Narrative (Details) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2021USD ($)employee	Sep. 30, 2020USD ($)employee	Jun. 30, 2020employee	Mar. 31, 2020employee	Sep. 30, 2021USD ($)employee	Sep. 30, 2020USD ($)	Dec. 31, 2020USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring charges (credits)	$ (2.3)	$ 2.3			$ (8.5)	$ 27
Number of positions with decreased severance-related reserve | employee	50				250
Number of positions eliminated | employee		100	550	90
Restructuring reserve	$ 21.8				$ 21.8		$ 43.4
Facility Idling
Restructuring Cost and Reserve [Line Items]
Restructuring charges (credits)					0.7
Employee Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges (credits)					(9.2)
Other Current Liabilities
Restructuring Cost and Reserve [Line Items]
Restructuring reserve	13.4				13.4
Other Noncurrent Liabilities
Restructuring Cost and Reserve [Line Items]
Restructuring reserve	$ 8.4				$ 8.4

Supplemental Financial Statem_5

Supplemental Financial Statement Information - Severance and Employee Benefit Costs Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Restructuring Reserve [Roll Forward]
Balance at December 31, 2020			$ 43.4
Payments			(12.4)
Balance at September 30, 2021	$ 21.8		21.8
Restructuring Cost and Reserve [Line Items]
Restructuring charges (credits)	$ (2.3)	$ 2.3	(8.5)	$ 27
Employee Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges (credits)			$ (9.2)

Debt Schedule of Debt (Details)

Debt Schedule of Debt (Details) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Dec. 31, 2020	Jul. 02, 2020	Feb. 15, 2016	Dec. 31, 2013
Debt Instrument [Line Items]
Debt Issuance Costs, Net	$ (22,800,000)	$ (22,800,000)	$ (14,500,000)
Total Debt	2,290,500,000	2,290,500,000	1,567,800,000
Short-term debt and current portion of long-term debt	606,900,000	606,900,000	17,800,000
Long-term debt	1,683,600,000	1,683,600,000	1,550,000,000
Allegheny Technologies 5.875% Notes due 2023 (a)
Debt Instrument [Line Items]
Debt, Gross	$ 500,000,000	$ 500,000,000	$ 500,000,000
Debt Instrument, Issuer		Allegheny Technologies	Allegheny Technologies
Interest rate	7.875%	7.875%	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	$ 350,000,000
Debt Instrument, Issuer		Allegheny Technologies	Allegheny Technologies
Interest rate	5.875%	5.875%	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	$ 291,400,000	$ 291,400,000
Debt Issuance Costs, Net	(7,000,000)	(7,000,000)	(8,200,000)
Equity component of convertible debt	$ 0	$ 0	$ (46,800,000)	$ (51,400,000)
Debt Instrument, Issuer		Allegheny Technologies	Allegheny Technologies
Interest rate	3.50%	3.50%	3.50%
Debt Instrument, Maturity Date		Jun. 15, 2025	Jun. 15, 2025
Allegheny Technologies 4.75% Convertible Senior Notes due 2022
Debt Instrument [Line Items]
Debt, Gross	$ 84,200,000	$ 84,200,000	$ 84,200,000
Debt Instrument, Issuer		Allegheny Technologies	Allegheny Technologies
Interest rate	4.75%	4.75%	4.75%
Debt Instrument, Maturity Date		Jul. 1, 2022	Jul. 1, 2022
Allegheny Technologies 4.75% Convertible Senior Notes due 2022 | Convertible Debt
Debt Instrument [Line Items]
Debt Issuance Costs, Net	$ (400,000)	$ (400,000)	$ (700,000)
Allegheny Ludlum 6.95% Debentures due 2025 (b)
Debt Instrument [Line Items]
Debt, Gross	$ 150,000,000	$ 150,000,000	$ 150,000,000
Debt Instrument, Issuer		Allegheny Ludlum	Allegheny Ludlum
Interest rate	6.95%	6.95%	6.95%
Debt Instrument, Maturity Date		Dec. 15, 2025	Dec. 15, 2025
2024 Term Loan
Debt Instrument [Line Items]
Debt, Gross	$ 200,000,000	$ 200,000,000	$ 200,000,000
Debt Instrument, Issuer		Allegheny Technologies	Allegheny Technologies
Interest rate	4.21%	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	$ 0
Foreign credit facilities
Debt Instrument [Line Items]
Short-term Debt	9,200,000	9,200,000	5,500,000
Finance leases and other
Debt Instrument [Line Items]
Debt, Gross	53,500,000	53,500,000	48,000,000
ATI 2031 Notes
Debt Instrument [Line Items]
Debt, Gross	$ 350,000,000	$ 350,000,000	0
Debt Instrument, Issuer	Allegheny Technologies
Interest rate	5.125%	5.125%
Debt Instrument, Maturity Date		Oct. 1, 2031
ATI 2029 Notes
Debt Instrument [Line Items]
Debt, Gross	$ 325,000,000	$ 325,000,000	$ 0
Debt Instrument, Issuer	Allegheny Technologies
Interest rate	4.875%	4.875%
Debt Instrument, Maturity Date		Oct. 1, 2029

