ATI (ATI) 10-Q2 Nov 232023 Q3 Quarterly reportFinancial data

Company Profile

Cover

Cover - shares	9 Months Ended
Cover - shares	Sep. 30, 2023	Oct. 13, 2023
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2023
Document Transition Report	false
Entity File Number	1-12001
Entity Registrant Name	ATI Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	25-1792394
Entity Address, Address Line One	2021 McKinney Avenue
Entity Address, City or Town	Dallas,
Entity Address, State or Province	TX
Entity Address, Postal Zip Code	75201
City Area Code	800
Local Phone Number	289-7454
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)		127,577,212
Amendment Flag	false
Entity Central Index Key	0001018963
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q3

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Current Assets:
Cash and cash equivalents	$ 432.9	$ 584
Accounts receivable, net	683	579.2
Short-term contract assets	56.6	64.1
Inventories, net	1,353.9	1,195.7
Prepaid expenses and other current assets	73.3	53.4
Total current assets	2,599.7	2,476.4
Property, plant and equipment, net	1,626.3	1,549.1
Goodwill	227.2	227.2
Other assets	277.9	192.9
Total assets	4,731.1	4,445.6
Current Liabilities:
Accounts payable	435	553.3
Short-term contract liabilities	110.2	149.1
Short-term debt and current portion of long-term debt	37.5	41.7
Other current liabilities	246.6	219.8
Total current liabilities	829.3	963.9
Long-term debt	2,147.7	1,706.3
Accrued postretirement benefits	173.2	184.9
Pension liabilities	39.7	225.6
Other long-term liabilities	185.3	207.7
Total liabilities	3,375.2	3,288.4
ATI Stockholders' Equity:
Preferred stock, par value $0.10: authorized-50,000,000 shares; issued-none	0	0
Common stock, par value $0.10: authorized-500,000,000 shares; issued-132,292,666 shares at September 30, 2023 and 131,392,262 shares at December 31, 2022; outstanding-127,576,395 shares at September 30, 2023 and 128,273,042 shares at December 31, 2022	13.2	13.1
Additional paid-in capital	1,689.5	1,668.1
Retained earnings	398.7	176.9
Treasury stock: 4,716,271 shares at September 30, 2023 and 3,119,220 shares at December 31, 2022	(153.6)	(87)
Accumulated other comprehensive loss, net of tax	(708.9)	(725.2)
Total ATI stockholders’ equity	1,238.9	1,045.9
Noncontrolling interests
Noncontrolling interests	117	111.3
Total Equity	1,355.9	1,157.2
Total Liabilities and Equity	$ 4,731.1	$ 4,445.6

CONSOLIDATED BALANCE SHEETS (PA

CONSOLIDATED BALANCE SHEETS (PARENTHETICAL) - $ / shares	Sep. 30, 2023	Dec. 31, 2022
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)	132,292,666	131,392,262
Common stock, outstanding (in shares)	127,576,395	128,273,042
Treasury stock (in shares)	4,716,271	3,119,220

CONSOLIDATED STATEMENTS OF INCO

CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Income Statement [Abstract]
Sales	$ 1,025.6	$ 1,032	$ 3,109.7	$ 2,825.6
Cost of sales	831	848.2	2,512.8	2,297.1
Gross profit	194.6	183.8	596.9	528.5
Selling and administrative expenses	69.8	73.2	235.8	220.7
Restructuring charges (credits)	(0.5)	(2.6)	2.2	(5)
Loss on asset sales and sales of businesses, net	0.1	0	0.8	134.2
Operating income	125.2	113.2	358.1	178.6
Nonoperating retirement benefit expense	(16.9)	(6.5)	(50.6)	(18.9)
Interest expense, net	(23.8)	(20.8)	(65)	(67.8)
Other income (expense), net	0	(18.5)	1.3	(15.3)
Income before income taxes	84.5	67.4	243.8	76.6
Income tax provision	4.9	3	12.9	11.3
Net income	79.6	64.4	230.9	65.3
Less: Net income attributable to noncontrolling interests	3.9	3.3	9.1	11.3
Net income attributable to ATI	$ 75.7	$ 61.1	$ 221.8	$ 54
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.59	$ 0.47	$ 1.73	$ 0.43
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.52	$ 0.42	$ 1.53	$ 0.42

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Statement of Comprehensive Income [Abstract]
Net income	$ 79.6	$ 64.4	$ 230.9	$ 65.3
Currency translation adjustment
Unrealized net change arising during the period	(9.2)	(30)	(14)	(59.2)
Reclassification adjustment included in net income	0	0	0	20
Total	(9.2)	(30)	(14)	(39.2)
Derivatives
Net derivatives gain (loss) on hedge transactions	(3.2)	7.7	(20.6)	48.4
Reclassification to net income of net realized loss (gain)	1.8	(7.1)	0.1	(32.5)
Income taxes on derivative transactions	0	0	0	0
Total	(1.4)	0.6	(20.5)	15.9
Postretirement benefit plans
Amortization of net actuarial loss	16	19.1	47.8	57.6
Amortization to net income of net prior service credits	(0.1)	(0.1)	(0.4)	(0.4)
Settlement loss included in net income	0	0	0	29.5
Income taxes on postretirement benefit plans	0	0	0	0
Total	15.9	19	47.4	86.7
Other comprehensive income (loss), net of tax	5.3	(10.4)	12.9	63.4
Comprehensive income	84.9	54	243.8	128.7
Less: Comprehensive income (loss) attributable to noncontrolling interests	1.8	(6.1)	5.7	(7.1)
Comprehensive income attributable to ATI	$ 83.1	$ 60.1	$ 238.1	$ 135.8

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Operating Activities:
Net income	$ 230.9	$ 65.3
Adjustments to reconcile net income to net cash used in operating activities:
Depreciation and amortization	106.6	107.1
Share-based compensation	21.5	20.1
Deferred taxes	2.2	2.2
Net gains from disposal of property, plant and equipment	(0.1)	(1)
Loss on sale of business	0.6	141
Changes in operating assets and liabilities:
Inventories	(158.2)	(212)
Accounts receivable	(104)	(224.4)
Accounts payable	(108.2)	52.2
Retirement benefits	(241)	2.4
Accrued liabilities and other	(81.6)	(52.3)
Cash used in operating activities	(331.3)	(99.4)
Investing Activities:
Purchases of property, plant and equipment	(147.3)	(100.5)
Proceeds from disposal of property, plant and equipment	3.3	1.5
Transaction costs from sales of businesses, net of proceeds	(0.3)	(2.8)
Other	1.1	0.8
Cash used in investing activities	(143.2)	(101)
Financing Activities:
Borrowings on long-term debt	425	0
Payments on long-term debt and finance leases	(22)	(16.6)
Net payments under credit facilities	(7.3)	(16)
Debt issuance costs	(6.1)	0
Purchase of treasury stock	(55.1)	(104.9)
Sale of noncontrolling interests	0	0.9
Dividends paid to noncontrolling interests	0	(16)
Shares repurchased for income tax withholding on share-based compensation and other	(11.1)	(5.6)
Cash provided by (used in) financing activities	323.4	(158.2)
Decrease in cash and cash equivalents
Decrease in cash and cash equivalents	(151.1)	(358.6)
Cash and cash equivalents at beginning of period	584	687.7
Cash and cash equivalents at end of period	$ 432.9	$ 329.1