Debt - Revolving Credit Facilit

Debt - Revolving Credit Facility Narrative (Details) - USD ($)	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2024	Dec. 31, 2020	Feb. 15, 2016	Dec. 31, 2013
Allegheny Technologies 5.875% Notes due 2023 (a)
Debt Instrument [Line Items]
Outstanding borrowings	$ 500,000,000			$ 500,000,000
Interest rate	7.875%			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	$ 42,800,000
Average borrowings during period	0	$ 38,000,000
Short-term Debt	0			$ 0
Interest rate during period		2.20%
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			200,000,000
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%
Allegheny Technologies 4.75% Convertible Senior Notes due 2022
Debt Instrument [Line Items]
Outstanding borrowings	$ 84,200,000			$ 84,200,000
Interest rate	4.75%			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

Debt - Convertible Notes Narrat

Debt - Convertible Notes Narrative (Details)	3 Months Ended			9 Months Ended
	Dec. 31, 2021USD ($)	Sep. 30, 2021USD ($)tradingDays$ / shares	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)tradingDays$ / shares	Sep. 30, 2020USD ($)	Jun. 30, 2021USD ($)	Jan. 01, 2021USD ($)	Dec. 31, 2020USD ($)	Jul. 02, 2020USD ($)	Jun. 30, 2020USD ($)	Dec. 31, 2019USD ($)	Feb. 15, 2016	Dec. 31, 2013
Debt Instrument [Line Items]
Payment for early debt extinguishment				$ 0	$ 19,100,000
Debt issuance costs				9,300,000	9,100,000
Equity adjustment for adoption		$ 680,200,000	$ 1,849,800,000	680,200,000	1,849,800,000	$ 606,100,000		$ 641,400,000		$ 1,842,300,000	$ 2,193,200,000
Debt issuance costs		22,800,000		22,800,000				14,500,000
Interest expense, net		(25,100,000)	(25,100,000)	(72,200,000)	(68,700,000)
Debt extinguishment charge		0	0	0	21,500,000
Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt		2,558,700,000		2,558,700,000				1,847,700,000
Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt		2,296,000,000		2,296,000,000				1,594,200,000
Forecast
Debt Instrument [Line Items]
Payment for early debt extinguishment	$ 64,500,000
Debt extinguishment charge	65,500,000
Additional Paid-in Capital [Member]
Debt Instrument [Line Items]
Equity adjustment for adoption		1,592,100,000	$ 1,622,200,000	1,592,100,000	$ 1,622,200,000	$ 1,587,500,000		1,625,500,000		$ 1,617,800,000	$ 1,618,000,000
Additional Paid-in Capital [Member] | Accounting Standards Update 2020-06
Debt Instrument [Line Items]
Equity adjustment for adoption							$ (49,800,000)
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt
Debt Instrument [Line Items]
Debt, Gross		$ 291,400,000		$ 291,400,000				$ 291,400,000
Interest rate		3.50%		3.50%				3.50%
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 instrument, convertible, conversion ratio				0.0645745
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares		$ 15.49		$ 15.49
Debt instrument, convertible, number of equity instruments (in shares)				18,800,000
Equity component of convertible debt		$ 0		$ 0				$ 46,800,000	$ 51,400,000
Debt issuance costs		$ 7,000,000		$ 7,000,000				8,200,000
Debt instrument, interest rate, effective percentage		4.20%	8.40%	4.20%	8.40%