STATEMENTS OF CHANGES IN CONSOL

STATEMENTS OF CHANGES IN CONSOLIDATED EQUITY - USD ($) $ in Millions	Total	[CommonStockMember]	Additional Paid-in Capital [Member]	[RetainedEarningsMember]	Treasury Stock	[AccumulatedOtherComprehensiveIncomeMember]	[NoncontrollingInterestMember]
Total Stockholders' Equity at Dec. 31, 2021	$ 832.7	$ 12.7	$ 1,596.7	$ 72.7	$ (4.8)	$ (991.7)	$ 147.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	65.3			54			11.3
Other comprehensive income	63.4					81.8	(18.4)
Purchase of treasury stock	(104.9)				(104.9)
Conversion of convertible notes	82.5	0.3	45.4	(26.7)	63.5
Dividends paid to noncontrolling interest	(16)						(16)
Sale of subsidiary shares to noncontrolling interest	0.9						0.9
Employee stock plans	14.5	0.1	20.1	0	(5.7)
Total Stockholders' Equity at Sep. 30, 2022	938.4	13.1	1,662.2	100	(51.9)	(909.9)	124.9
Total Stockholders' Equity at Jun. 30, 2022	893.9	13.1	1,656.6	38.9	(36.8)	(908.9)	131
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	64.4			61.1			3.3
Other comprehensive income	(10.4)					(1)	(9.4)
Purchase of treasury stock	(15)				(15)
Employee stock plans	5.5	0	5.6	0	(0.1)
Total Stockholders' Equity at Sep. 30, 2022	938.4	13.1	1,662.2	100	(51.9)	(909.9)	124.9
Total Stockholders' Equity at Dec. 31, 2022	1,157.2	13.1	1,668.1	176.9	(87)	(725.2)	111.3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	230.9			221.8			9.1
Other comprehensive income	12.9					16.3	(3.4)
Purchase of treasury stock	(55.5)				(55.5)
Employee stock plans	10.4	0.1	21.4	0	(11.1)
Total Stockholders' Equity at Sep. 30, 2023	1,355.9	13.2	1,689.5	398.7	(153.6)	(708.9)	117
Total Stockholders' Equity at Jun. 30, 2023	1,309.2	13.2	1,682	323	(107.9)	(716.3)	115.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	79.6			75.7			3.9
Other comprehensive income	5.3					7.4	(2.1)
Purchase of treasury stock	(45.4)				(45.4)
Employee stock plans	7.2	0	7.5	0	(0.3)
Total Stockholders' Equity at Sep. 30, 2023	$ 1,355.9	$ 13.2	$ 1,689.5	$ 398.7	$ (153.6)	$ (708.9)	$ 117

Accounting Policies

Accounting Policies	9 Months Ended
Accounting Policies	Sep. 30, 2023
Accounting Policies [Abstract]
Accounting Policies	Accounting Policies The interim consolidated financial statements include the accounts of ATI Inc. and its subsidiaries. Unless the context requires otherwise, “ATI” and “the Company” refer to ATI Inc. 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 2022 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, 2022 financial information has been derived from the Company’s audited consolidated financial statements. New Accounting Pronouncements Adopted In September 2022, the Financial Accounting Standards Board (FASB) issued new accounting guidance related to disclosures about supplier finance programs. Supplier finance programs allow a buyer to offer its suppliers the option for access to payment in advance of an invoice due date, which is paid by a third-party finance provider or intermediary on the basis of invoices that the buyer has confirmed as valid. This new guidance requires a buyer in a supplier finance program to disclose sufficient information about the program to allow a user of financial statements to understand the program’s nature, activity during the period, changes from period to period, and potential magnitude, using both qualitative and quantitative information about its supplier finance programs. This new guidance, with the exception of disclosures on rollforward information, is effective for the Company in fiscal year 2023. The Company adopted this new accounting guidance effective January 1, 2023. The rollforward information disclosures are effective for the Company in fiscal year 2024, with early adoption permitted. The Company did not early adopt this guidance. The adoption of these changes did not have an impact on the Company’s consolidated financial statements other than disclosure requirements which are included in Note 6.

Revenue from Contracts with Cus

Revenue from Contracts with Customers	9 Months Ended
Revenue from Contracts with Customers	Sep. 30, 2023
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 regarding the Company’s overall revenues (in millions) by global and geographical markets for the third quarters and nine months ended September 30, 2023 and 2022 is included in the following tables. (in millions) Third quarter ended September 30, 2023 September 30, 2022 HPMC AA&S Total HPMC AA&S Total Diversified Global Markets: Aerospace & Defense: Jet Engines- Commercial $ 312.5 $ 16.9 $ 329.4 $ 285.7 $ 26.9 $ 312.6 Airframes- Commercial 104.0 99.6 203.6 49.8 81.6 131.4 Defense 39.7 53.1 92.8 38.5 47.6 86.1 Total Aerospace & Defense 456.2 169.6 625.8 374.0 156.1 530.1 Energy: Oil & Gas 2.5 84.5 87.0 6.9 120.2 127.1 Specialty Energy 20.2 41.7 61.9 26.4 39.2 65.6 Total Energy 22.7 126.2 148.9 33.3 159.4 192.7 Automotive 7.2 40.9 48.1 3.5 66.1 69.6 Medical 28.9 18.6 47.5 22.3 25.1 47.4 Electronics 0.6 44.2 44.8 0.7 47.8 48.5 Construction/Mining 7.7 32.3 40.0 9.5 38.3 47.8 Food Equipment & Appliances — 16.2 16.2 0.2 45.0 45.2 Other 16.2 38.1 54.3 14.1 36.6 50.7 Total $ 539.5 $ 486.1 $ 1,025.6 $ 457.6 $ 574.4 $ 1,032.0 (in millions) Nine months ended September 30, 2023 September 30, 2022 HPMC AA&S Total HPMC AA&S Total Diversified Global Markets: Aerospace & Defense: Jet Engines- Commercial $ 914.2 $ 67.0 $ 981.2 $ 696.1 $ 61.8 $ 757.9 Airframes- Commercial 242.0 295.7 537.7 130.9 200.3 331.2 Defense 131.6 157.8 289.4 120.6 123.6 244.2 Total Aerospace & Defense 1,287.8 520.5 1,808.3 947.6 385.7 1,333.3 Energy: Oil & Gas 8.6 317.2 325.8 32.1 323.3 355.4 Specialty Energy 75.0 137.8 212.8 88.0 109.3 197.3 Total Energy 83.6 455.0 538.6 120.1 432.6 552.7 Automotive 19.5 140.8 160.3 8.7 227.4 236.1 Construction/Mining 26.9 101.9 128.8 25.8 113.9 139.7 Medical 70.7 53.7 124.4 52.3 70.8 123.1 Electronics 1.8 113.4 115.2 1.9 147.6 149.5 Food Equipment & Appliances — 58.6 58.6 0.2 141.7 141.9 Other 47.4 128.1 175.5 38.7 110.6 149.3 Total $ 1,537.7 $ 1,572.0 $ 3,109.7 $ 1,195.3 $ 1,630.3 $ 2,825.6 (in millions) Third quarter ended September 30, 2023 September 30, 2022 HPMC AA&S Total HPMC AA&S Total Primary Geographical Market: United States $ 242.6 $ 314.2 $ 556.8 $ 221.6 $ 395.5 $ 617.1 Europe 217.7 49.7 267.4 147.0 53.2 200.2 Asia 37.8 103.4 141.2 60.7 108.9 169.6 Canada 13.1 10.4 23.5 10.0 9.5 19.5 South America, Middle East and other 28.3 8.4 36.7 18.3 7.3 25.6 Total $ 539.5 $ 486.1 $ 1,025.6 $ 457.6 $ 574.4 $ 1,032.0 (in millions) Nine months ended September 30, 2023 September 30, 2022 HPMC AA&S Total HPMC AA&S Total Primary Geographical Market: United States $ 661.1 $ 1,052.3 $ 1,713.4 $ 557.8 $ 1,105.6 $ 1,663.4 Europe 619.7 137.7 757.4 403.3 140.6 543.9 Asia 132.8 316.4 449.2 155.4 328.4 483.8 Canada 41.7 34.9 76.6 33.0 29.9 62.9 South America, Middle East and other 82.4 30.7 113.1 45.8 25.8 71.6 Total $ 1,537.7 $ 1,572.0 $ 3,109.7 $ 1,195.3 $ 1,630.3 $ 2,825.6 Comparative information regarding the Company’s major products based on their percentages of sales is included in the following table. Hot-Rolling and Processing Facility (HRPF) conversion service sales in the AA&S segment are excluded from this presentation. Third quarter ended September 30, 2023 September 30, 2022 HPMC AA&S Total HPMC AA&S Total Diversified Products and Services: Nickel-based alloys and specialty alloys 42 % 52 % 47 % 52 % 57 % 54 % Titanium and titanium-based alloys 24 % 13 % 19 % 16 % 7 % 11 % Precision forgings, castings and components 33 % — % 18 % 32 % — % 15 % Precision rolled strip products 1 % 19 % 9 % — % 22 % 12 % Zirconium and related alloys — % 16 % 7 % — % 14 % 8 % Total 100 % 100 % 100 % 100 % 100 % 100 % Nine months ended September 30, 2023 September 30, 2022 HPMC AA&S Total HPMC AA&S Total Diversified Products and Services: Nickel-based alloys and specialty alloys 45 % 56 % 50 % 50 % 55 % 52 % Precision forgings, castings and components 33 % — % 17 % 34 % — % 15 % Titanium and titanium-based alloys 21 % 11 % 16 % 16 % 6 % 11 % Precision rolled strip products 1 % 18 % 9 % — % 25 % 14 % Zirconium and related alloys — % 15 % 8 % — % 14 % 8 % Total 100 % 100 % 100 % 100 % 100 % 100 % The Company maintained a backlog of confirmed orders totaling $3.6 billion and $2.7 billion at September 30, 2023 and 2022, respectively. Due to the structure of the Company’s long-term agreements, approximately 70% of this backlog at September 30, 2023 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, 2023 and December 31, 2022, accounts receivable with customers were $686.7 million and $586.9 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, 2023 and 2022: (in millions) Accounts Receivable - Reserve for Doubtful Accounts September 30, September 30, Balance as of beginning of fiscal year $ 7.7 $ 3.8 Expense to increase the reserve 0.2 0.7 Write-off of uncollectible accounts (4.2) (0.6) Balance as of period end $ 3.7 $ 3.9 (in millions) Contract Assets Short-term September 30, September 30, Balance as of beginning of fiscal year $ 64.1 $ 53.9 Recognized in current year 66.6 74.1 Reclassified to accounts receivable (74.1) (57.9) Reclassification to/from long-term and contract liability — 0.1 Balance as of period end $ 56.6 $ 70.2 (in millions) Contract Liabilities Short-term September 30, September 30, Balance as of beginning of fiscal year $ 149.1 $ 116.2 Recognized in current year 61.4 105.3 Amounts in beginning balance reclassified to revenue (86.7) (80.8) Current year amounts reclassified to revenue (40.9) (42.9) Other (0.1) 0.8 Reclassification to/from long-term and contract asset 27.4 22.1 Balance as of period end $ 110.2 $ 120.7 Long-term (a) September 30, September 30, Balance as of beginning of fiscal year $ 66.8 $ 84.4 Recognized in current year 1.0 9.6 Reclassification to/from short-term (27.4) (22.0) Balance as of period end $ 40.4 $ 72.0 (a) Long-term contract liabilities are included in Other long-term liabilities on the consolidated balance sheets. Contract costs for obtaining and fulfilling a contract were $7.7 million and $7.3 million as of September 30, 2023 and December 31, 2022, 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, 2023 was $0.2 million and $0.9 million, respectively. Contract cost amortization expense for the three and nine months ended September 30, 2022 was $0.2 million and $0.7 million, respectively.

Inventories

Inventories	9 Months Ended
Inventories	Sep. 30, 2023
Inventory Disclosure [Abstract]
Inventories	Inventories Inventories at September 30, 2023 and December 31, 2022 were as follows (in millions): September 30, December 31, Raw materials and supplies $ 255.4 $ 213.6 Work-in-process 1,056.0 941.1 Finished goods 127.6 111.9 1,439.0 1,266.6 Inventory valuation reserves (85.1) (70.9) Total inventories, net $ 1,353.9 $ 1,195.7

Property Plant And Equipment

Property Plant And Equipment	9 Months Ended
Property Plant And Equipment	Sep. 30, 2023
Property, Plant and Equipment [Abstract]
Property Plant and Equipment	Property, Plant and Equipment Property, plant and equipment at September 30, 2023 and December 31, 2022 was as follows (in millions): September 30, December 31, Land $ 32.2 $ 31.5 Buildings and leasehold improvements 655.9 601.6 Equipment 2,984.7 2,895.5 3,672.8 3,528.6 Accumulated depreciation and amortization (2,046.5) (1,979.5) Total property, plant and equipment, net $ 1,626.3 $ 1,549.1

Joint Ventures

Joint Ventures	9 Months Ended
Joint Ventures	Sep. 30, 2023
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, 2023 were $85.0 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. Next Gen Alloys LLC funds its development activities through the sale of shares to the two joint venture partners, and in the first quarter of 2022 the Company received $0.9 million from sales of noncontrolling interests to its joint venture partner, which is reported as a financing activity on the consolidated statements of cash flows. Cash and cash equivalents held by this joint venture as of September 30, 2023 were $1.0 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 was received by ATI through December 31, 2021 and the remaining $5.5 million was received in the fourth quarter of 2022. 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 United States 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 United States Department of Commerce in the second quarter of 2020, and the 25% tariff remained 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 were losses of $0.5 million and $1.3 million for the three and nine months ended September 30, 2023, respectively, and a loss of $0.5 million for the three months ended September 30, 2022 and income of $8.7 million for the nine months ended September 30, 2022, which is included within other income/expense, net, on the consolidated statements of operations and in the AA&S segment’s operating results. ATI’s share of A&T Stainless results for the nine months ended September 30, 2022 included $9.9 million for ATI’s share of the $19.7 million tariff refund and accrued interest recognized as income by the joint venture in the second quarter of 2022 resulting from a settlement agreement with the U.S. pursuant to which the U.S., without admitting liability, agreed to refund a substantial portion of the Section 232 tariffs previously paid by A&T Stainless. As of September 30, 2023 and December 31, 2022, ATI had net receivables for working capital advances and administrative services from A&T Stainless of $1.9 million and $3.2 million, respectively. For the September 30, 2023 balance of net receivables, $0.5 million was reported in prepaid expenses and other current assets and $1.4 million in other long-term assets on the consolidated balance sheet, while for December 31, 2022, $0.4 million was reported in prepaid expenses and other current assets and $2.8 million in other long-term assets. 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 results were losses of $0.2 million for the three months ended September 30, 2023 and income of $0.3 million for the nine months ended September 30, 2023, and income of $1.3 million and $2.5 million for the three and nine months ended September 30, 2022, 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
Supplemental Financial Statement Information - (Notes)	Sep. 30, 2023
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, 2023 and 2022 was as follows: (in millions) Three months ended September 30, Nine months ended September 30, 2023 2022 2023 2022 Rent and royalty income $ 0.7 $ 0.5 $ 2.0 $ 1.7 Gains from disposal of property, plant and equipment, net — 0.1 0.3 0.3 Net equity income (loss) on joint ventures (See Note 5) (0.7) 0.8 (1.0) 11.2 Litigation reserve — (19.9) — (28.5) Total other income (expense), net $ — $ (18.5) $ 1.3 $ (15.3) Restructuring Restructuring charges for the third quarter ended September 30, 2023 were a credit of $0.5 million for a reduction in severance-related reserves related to approximately 10 employees based on changes in planned operating rates and revised workforce reduction estimates. Restructuring charges for the nine months ended September 30, 2023 were a charge of $2.2 million and represent severance for the involuntary reduction of approximately 40 employees across ATI’s domestic operations in conjunction with the continued transformation, partially offset by the credit in the third quarter 2023 discussed above for a reduction in severance-related reserves. Restructuring charges for the third quarter and nine months ended September 30, 2022 were a net credit of $2.6 million and $5.0 million, respectively, for a reduction in severance-related reserves related to approximately 60 and 110 employees, respectively, based on changes in planned operating rates and revised workforce reduction estimates. These amounts were presented as a restructuring credit in the consolidated statements of operations and are excluded from segment EBITDA. Restructuring reserves for severance cost activity is as follows: Severance and Employee Benefit Costs Balance at December 31, 2022 $ 9.8 Additions, net 2.2 Payments (1.1) Balance at September 30, 2023 $ 10.9 The $10.9 million restructuring reserve balance at September 30, 2023 includes $6.7 million recorded in other current liabilities and $4.2 million recorded in other long-term liabilities on the consolidated balance sheet. Supplier Financing