Cap price (in dollars per share) | $ / shares		$ 19.76		$ 19.76
Interest Expense, Debt, Excluding Amortization		$ 2,500,000	$ 2,400,000	$ 7,600,000	$ 2,400,000
Amortization of Debt Issuance Costs		500,000	400,000	1,300,000	400,000
Interest expense, net		3,000,000	$ 2,800,000	8,900,000	$ 2,800,000
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt | Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt		402,000,000		402,000,000
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt | Accounting Standards Update 2020-06
Debt Instrument [Line Items]
Equity adjustment for adoption							$ (45,400,000)
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt | Additional Paid-in Capital [Member]
Debt Instrument [Line Items]
Debt issuance costs								1,600,000
Allegheny Technologies 4.75% Convertible Senior Notes due 2022
Debt Instrument [Line Items]
Debt, Gross		$ 84,200,000		$ 84,200,000				$ 84,200,000
Interest rate		4.75%		4.75%				4.75%
Allegheny Technologies 4.75% Convertible Senior Notes due 2022 | Convertible Debt
Debt Instrument [Line Items]
Debt instrument, convertible, redemption price as a percent of principal amount				100.00%
Debt instrument, convertible, conversion ratio				0.0692042
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares		$ 14.45		$ 14.45
Debt instrument, convertible, number of equity instruments (in shares)				5,800,000
Debt issuance costs		$ 400,000		$ 400,000				$ 700,000
Debt instrument, interest rate, effective percentage		5.40%	5.40%	5.40%	5.40%
Interest Expense, Debt, Excluding Amortization		$ 1,000,000	$ 1,000,000	$ 3,000,000	$ 7,600,000
Amortization of Debt Issuance Costs		200,000	100,000	400,000	900,000
Interest expense, net		1,200,000	$ 1,100,000	3,400,000	$ 8,500,000
Debt instrument, repurchased face amount								203,200,000
Allegheny Technologies 4.75% Convertible Senior Notes due 2022 | Convertible Debt | Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt		115,000,000		115,000,000
Allegheny Technologies 5.875% Notes due 2023 (a)
Debt Instrument [Line Items]
Debt, Gross		$ 500,000,000		$ 500,000,000				$ 500,000,000
Interest rate		7.875%		7.875%				7.875%				7.875%	5.875%
Debt instrument, notice period required for redemption | tradingDays		30
Allegheny Technologies 5.875% Notes due 2023 (a) | Forecast
Debt Instrument [Line Items]
Write off of deferred debt issue costs	$ 1,000,000
ATI 2029 and 2031 Notes
Debt Instrument [Line Items]
Debt issuance costs		$ 4,700,000
Debt instrument, redemption price, percentage		101.00%
Proceeds from Debt, Net of Issuance Costs		$ 665,700,000
ATI 2029 and 2031 Notes | Minimum
Debt Instrument [Line Items]
Debt instrument, notice period required for redemption | tradingDays		15
ATI 2029 and 2031 Notes | Maximum
Debt Instrument [Line Items]
Debt instrument, notice period required for redemption | tradingDays		60
ATI 2029 Notes
Debt Instrument [Line Items]
Debt, Gross		$ 325,000,000		$ 325,000,000				$ 0
Interest rate		4.875%		4.875%
Debt instrument, amortization period		8 years
ATI 2031 Notes
Debt Instrument [Line Items]
Debt, Gross		$ 350,000,000		$ 350,000,000				$ 0
Interest rate		5.125%		5.125%
Debt instrument, amortization period		10 years