Debt

Debt	9 Months Ended
Debt	Sep. 30, 2023
Debt Disclosure [Abstract]
Debt	Debt Debt at September 30, 2023 and December 31, 2022 was as follows (in millions): September 30, December 31, ATI Inc. 7.25% Notes due 2030 $ 425.0 $ — ATI Inc. 5.875% Notes due 2027 350.0 350.0 ATI Inc. 5.125% Notes due 2031 350.0 350.0 ATI Inc. 4.875% Notes due 2029 325.0 325.0 ATI Inc. 3.5% Convertible Senior Notes due 2025 291.4 291.4 Allegheny Ludlum 6.95% Debentures due 2025 (a) 150.0 150.0 ABL Term Loan 200.0 200.0 U.S. revolving credit facility — — Foreign credit facilities 11.2 19.4 Finance leases and other 103.3 79.4 Debt issuance costs (20.7) (17.2) Debt 2,185.2 1,748.0 Short-term debt and current portion of long-term debt 37.5 41.7 Long-term debt $ 2,147.7 $ 1,706.3 (a) 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 operations. The ABL facility also provides the Company with the option of including certain machinery and equipment as additional collateral for purposes of determining availability under the facility. The ABL facility, which matures in September 2027, includes a $600 million revolving credit facility, a letter of credit sub-facility of up to $200 million, a $200 million term loan (Term Loan), and a swing loan facility of up to $60 million. The Term Loan has an interest rate of 2.0% above adjusted Secured Overnight Financing Rate (SOFR) 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 $300 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 SOFR-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) 10% of the then applicable maximum loan amount under the revolving credit portion of the ABL and the outstanding Term Loan balance, or (ii) $60.0 million. The Company was in compliance with the fixed charge coverage ratio as of September 30, 2023. Additionally, the Company must demonstrate minimum liquidity specified by the facility during the 90-day period immediately preceding the stated maturity date of its 3.5% Convertible Senior Notes due 2025 and the 6.95% Debentures due 2025 issued by the Company’s wholly owned subsidiary, Allegheny Ludlum LLC. 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) $120 million or (b) 20% of the sum of the maximum loan amount under the revolving credit portion of the ABL and the outstanding Term Loan balance. As of September 30, 2023, there were no outstanding borrowings under the revolving portion of the ABL facility, and $31.7 million was utilized to support the issuance of letters of credit. There were average revolving credit borrowings of $17 million bearing an average annual interest rate of 6.5% under the ABL facility for the first nine months of 2023. There were no revolving credit borrowings under the ABL facility during the first nine months of 2022. The Company also has foreign credit facilities, primarily in China, that total $57 million based on September 30, 2023 foreign exchange rates, under which $11.2 million and $19.4 million was drawn as of September 30, 2023 and December 31, 2022, respectively. 2030 Notes In August 2023, ATI issued $425 million aggregate principal amount of 7.25% Senior Notes due 2030 (2030 Notes). Interest on the 2030 Notes is payable semi-annually in arrears at a rate of 7.25% per year. The 2030 Notes will mature on August 15, 2030. Net proceeds were $418.8 million from this issuance, of which $222 million was used to fund ATI’s U.S. qualified defined benefit pension plan in order to facilitate a pension derisking strategy (see Note 16), and the remaining proceeds were used for liquidity and general corporate purposes. Underwriting fees and other third-party expenses for the issuance of the 2030 Notes were $6.2 million, and are being amortized to interest expense over the 7-year term of the 2030 Notes. The 2030 Notes are unsecured and unsubordinated obligations of the Company and equally ranked with all of its existing and future senior unsecured debt. The 2030 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 2030 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 2030 Notes. The 2030 Notes are subject to repurchase upon the occurrence of a change in control repurchase event (as defined in the 2030 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 2030 Notes repurchased. 2025 Convertible Notes As of September 30, 2023, the Company had $291.4 million aggregate principal amount of 3.5% Convertible Notes due 2025 (2025 Convertible Notes) outstanding, which mature on June 15, 2025. As of September 30, 2023 and December 31, 2022, the fair value of the 2025 Convertible Notes was $786 million and $590 million, respectively, 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 third quarters and nine months ended September 30, 2023 and 2022. Remaining deferred issuance costs were $3.4 million and $4.8 million at September 30, 2023 and December 31, 2022, respectively. Interest expense on the 2025 Convertible Notes was as follows: Three months ended September 30, Nine months ended September 30, (in millions) 2023 2022 2023 2022 Contractual coupon rate $ 2.5 $ 2.5 $ 7.6 $ 7.6 Amortization of debt issuance costs 0.5 0.4 1.4 1.3 Total interest expense $ 3.0 $ 2.9 $ 9.0 $ 8.9 Currently, 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. 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.

Derivative Financial Instrument

Derivative Financial Instruments and Hedging	9 Months Ended
Derivative Financial Instruments and Hedging	Sep. 30, 2023
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, 2023, 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 6 million pounds of nickel with hedge dates through 2024. The aggregate notional amount hedged is approximately 9% of a single year’s estimated nickel raw material purchase requirements. These derivative instruments are used to hedge the variability of a selling price that is based on the London Metal Exchange (LME) index for nickel, as well as to hedge the variability of the purchase cost of nickel based on this LME index. Any gain or loss associated with these hedging arrangements is included in sales or cost of sales, depending on whether the underlying risk being hedged was the variable selling price or the variable raw material cost, respectively. At September 30, 2023, the outstanding financial derivatives used to hedge the Company’s exposure to energy cost volatility included natural gas cost hedges. At September 30, 2023, the Company hedged approximately 80% of its forecasted domestic requirements for natural gas for the remainder of 2023, approximately 55% for 2024 and approximately 15% for 2025. 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, 2023, 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 contain 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: Interest rate swap Prepaid expenses and other current assets $ 1.2 $ 1.4 Foreign exchange contracts Prepaid expenses and other current assets 0.2 — Nickel and other raw material contracts Prepaid expenses and other current assets 2.5 12.5 Natural gas contracts Prepaid expenses and other current assets 0.1 2.4 Interest rate swap Other assets — 0.5 Nickel and other raw material contracts Other assets — 0.5 Natural gas contracts Other assets 0.2 0.7 Total derivatives designated as hedging instruments $ 4.2 $ 18.0 Liability derivatives Balance sheet location Derivatives designated as hedging instruments: Natural gas contracts Other current liabilities 3.3 2.0 Nickel and other raw material contracts Other current liabilities 5.0 2.1 Natural gas contracts Other long-term liabilities 0.5 0.5 Nickel and other raw material contracts Other long-term liabilities 0.5 — Total derivatives designated as hedging instruments $ 9.3 $ 4.6 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, 2023. 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 14 for further explanation). Assuming market prices remain constant with those at September 30, 2023, a pre-tax loss of $4.3 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, 2023 and 2022 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 2023 2022 2023 2022 Nickel and other raw material contracts $ (1.6) $ (0.7) $ (0.4) $ 0.8 Natural gas contracts (1.0) 5.4 (1.4) 4.1 Foreign exchange contracts 0.1 0.5 0.1 0.5 Interest rate swap 0.1 0.6 0.3 — Total $ (2.4) $ 5.8 $ (1.4) $ 5.4 Amount of Gain (Loss) Amount of Gain (Loss) Nine months ended September 30, Nine months ended September 30, Derivatives in Cash Flow Hedging Relationships 2023 2022 2023 2022 Nickel and other raw material contracts $ (8.5) $ 16.6 $ 3.5 $ 15.3 Natural gas contracts (7.8) 17.2 (4.6) 9.1 Foreign exchange contracts 0.3 0.9 0.2 0.6 Interest rate swap 0.3 2.1 0.8 (0.3) Total $ (15.7) $ 36.8 $ (0.1) $ 24.7 (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 sales and 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
Fair Value Of Financial Instruments	Sep. 30, 2023
Fair Value Disclosures [Abstract]
Fair Value Of Financial Instruments	Fair Value of Financial Instruments The estimated fair value of financial instruments at September 30, 2023 was as follows: Fair Value Measurements at Reporting Date Using (In millions) Total Total Quoted Prices in Significant Cash and cash equivalents $ 432.9 $ 432.9 $ 432.9 $ — Derivative financial instruments: Assets 4.2 4.2 — 4.2 Liabilities 9.3 9.3 — 9.3 Debt (a) 2,205.9 2,359.3 2,044.8 314.5 The estimated fair value of financial instruments at December 31, 2022 was as follows: Fair Value Measurements at Reporting Date Using (In millions) Total Total Quoted Prices in Significant Cash and cash equivalents $ 584.0 $ 584.0 $ 584.0 $ — Derivative financial instruments: Assets 18.0 18.0 — 18.0 Liabilities 4.6 4.6 — 4.6 Debt (a) 1,765.2 1,964.5 1,665.7 298.8 (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. 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 prioritize 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
Retirement Benefits	Sep. 30, 2023
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. The Company also sponsors several postretirement plans covering certain collectively-bargained salaried and hourly employees. The plans provide health care and life insurance benefits for eligible retirees. In most retiree health care plans, Company contributions towards premiums are capped based on the cost as of a certain date, thereby creating a defined contribution. All defined benefit pension and retiree health care plans are closed to new entrants. For the three month periods ended September 30, 2023 and 2022, the components of pension and other postretirement benefit expense for the Company’s defined benefit plans included the following (in millions): Pension Benefits Other Postretirement Benefits Three months ended September 30, Three months ended September 30, 2023 2022 2023 2022 Service cost - benefits earned during the year $ 1.5 $ 3.0 $ 0.2 $ 0.3 Interest cost on benefits earned in prior years 24.0 17.2 2.7 1.9 Expected return on plan assets (25.7) (31.6) — — Amortization of prior service cost (credit) 0.1 0.1 (0.2) (0.2) Amortization of net actuarial loss 14.5 15.8 1.5 3.3 Total retirement benefit expense $ 14.4 $ 4.5 $ 4.2 $ 5.3 For the nine month periods ended September 30, 2023 and 2022, 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, 2023 2022 2023 2022 Service cost - benefits earned during the year $ 4.7 $ 9.0 $ 0.5 $ 0.8 Interest cost on benefits earned in prior years 72.0 52.5 8.2 5.8 Expected return on plan assets (77.0) (96.6) — — Amortization of prior service cost (credit) 0.3 0.3 (0.7) (0.7) Amortization of net actuarial loss 43.3 47.7 4.5 9.9 Settlement loss — 29.5 — — Total retirement benefit expense $ 43.3 $ 42.4 $ 12.5 $ 15.8 In May 2022, the Company completed the sale of its Sheffield, UK operations. As a result of this sale, ATI recognized a $29.5 million settlement loss in the second quarter of 2022, which is recorded in loss on asset sales and sales of businesses, net, on the consolidated statement of operations, related to the amount in accumulated other comprehensive loss for the UK defined benefit pension plan that transferred as part of the sale.