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, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)lb	Sep. 30, 2020USD ($)	Sep. 30, 2021EUR (€)
Derivative Instruments Gain Loss [Line Items]
Percentage of estimated annual nickel requirements			10.00%
Percentage Of Forecasted Natural Gas Usage Hedged for 2021			80.00%
Percentage Of Forecasted Natural Gas Usage Hedged for 2022			75.00%
Percentage Of Forecasted Natural Gas Usage Hedged for 2023			35.00%
Percentage of Forecasted Natural Gas Usage Hedged for 2024			5.00%
Reclassification of Cash Flow Hedge Gain (Loss) [Abstract]
Cash Flow Hedge Gain (Loss) To Be Reclassified Within Twelve Months			$ 8.6
Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Net derivative gain (loss) on hedge transactions	$ 5.2	$ 4.7	10.6	$ (3.1)
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	2.3	(0.3)	$ 4.7	(4.9)
Nickel
Derivative Instruments Gain Loss [Line Items]
Notional amount of nickel hedge (in pounds of nickel) | lb			4
Interest Rate Swap | Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Net derivative gain (loss) on hedge transactions	(0.2)	0.1	$ (0.1)	(2.1)
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)	(0.2)	(0.6)	(0.7)
Nickel and other raw material contracts | Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Net derivative gain (loss) on hedge transactions	(0.2)	2.1	2.6	(2.1)
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	1.5	0.7	4.1	(1.3)
Natural gas contracts | Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Net derivative gain (loss) on hedge transactions	5.6	2.5	8	1.2
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	1	(0.8)	1.2	(2.8)
Foreign exchange contracts | Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Net derivative gain (loss) on hedge transactions	0	0	0.1	(0.1)
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	$ 0	$ (0.1)
Designated as Hedging Instrument | Foreign exchange forward | 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) - Designated as Hedging Instrument - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Asset	$ 12.2	$ 4.8
Derivative Fair Value Of Derivative Liability	3.6	4
Nickel and other raw material contracts | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Asset	2.5	3.7
Nickel and other raw material contracts | Other assets
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Asset	0.2	0.7
Nickel and other raw material contracts | Accrued liabilities
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Liability	0.6	0.1
Nickel and other raw material contracts | Other long-term liabilities
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Liability	0.1	0
Interest Rate Swap | Accrued liabilities
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Liability	1	1
Interest Rate Swap | Other long-term liabilities
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Liability	1.4	2.5
Natural gas contracts | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Asset	8	0.2
Natural gas contracts | Other assets
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Asset	1.5	0.2
Natural gas contracts | Accrued liabilities
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Liability	0.3	0.3
Natural gas contracts | Other long-term liabilities
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Liability	$ 0.2	$ 0.1

Fair Value of Financial Instr_3

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Jul. 02, 2020
Carrying Value Reported Amount Fair Value Disclosure
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 1,006.8	$ 645.9
Derivative financial instruments, Assets	12.2	4.8
Derivative Financial Instruments, liabilities	3.6	4
Debt	2,313.3	1,629.1
Estimate Of Fair Value Fair Value Disclosure
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	1,006.8	645.9
Derivative financial instruments, Assets	12.2	4.8
Derivative Financial Instruments, liabilities	3.6	4
Debt	2,558.7	1,847.7
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	1,006.8	645.9
Derivative financial instruments, Assets	0	0
Derivative Financial Instruments, liabilities	0	0
Debt	2,296	1,594.2
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	12.2	4.8
Derivative Financial Instruments, liabilities	3.6	4
Debt	262.7	253.5
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	0	$ 46.8	$ 51.4
Convertible Debt | Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Estimate Of Fair Value Fair Value Disclosure | Fair Value Inputs Level 1
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt	$ 402

Retirement Benefits (Details1)

Retirement Benefits (Details1) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Plan Disclosure [Line Items]
Settlement gain	$ (64.9)	$ 0	$ (64.9)	$ 0
Pension Plans Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the year	3.8	3.2	11.3	9.5
Interest cost on benefits earned in prior years	17.1	21.6	51.3	64.7
Expected return on plan assets	(34.1)	(33.6)	(102.3)	(100.8)
Amortization of prior service cost (credit)	0.2	0.1	0.5	0.5
Amortization of net actuarial loss	18.9	18.6	56.7	55.8
Settlement gain	0	0	0	0
Total retirement benefit expense (income)	5.9	9.9	17.5	29.7
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the year	0.4	0.6	1.1	1.7
Interest cost on benefits earned in prior years	1.9	2.6	6.1	8
Expected return on plan assets	0	0	0	0
Amortization of prior service cost (credit)	(0.6)	(0.9)	(1.9)	(2.8)
Amortization of net actuarial loss	3.6	2.7	10.2	8.1
Settlement gain	(64.9)	0	(64.9)	0
Total retirement benefit expense (income)	$ (59.6)	$ 5	$ (49.4)	$ 15