Income Taxes

Income Taxes	9 Months Ended
Income Taxes	Sep. 30, 2023
Income Tax Disclosure [Abstract]
Income Taxes	Income TaxesThe provision for income taxes for the third quarter and nine months ended September 30, 2023 was $4.9 million and $12.9 million, respectively. The provision for income taxes for the third quarter and nine months ended September 30, 2022 was $3.0 million and $11.3 million, respectively. Tax expense in both periods is mainly attributable to the Company’s foreign operations and was based on an estimated annual effective tax rate calculation which included foreign, non-valuation allowance operations combined with the U.S. jurisdiction. The 2022 calculation excluded the results related to the Company’s Sheffield, UK operations, which was sold in the second quarter of 2022. 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 deferred tax asset valuation allowance, therefore limiting sources of income as part of the analysis. ATI continues to maintain valuation allowances on its U.S. federal and state deferred tax assets, as well as for certain foreign jurisdictions.

Business Segments

Business Segments	9 Months Ended
Business Segments	Sep. 30, 2023
Segment Reporting [Abstract]
Business Segments	Business Segments The Company operates in two business segments: High Performance Materials & Components (HPMC) and Advanced Alloys & Solutions (AA&S). The measure of segment EBITDA categorically excludes 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 gains or losses on asset sales and sales of businesses. 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, 2023 2022 2023 2022 Total sales: High Performance Materials & Components $ 578.1 $ 505.0 $ 1,670.5 $ 1,325.6 Advanced Alloys & Solutions 551.8 646.8 1,791.0 1,805.5 1,129.9 1,151.8 3,461.5 3,131.1 Intersegment sales: High Performance Materials & Components 38.6 47.4 132.8 130.3 Advanced Alloys & Solutions 65.7 72.4 219.0 175.2 104.3 119.8 351.8 305.5 Sales to external customers: High Performance Materials & Components 539.5 457.6 1,537.7 1,195.3 Advanced Alloys & Solutions 486.1 574.4 1,572.0 1,630.3 $ 1,025.6 $ 1,032.0 $ 3,109.7 $ 2,825.6 Three months ended September 30, Nine months ended September 30, 2023 2022 2023 2022 EBITDA: High Performance Materials & Components $ 115.7 $ 85.8 $ 303.9 $ 214.2 Advanced Alloys & Solutions 50.4 75.8 186.3 255.7 Total segment EBITDA 166.1 161.6 490.2 469.9 Corporate expenses (12.9) (14.2) (48.3) (47.9) Closed operations and other expense (5.1) (6.3) (11.3) (12.8) Depreciation & amortization (a) (35.6) (35.6) (106.6) (107.1) Interest expense, net (23.8) (20.8) (65.0) (67.8) Restructuring and other charges (4.2) (17.3) (14.6) (23.5) Loss on asset sales and sales of businesses, net — — (0.6) (134.2) Income before income taxes $ 84.5 $ 67.4 $ 243.8 $ 76.6 a) The following is depreciation & amortization by each business segment: Three months ended September 30, Nine months ended September 30, 2023 2022 2023 2022 High Performance Materials & Components $ 16.5 $ 16.7 $ 51.8 $ 51.5 Advanced Alloys & Solutions 17.3 17.1 49.6 50.0 Other 1.8 1.8 5.2 5.6 $ 35.6 $ 35.6 $ 106.6 $ 107.1 Beginning in 2020, the U.S. government enacted various relief packages in response to the COVID-19 pandemic. Results for the nine months ended September 30, 2022 include $34.3 million related to this government sponsored COVID relief in segment EBITDA. HPMC segment nine month 2022 results include $27.5 million for the Aviation Manufacturing Jobs Protection Program and employee retention credits, and AA&S segment nine month 2022 results include $6.8 million in employee retention credits. Restructuring and other charges of $4.2 million for the third quarter ended September 30, 2023 include $2.8 million of start up costs and $1.9 million of costs associated with an unplanned outage at our Lockport, NY facility, both of which are included within cost of sales on the consolidated statements of operations. These charges were partially offset by a $0.5 million pre-tax credit for restructuring charges, primarily related to lowered severance-related reserves based on changes in planned operating rates and revised workforce reduction estimates (see Note 6). Restructuring and other charges of $14.6 million for the nine months ended September 30, 2023 include $2.2 million of severance-related restructuring charges (see Note 6) as well as $8.5 million of start up costs, $1.9 million of costs associated with an unplanned outage at our Lockport, NY facility, and $2.0 million primarily for asset write-offs for the closure of our Robinson, PA operations, all of which are included within cost of sales on the consolidated statements of operations. Restructuring and other charges for the third quarter and nine months ended September 30, 2022 include a $19.9 million and $28.5 million charge, respectively, for a litigation reserve, which is reported in other nonoperating income (expense) on the consolidated statement of operations, partially offset by $2.6 million and $5.0 million, respectively, of restructuring credits for a reduction in severance-related reserves (see Note 6). Depreciation expense in the nine months ended September 30, 2023 includes $0.8 million of accelerated depreciation on fixed assets for the closure of our Robinson, PA operations.

Per Share Information

Per Share Information	9 Months Ended
Per Share Information	Sep. 30, 2023
Earnings Per Share [Abstract]
Per Share Information	Per Share Information The following table sets forth the computation of basic and diluted income per common share: (In millions, except per share amounts) Three months ended September 30, Nine months ended September 30, 2023 2022 2023 2022 Numerator: Numerator for basic income per common share – Net income attributable to ATI $ 75.7 $ 61.1 $ 221.8 $ 54.0 Effect of dilutive securities: 3.5% Convertible Senior Notes due 2025 2.7 2.8 7.9 — Numerator for diluted net income per common share – Net income attributable to ATI after assumed conversions $ 78.4 $ 63.9 $ 229.7 $ 54.0 Denominator: Denominator for basic net income per common share – weighted average shares 128.1 129.8 128.4 126.9 Effect of dilutive securities: Share-based compensation 3.3 2.2 2.9 2.0 3.5% Convertible Senior Notes due 2025 18.8 18.8 18.8 — Denominator for diluted net income per common share – adjusted weighted average shares and assumed conversions 150.2 150.8 150.1 128.9 Basic net income attributable to ATI per common share $ 0.59 $ 0.47 $ 1.73 $ 0.43 Diluted net income attributable to ATI per common share $ 0.52 $ 0.42 $ 1.53 $ 0.42 Common stock that would be issuable upon the assumed conversion of 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 and nine months ended September 30, 2023. There were no anti-dilutive shares for the three months ended September 30, 2022, and 22.5 million anti-dilutive shares for the nine months ended September 30, 2022.

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income (Loss)	9 Months Ended
Accumulated Other Comprehensive Income (Loss)	Sep. 30, 2023
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, 2023 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, June 30, 2023 $ (712.0) $ (73.6) $ (1.1) $ 70.4 $ (716.3) OCI before reclassifications — (7.1) (2.4) — (9.5) Amounts reclassified from AOCI (a) 12.1 (b) — (c) 1.4 (d) 3.4 16.9 Net current-period OCI 12.1 (7.1) (1.0) 3.4 7.4 Balance, September 30, 2023 $ (699.9) $ (80.7) $ (2.1) $ 73.8 $ (708.9) Attributable to noncontrolling interests: Balance, June 30, 2023 $ — $ 6.4 $ — $ — $ 6.4 OCI before reclassifications — (2.1) — — (2.1) Amounts reclassified from AOCI — (b) — — — — Net current-period OCI — (2.1) — — (2.1) Balance, September 30, 2023 $ — $ 4.3 $ — $ — $ 4.3 The changes in AOCI by component, net of tax, for the nine month period ended September 30, 2023 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, December 31, 2022 $ (736.1) $ (70.1) $ 13.5 $ 67.5 $ (725.2) OCI before reclassifications — (10.6) (15.7) — (26.3) Amounts reclassified from AOCI (a) 36.2 (b) — (c) 0.1 (d) 6.3 42.6 Net current-period OCI 36.2 (10.6) (15.6) 6.3 16.3 Balance, September 30, 2023 $ (699.9) $ (80.7) $ (2.1) $ 73.8 $ (708.9) Attributable to noncontrolling interests: Balance, December 31, 2022 $ — $ 7.7 $ — $ — $ 7.7 OCI before reclassifications — (3.4) — — (3.4) Amounts reclassified from AOCI — (b) — — — — Net current-period OCI — (3.4) — — (3.4) Balance, September 30, 2023 $ — $ 4.3 $ — $ — $ 4.3 (a) Amounts were included in net periodic benefit cost for pension and other postretirement benefit plans (see Note 11). (b) No amounts were reclassified to earnings. (c) Amounts related to derivatives are included in sales, cost of goods sold or interest expense in the period or periods the hedged item affects earnings (see Note 8). (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, 2022 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, June 30, 2022 $ (883.0) $ (65.1) $ 16.8 $ 22.4 $ (908.9) OCI before reclassifications — (20.6) 5.8 — (14.8) Amounts reclassified from AOCI (a) 14.5 (c) — (d) (5.4) (e) 4.7 13.8 Net current-period OCI 14.5 (20.6) 0.4 4.7 (1.0) Balance, September 30, 2022 $ (868.5) $ (85.7) $ 17.2 $ 27.1 $ (909.9) Attributable to noncontrolling interests: Balance, June 30, 2022 $ — $ 17.0 $ — $ — $ 17.0 OCI before reclassifications — (9.4) — — (9.4) Amounts reclassified from AOCI — (c) — — — — Net current-period OCI — (9.4) — — $ (9.4) Balance, September 30, 2022 $ — $ 7.6 $ — $ — $ 7.6 The changes in AOCI by component, net of tax, for the nine month period ended September 30, 2022 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, December 31, 2021 $ (947.7) $ (64.9) $ 5.1 $ 15.8 $ (991.7) OCI before reclassifications — (40.8) 36.8 — (4.0) Amounts reclassified from AOCI (a) 79.2 (b) 20.0 (d) (24.7) (e) 11.3 85.8 Net current-period OCI 79.2 (20.8) 12.1 11.3 81.8 Balance, September 30, 2022 $ (868.5) $ (85.7) $ 17.2 $ 27.1 $ (909.9) Attributable to noncontrolling interests: Balance, December 31, 2021 $ — $ 26.0 $ — $ — $ 26.0 OCI before reclassifications — (18.4) — — (18.4) Amounts reclassified from AOCI — (c) — — — — Net current-period OCI — (18.4) — — $ (18.4) Balance, September 30, 2022 $ — $ 7.6 $ — $ — $ 7.6 (a) Amounts were included in net periodic benefit cost for pension and other postretirement benefit plans (see Note 11) and/or loss on asset sales and sales of businesses, net, as part of the loss on sale of the Sheffield, UK operations. (b) Amounts were included in loss on asset sales and sales of businesses, net, as part of the loss on sale of the Sheffield, UK operations. (c) No amounts were reclassified to earnings. (d) Amounts related to derivatives are included in sales, cost of goods sold or interest expense in the period or periods the hedged item affects earnings (see Note 8). (e) 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, 2023 and 2022 were as follows: Details about AOCI Components (In millions) Three months ended September 30, 2023 Three months ended September 30, 2022 Nine months ended September 30, 2023 Nine months ended September 30, 2022 Affected line item in the statements Postretirement benefit plans Prior service credit $ 0.1 0.1 $ 0.4 $ 0.4 (a) Actuarial losses (16.0) (19.1) (47.8) (57.6) (a) Settlement loss — — — (29.5) (b) (15.9) (19.0) (47.4) (86.7) (d) Total before tax (3.8) (4.5) (11.2) (7.5) Tax expense (benefit) (e) $ (12.1) $ (14.5) $ (36.2) $ (79.2) Net of tax Currency translation adjustment $ — $ — $ — $ (20.0) (b,d) Derivatives Nickel and other raw material contracts $ (0.5) $ 1.0 $ 4.6 $ 20.1 (c) Natural gas contracts (1.8) 5.4 (6.0) 12.0 (c) Foreign exchange contracts 0.2 0.7 0.3 0.8 (c) Interest rate swap 0.3 — 1.0 (0.4) (c) (1.8) 7.1 (0.1) 32.5 (d) Total before tax (0.4) 1.7 — 7.8 Tax expense (benefit) (e) $ (1.4) $ 5.4 $ (0.1) $ 24.7 Net of tax (a) Amounts are reported in nonoperating retirement benefit expense (see Note 11). (b) Amounts in 2022 were included in loss on asset sales and sales of businesses, net, as part of the loss on sale of the Sheffield UK operations. (c) Amounts related to derivatives, with the exception of the interest rate swap, are included in sales or cost of goods sold in the period or periods the hedged item affects earnings. Amounts related to the interest rate swap are included in interest expense in the same period as the interest expense on the Term Loan is recognized in earnings (see Note 8). (d) For pre-tax 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. (e) These amounts exclude the impact of any deferred tax asset valuation allowances, when applicable.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
Commitments and Contingencies	Sep. 30, 2023
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, 2023, the Company’s reserves for environmental remediation obligations totaled approximately $13 million, of which $6 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; $7 million for formerly owned or operated sites for which the Company has remediation or indemnification obligations; and $3 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 and cash flows. 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.