Retirement Benefits Narrative (

Retirement Benefits Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Jul. 31, 2021	Dec. 31, 2020
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accrued postretirement benefits	$ 269.4		$ 269.4			$ 326.7
Income tax provision	22	$ 0.8	31.5	$ 104.2
Settlement gain	(64.9)	0	(64.9)	0
Collective-Bargaining Arrangement, Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Income tax provision	15.5
Pension Plans Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year						$ 87
Defined Benefit Plan, Plan Assets, Contributions by Employer	50		67.5
Settlement gain	0	0	0	0
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Settlement gain	$ (64.9)	$ 0	$ (64.9)	$ 0
Other Postretirement Benefit Plan, Defined Benefit | Collective-Bargaining Arrangement, Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accrued postretirement benefits					$ 43
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax					$ 21.9

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Investments, Owned, Federal Income Tax Note [Line Items]
Income tax (provision) benefit	$ (22)	$ (0.8)		$ (31.5)	$ (104.2)
Effective income tax rate		(1.70%)			(30.70%)
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount			$ 99
Collective-Bargaining Arrangement, Other
Investments, Owned, Federal Income Tax Note [Line Items]
Income tax (provision) benefit	$ (15.5)

Business Segments (Details)

Business Segments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Sales	$ 725.7	$ 598	$ 2,034.4	$ 2,323.8
Segment EBITDA	94.2	27.8	241.7	207.6
LIFO and net realizable value reserves	0	0	0	0
Corporate expenses	(12.9)	(10.2)	(41)	(29.7)
Closed operations and other expense	(1.4)	(1)	(4.5)	(4.6)
Total ATI Adjusted EBITDA	79.9	16.6	196.2	173.3
Depreciation & amortization	(35.6)	(35.4)	(108)	(108.3)
Interest expense, net	(25.1)	(25.1)	(72.2)	(68.7)
Restructuring charges (credits)	(2.3)	2.3	(8.5)	27
Strike related costs	(22.9)	0	(63.2)	0
Retirement benefit settlement gain	64.9	0	64.9	0
Impairment of goodwill	0	0	0	(287)
Income (loss) from equity investments	0.4	(1)	0.4	(6.6)
Debt extinguishment charge	0	0	0	(21.5)
Gain on asset sales and sale of business	13.7	0	13.7	2.5
Income (loss) before income taxes	77.2	(46.2)	39.9	(339.1)
Employee Severance [Member]
Segment Reporting Information [Line Items]
Restructuring charges (credits)			(9.2)
Allegheny & Tsingshan Stainless | Employee Severance [Member]
Segment Reporting Information [Line Items]
Income (loss) from equity investments	0	0	0	(2.4)
Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless
Segment Reporting Information [Line Items]
Income (loss) from equity investments	0.3	(1.6)	(0.6)	(10.1)
Operating Segments
Segment Reporting Information [Line Items]
Sales	789.7	623.4	2,189	2,482.4
Depreciation & amortization	(35.6)	(35.4)	(108)	(108.3)
Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	325	230.1	908.9	995.3
Segment EBITDA	37.4	16.8	99.2	122.1
Depreciation & amortization	(18.2)	(19.5)	(57)	(58.5)
Strike related costs	(1.4)		(3.5)
Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Sales	464.7	393.3	1,280.1	1,487.1
Segment EBITDA	56.8	11	142.5	85.5
Depreciation & amortization	(16.3)	(15.1)	(47.9)	(47.4)
Strike related costs	(21.5)		(59.7)
Operating Segments | Corporate Segment
Segment Reporting Information [Line Items]
Depreciation & amortization	(1.1)	(0.8)	(3.1)	(2.4)
Internal Customers | Operating Segments
Segment Reporting Information [Line Items]
Sales	64	25.4	154.6	158.6
Internal Customers | Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	25	8.8	67.4	53
Internal Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Sales	39	16.6	87.2	105.6
External Customers | Operating Segments
Segment Reporting Information [Line Items]
Sales	725.7	598	2,034.4	2,323.8
External Customers | Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	300	221.3	841.5	942.3
External Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Sales	$ 425.7	$ 376.7	$ 1,192.9	$ 1,381.5

Business Segments - Additional

Business Segments - Additional Information (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)segment	Sep. 30, 2020USD ($)	Feb. 28, 2021employee	Dec. 31, 2020USD ($)
Restructuring Cost and Reserve [Line Items]
Number of business segments | segment			2
Strike related costs	$ 22,900,000	$ 0	$ 63,200,000	$ 0
Impairment of goodwill	0	0	0	(287,000,000)
Debt extinguishment charge	0	$ 0	0	(21,500,000)
Gain on the sale of oil and gas rights				$ 2,500,000
Entity number of employees covered by USW Collective Bargaining Agreements that expired February 28 2021 | employee					1,100
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt
Restructuring Cost and Reserve [Line Items]
Debt, Gross	291,400,000		291,400,000			$ 291,400,000
High Performance Materials & Components | Operating Segments
Restructuring Cost and Reserve [Line Items]
Strike related costs	1,400,000		3,500,000
Advanced Alloys & Solutions [Member] | Operating Segments
Restructuring Cost and Reserve [Line Items]
Strike related costs	$ 21,500,000		$ 59,700,000

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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income (loss) attributable to ATI	$ 48.7	$ (50.1)	$ (8.4)	$ (451.6)
Net income (loss) attributable to ATI after assumed conversions	$ 52.7	$ (50.1)	$ (8.4)	$ (451.6)
Denominator for basic net income (loss) per common share – weighted average shares	127.2	126.6	127	126.5
Share-based compensation	0.8
Denominator for diluted net income (loss) per common share – adjusted weighted average shares and assumed conversions	152.6	126.6	127	126.5
Basic net (loss) income attributable to ATI per common share (in dollars per share)	$ 0.38	$ (0.40)	$ (0.07)	$ (3.57)
Diluted net (loss) income attributable to ATI per common share (in dollars per share)	$ 0.35	$ (0.40)	$ (0.07)	$ (3.57)
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	0	25	25.6	21.9
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	$ 1.1
Convertible Senior Notes- Effect on denominator	5.8
Interest rate	4.75%		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	$ 2.9
Convertible Senior Notes- Effect on denominator	18.8