Subsequent Events

Subsequent Events	9 Months Ended
Subsequent Events	Sep. 30, 2023
Subsequent Events [Abstract]
Subsequent Event	Subsequent Event In October 2023, the Company purchased group annuity contracts from an insurer covering approximately 85% of the Company’s U.S. qualified defined benefit pension plan obligations. Under these contracts, the Company transferred the pension obligations and associated assets for approximately 8,200 plan participants to the selected insurance company. This transaction had no impact on the amount, timing or form of the retirement benefit payments to the affected retirees and beneficiaries. To facilitate this pension derisking strategy, the Company completed a voluntary cash out for term vested employees and contributed $222 million to its pension plan in the third quarter of 2023, to fully fund remaining pension liabilities ahead of this annuity transaction. After these actions, the Company’s U.S. qualified defined benefit plan will include approximately 1,900 participants. In connection with this transaction, the Company expects to recognize a non-cash, non-operating settlement gain in the fourth quarter 2023. The actual settlement gain will depend on the finalization of the actuarial calculations.

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Millions	3 Months Ended		9 Months Ended
Pay vs Performance Disclosure - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Pay vs Performance Disclosure
Net Income (Loss) Attributable to Parent	$ 75.7	$ 61.1	$ 221.8	$ 54

Insider Trading Arrangements

Insider Trading Arrangements	3 Months Ended	9 Months Ended
Insider Trading Arrangements	Sep. 30, 2023 shares	Sep. 30, 2023 shares
Trading Arrangements, by Individual
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false
Elliot S. Davis [Member]
Trading Arrangements, by Individual
Material Terms of Trading Arrangement		Elliot S. Davis, the Company’s Senior Vice President, Chief Legal and Compliance Officer, entered into a pre-arranged stock trading plan on September 15, 2023, which provides for the potential sale of up to 84,098 shares of the Company’s Common Stock between December 15, 2023 and May 3, 2024.
Name	Elliot S. Davis
Title	Senior Vice President, Chief Legal and Compliance Officer
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	September 15, 2023
Aggregate Available	84,098	84,098
James C. Diggs [Member]
Trading Arrangements, by Individual
Arrangement Duration	140 days

Accounting Policies (Policies)

Accounting Policies (Policies)	9 Months Ended
Accounting Policies (Policies)	Sep. 30, 2023
Accounting Policies [Abstract]
Basis Of Accounting	The interim consolidated financial statements include the accounts of ATI Inc. and its subsidiaries. Unless the context requires otherwise, “ATI” and “the Company” refer to ATI Inc. 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 2022 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, 2022 financial information has been derived from the Company’s audited consolidated financial statements.
New Accounting Pronouncements Adopted	New Accounting Pronouncements Adopted In September 2022, the Financial Accounting Standards Board (FASB) issued new accounting guidance related to disclosures about supplier finance programs. Supplier finance programs allow a buyer to offer its suppliers the option for access to payment in advance of an invoice due date, which is paid by a third-party finance provider or intermediary on the basis of invoices that the buyer has confirmed as valid. This new guidance requires a buyer in a supplier finance program to disclose sufficient information about the program to allow a user of financial statements to understand the program’s nature, activity during the period, changes from period to period, and potential magnitude, using both qualitative and quantitative information about its supplier finance programs. This new guidance, with the exception of disclosures on rollforward information, is effective for the Company in fiscal year 2023. The Company adopted this new accounting guidance effective January 1, 2023. The rollforward information disclosures are effective for the Company in fiscal year 2024, with early adoption permitted. The Company did not early adopt this guidance. The adoption of these changes did not have an impact on the Company’s consolidated financial statements other than disclosure requirements which are included in Note 6.
Inventory	Inventories are stated at the lower of cost (first-in, first-out (FIFO) and average cost methods) or net realizable value.
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, 2023, 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 6 million pounds of nickel with hedge dates through 2024. The aggregate notional amount hedged is approximately 9% of a single year’s estimated nickel raw material purchase requirements. These derivative instruments are used to hedge the variability of a selling price that is based on the London Metal Exchange (LME) index for nickel, as well as to hedge the variability of the purchase cost of nickel based on this LME index. Any gain or loss associated with these hedging arrangements is included in sales or cost of sales, depending on whether the underlying risk being hedged was the variable selling price or the variable raw material cost, respectively. At September 30, 2023, the outstanding financial derivatives used to hedge the Company’s exposure to energy cost volatility included natural gas cost hedges. At September 30, 2023, the Company hedged approximately 80% of its forecasted domestic requirements for natural gas for the remainder of 2023, approximately 55% for 2024 and approximately 15% for 2025. 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, 2023, 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 contain 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. 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
Inventory (Policies)	Sep. 30, 2023
Inventory Disclosure [Abstract]
Inventory	Inventories are stated at the lower of cost (first-in, first-out (FIFO) and average cost methods) or net realizable value.

Derivative Instruments and Hedg

Derivative Instruments and Hedging Activities (Policies)	9 Months Ended
Derivative Instruments and Hedging Activities (Policies)	Sep. 30, 2023
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, 2023, 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 6 million pounds of nickel with hedge dates through 2024. The aggregate notional amount hedged is approximately 9% of a single year’s estimated nickel raw material purchase requirements. These derivative instruments are used to hedge the variability of a selling price that is based on the London Metal Exchange (LME) index for nickel, as well as to hedge the variability of the purchase cost of nickel based on this LME index. Any gain or loss associated with these hedging arrangements is included in sales or cost of sales, depending on whether the underlying risk being hedged was the variable selling price or the variable raw material cost, respectively. At September 30, 2023, the outstanding financial derivatives used to hedge the Company’s exposure to energy cost volatility included natural gas cost hedges. At September 30, 2023, the Company hedged approximately 80% of its forecasted domestic requirements for natural gas for the remainder of 2023, approximately 55% for 2024 and approximately 15% for 2025. 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, 2023, 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 contain 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.

Compensation Related Costs, Ret

Compensation Related Costs, Retirement Benefits (Policies)	9 Months Ended
Compensation Related Costs, Retirement Benefits (Policies)	Sep. 30, 2023
Retirement Benefits [Abstract]
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. 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.