Accumulated Other Comprehensi_3

Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		$ (1,176.3)	$ (1,172.8)	$ (1,223.6)	$ (1,201.7)
OCI before reclassifications		1	21.9	6.1	6.2
Amounts reclassified from AOCI		14.2	22.3	56.4	66.9
Net current-period OCI		15.2	44.2	62.5	73.1
Ending balance		(1,161.1)	(1,128.6)	(1,161.1)	(1,128.6)
Increase (Decrease) in Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Beginning balance		23.7	8.5	21.2	9.8
OCI before reclassifications		(0.8)	6	1.7	4.7
Amounts reclassified from AOCI		0	0	0	0
Net current-period OCI		(0.8)	6	1.7	4.7
Ending balance		22.9	14.5	22.9	14.5
Post- retirement benefit plans [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		(1,086.9)	(1,051.8)	(1,119.9)	(1,083.1)
OCI before reclassifications		0	0	0	0
Amounts reclassified from AOCI	[1]	4	15.6	37	46.9
Net current-period OCI		4	15.6	37	46.9
Ending balance		(1,082.9)	(1,036.2)	(1,082.9)	(1,036.2)
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		(55.8)	(84.5)	(55.5)	(76.6)
OCI before reclassifications		(4.2)	17.2	(4.5)	9.3
Amounts reclassified from AOCI	[2]	0	0	0	0
Net current-period OCI		(4.2)	17.2	(4.5)	9.3
Ending balance		(60)	(67.3)	(60)	(67.3)
Increase (Decrease) in Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Beginning balance		23.7	8.5	21.2	9.8
OCI before reclassifications		(0.8)	6	1.7	4.7
Amounts reclassified from AOCI	[2]	0	0	0	0
Net current-period OCI		(0.8)	6	1.7	4.7
Ending balance		22.9	14.5	22.9	14.5
Derivatives [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		5.1	(3.7)	2.1	(0.5)
OCI before reclassifications		5.2	4.7	10.6	(3.1)
Amounts reclassified from AOCI	[3]	(2.3)	0.3	(4.7)	4.9
Net current-period OCI		2.9	5	5.9	1.8
Ending balance		8	1.3	8	1.3
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
Accumulated Deferred Tax Asset Valuation Allowance [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		(38.7)	(32.8)	(50.3)	(41.5)
OCI before reclassifications		0	0	0	0
Amounts reclassified from AOCI	[4]	12.5	6.4	24.1	15.1
Net current-period OCI		12.5	6.4	24.1	15.1
Ending balance		(26.2)	(26.4)	(26.2)	(26.4)
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 12).
[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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization to net income (loss) of net prior service credits		$ (0.4)	$ (0.8)	$ (1.4)	$ (2.3)
Amount reclassified from AOCI, Postretirement benefit plans, Actuarial losses		22.5	21.3	66.9	63.9
Settlement gain		(21.9)	0	(21.9)	0
Income (loss) before income taxes		77.2	(46.2)	39.9	(339.1)
Income tax provision		22	0.8	31.5	104.2
Cost of sales		(643.2)	(559.9)	(1,823.4)	(2,076.2)
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]	0.4	0.8	1.4	2.3
Amount reclassified from AOCI, Postretirement benefit plans, Actuarial losses	[1]	(22.5)	(21.3)	(66.9)	(63.9)
Settlement gain	[1]	21.9	0	21.9	0
Income (loss) before income taxes	[2]	(0.2)	(20.5)	(43.6)	(61.6)
Income tax provision	[3]	3.8	(4.9)	(6.6)	(14.7)
Net income (loss)		(4)	(15.6)	(37)	(46.9)
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]	3	(0.4)	6.2	(6.4)
Income tax provision	[3]	0.7	(0.1)	1.5	(1.5)
Net income (loss)		2.3	(0.3)	4.7	(4.9)
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]	2	0.9	5.4	(1.7)
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]	1.3	(1.1)	1.6	(3.7)
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	0	(0.1)
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]	$ (0.3)	$ (0.2)	$ (0.8)	$ (0.9)
[1]	Amounts are reported in nonoperating retirement benefit expense (see Note 12).
[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 9).

Commitments and Contingencies (

Commitments and Contingencies (Details) $ in Millions	Sep. 30, 2021USD ($)
Components of Environmental Loss Accrual [Abstract]
Accrual For Environmental Loss Contingencies	$ 13
Accrued Environmental Loss Contingencies Current	5
Federal Superfund and comparable state-managed sites	3
Formerly owned or operated sites	8
Owned or controlled sites at which Company operations have been or plan to be discontinued	2
Maximum
Loss Contingency, Estimate [Abstract]
Loss contingency maximum possible loss	$ 16

Subsequent Event - Narrative (D

Subsequent Event - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Jul. 31, 2021USD ($)	Feb. 28, 2021employee	Dec. 31, 2020USD ($)
Subsequent Event [Line Items]
Income tax provision	$ 22	$ 0.8	$ 31.5	$ 104.2
Accrued postretirement benefits	269.4		$ 269.4				$ 326.7
Entity number of employees covered by USW Collective Bargaining Agreements that expired February 28 2021 | employee						1,100
Collective-Bargaining Arrangement, Other
Subsequent Event [Line Items]
Income tax provision	$ 15.5
Other Postretirement Benefit Plan, Defined Benefit | Collective-Bargaining Arrangement, Other
Subsequent Event [Line Items]
Accrued postretirement benefits					$ 43
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax					$ (21.9)