Commitment and Contingencies (P

Commitment and Contingencies (Policies)	9 Months Ended
Commitment and Contingencies (Policies)	Sep. 30, 2023
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
Revenue from Contracts with Customers (Tables)	Sep. 30, 2023
Revenue from Contract with Customer [Abstract]
Schedule of Disaggregation of Revenue	Comparative information regarding the Company’s overall revenues (in millions) by global and geographical markets for the third quarters and nine months ended September 30, 2023 and 2022 is included in the following tables. (in millions) Third quarter ended September 30, 2023 September 30, 2022 HPMC AA&S Total HPMC AA&S Total Diversified Global Markets: Aerospace & Defense: Jet Engines- Commercial $ 312.5 $ 16.9 $ 329.4 $ 285.7 $ 26.9 $ 312.6 Airframes- Commercial 104.0 99.6 203.6 49.8 81.6 131.4 Defense 39.7 53.1 92.8 38.5 47.6 86.1 Total Aerospace & Defense 456.2 169.6 625.8 374.0 156.1 530.1 Energy: Oil & Gas 2.5 84.5 87.0 6.9 120.2 127.1 Specialty Energy 20.2 41.7 61.9 26.4 39.2 65.6 Total Energy 22.7 126.2 148.9 33.3 159.4 192.7 Automotive 7.2 40.9 48.1 3.5 66.1 69.6 Medical 28.9 18.6 47.5 22.3 25.1 47.4 Electronics 0.6 44.2 44.8 0.7 47.8 48.5 Construction/Mining 7.7 32.3 40.0 9.5 38.3 47.8 Food Equipment & Appliances — 16.2 16.2 0.2 45.0 45.2 Other 16.2 38.1 54.3 14.1 36.6 50.7 Total $ 539.5 $ 486.1 $ 1,025.6 $ 457.6 $ 574.4 $ 1,032.0 (in millions) Nine months ended September 30, 2023 September 30, 2022 HPMC AA&S Total HPMC AA&S Total Diversified Global Markets: Aerospace & Defense: Jet Engines- Commercial $ 914.2 $ 67.0 $ 981.2 $ 696.1 $ 61.8 $ 757.9 Airframes- Commercial 242.0 295.7 537.7 130.9 200.3 331.2 Defense 131.6 157.8 289.4 120.6 123.6 244.2 Total Aerospace & Defense 1,287.8 520.5 1,808.3 947.6 385.7 1,333.3 Energy: Oil & Gas 8.6 317.2 325.8 32.1 323.3 355.4 Specialty Energy 75.0 137.8 212.8 88.0 109.3 197.3 Total Energy 83.6 455.0 538.6 120.1 432.6 552.7 Automotive 19.5 140.8 160.3 8.7 227.4 236.1 Construction/Mining 26.9 101.9 128.8 25.8 113.9 139.7 Medical 70.7 53.7 124.4 52.3 70.8 123.1 Electronics 1.8 113.4 115.2 1.9 147.6 149.5 Food Equipment & Appliances — 58.6 58.6 0.2 141.7 141.9 Other 47.4 128.1 175.5 38.7 110.6 149.3 Total $ 1,537.7 $ 1,572.0 $ 3,109.7 $ 1,195.3 $ 1,630.3 $ 2,825.6 (in millions) Third quarter ended September 30, 2023 September 30, 2022 HPMC AA&S Total HPMC AA&S Total Primary Geographical Market: United States $ 242.6 $ 314.2 $ 556.8 $ 221.6 $ 395.5 $ 617.1 Europe 217.7 49.7 267.4 147.0 53.2 200.2 Asia 37.8 103.4 141.2 60.7 108.9 169.6 Canada 13.1 10.4 23.5 10.0 9.5 19.5 South America, Middle East and other 28.3 8.4 36.7 18.3 7.3 25.6 Total $ 539.5 $ 486.1 $ 1,025.6 $ 457.6 $ 574.4 $ 1,032.0 (in millions) Nine months ended September 30, 2023 September 30, 2022 HPMC AA&S Total HPMC AA&S Total Primary Geographical Market: United States $ 661.1 $ 1,052.3 $ 1,713.4 $ 557.8 $ 1,105.6 $ 1,663.4 Europe 619.7 137.7 757.4 403.3 140.6 543.9 Asia 132.8 316.4 449.2 155.4 328.4 483.8 Canada 41.7 34.9 76.6 33.0 29.9 62.9 South America, Middle East and other 82.4 30.7 113.1 45.8 25.8 71.6 Total $ 1,537.7 $ 1,572.0 $ 3,109.7 $ 1,195.3 $ 1,630.3 $ 2,825.6 Comparative information regarding the Company’s major products based on their percentages of sales is included in the following table. Hot-Rolling and Processing Facility (HRPF) conversion service sales in the AA&S segment are excluded from this presentation. Third quarter ended September 30, 2023 September 30, 2022 HPMC AA&S Total HPMC AA&S Total Diversified Products and Services: Nickel-based alloys and specialty alloys 42 % 52 % 47 % 52 % 57 % 54 % Titanium and titanium-based alloys 24 % 13 % 19 % 16 % 7 % 11 % Precision forgings, castings and components 33 % — % 18 % 32 % — % 15 % Precision rolled strip products 1 % 19 % 9 % — % 22 % 12 % Zirconium and related alloys — % 16 % 7 % — % 14 % 8 % Total 100 % 100 % 100 % 100 % 100 % 100 % Nine months ended September 30, 2023 September 30, 2022 HPMC AA&S Total HPMC AA&S Total Diversified Products and Services: Nickel-based alloys and specialty alloys 45 % 56 % 50 % 50 % 55 % 52 % Precision forgings, castings and components 33 % — % 17 % 34 % — % 15 % Titanium and titanium-based alloys 21 % 11 % 16 % 16 % 6 % 11 % Precision rolled strip products 1 % 18 % 9 % — % 25 % 14 % Zirconium and related alloys — % 15 % 8 % — % 14 % 8 % 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 $ 7.7 $ 3.8 Expense to increase the reserve 0.2 0.7 Write-off of uncollectible accounts (4.2) (0.6) Balance as of period end $ 3.7 $ 3.9
Schedule of Contract Assets and Liabilities	(in millions) Contract Assets Short-term September 30, September 30, Balance as of beginning of fiscal year $ 64.1 $ 53.9 Recognized in current year 66.6 74.1 Reclassified to accounts receivable (74.1) (57.9) Reclassification to/from long-term and contract liability — 0.1 Balance as of period end $ 56.6 $ 70.2 (in millions) Contract Liabilities Short-term September 30, September 30, Balance as of beginning of fiscal year $ 149.1 $ 116.2 Recognized in current year 61.4 105.3 Amounts in beginning balance reclassified to revenue (86.7) (80.8) Current year amounts reclassified to revenue (40.9) (42.9) Other (0.1) 0.8 Reclassification to/from long-term and contract asset 27.4 22.1 Balance as of period end $ 110.2 $ 120.7 Long-term (a) September 30, September 30, Balance as of beginning of fiscal year $ 66.8 $ 84.4 Recognized in current year 1.0 9.6 Reclassification to/from short-term (27.4) (22.0) Balance as of period end $ 40.4 $ 72.0 (a) Long-term contract liabilities are included in Other long-term liabilities on the consolidated balance sheets.

Inventories (Tables)

Inventories (Tables)	9 Months Ended
Inventories (Tables)	Sep. 30, 2023
Inventory Disclosure [Abstract]
Schedule of Inventories	Inventories at September 30, 2023 and December 31, 2022 were as follows (in millions): September 30, December 31, Raw materials and supplies $ 255.4 $ 213.6 Work-in-process 1,056.0 941.1 Finished goods 127.6 111.9 1,439.0 1,266.6 Inventory valuation reserves (85.1) (70.9) Total inventories, net $ 1,353.9 $ 1,195.7

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	9 Months Ended
Property, Plant and Equipment (Tables)	Sep. 30, 2023
Property, Plant and Equipment [Abstract]
Property Plant And Equipment	Property, plant and equipment at September 30, 2023 and December 31, 2022 was as follows (in millions): September 30, December 31, Land $ 32.2 $ 31.5 Buildings and leasehold improvements 655.9 601.6 Equipment 2,984.7 2,895.5 3,672.8 3,528.6 Accumulated depreciation and amortization (2,046.5) (1,979.5) Total property, plant and equipment, net $ 1,626.3 $ 1,549.1

Supplemental Financial Statem_2

Supplemental Financial Statement Information - (Tables)	9 Months Ended
Supplemental Financial Statement Information - (Tables)	Sep. 30, 2023
Other Income and Expenses [Abstract]
Schedule of Other Expense, Net	Other income (expense), net for the three and nine months ended September 30, 2023 and 2022 was as follows: (in millions) Three months ended September 30, Nine months ended September 30, 2023 2022 2023 2022 Rent and royalty income $ 0.7 $ 0.5 $ 2.0 $ 1.7 Gains from disposal of property, plant and equipment, net — 0.1 0.3 0.3 Net equity income (loss) on joint ventures (See Note 5) (0.7) 0.8 (1.0) 11.2 Litigation reserve — (19.9) — (28.5) Total other income (expense), net $ — $ (18.5) $ 1.3 $ (15.3)
Roll Forward of Restructuring Reserve Activity	Restructuring reserves for severance cost activity is as follows: Severance and Employee Benefit Costs Balance at December 31, 2022 $ 9.8 Additions, net 2.2 Payments (1.1) Balance at September 30, 2023 $ 10.9

Debt (Tables)

Debt (Tables)	9 Months Ended
Debt (Tables)	Sep. 30, 2023
Debt Disclosure [Abstract]
Schedule of Debt Instruments	Debt at September 30, 2023 and December 31, 2022 was as follows (in millions): September 30, December 31, ATI Inc. 7.25% Notes due 2030 $ 425.0 $ — ATI Inc. 5.875% Notes due 2027 350.0 350.0 ATI Inc. 5.125% Notes due 2031 350.0 350.0 ATI Inc. 4.875% Notes due 2029 325.0 325.0 ATI Inc. 3.5% Convertible Senior Notes due 2025 291.4 291.4 Allegheny Ludlum 6.95% Debentures due 2025 (a) 150.0 150.0 ABL Term Loan 200.0 200.0 U.S. revolving credit facility — — Foreign credit facilities 11.2 19.4 Finance leases and other 103.3 79.4 Debt issuance costs (20.7) (17.2) Debt 2,185.2 1,748.0 Short-term debt and current portion of long-term debt 37.5 41.7 Long-term debt $ 2,147.7 $ 1,706.3 (a) 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) 2023 2022 2023 2022 Contractual coupon rate $ 2.5 $ 2.5 $ 7.6 $ 7.6 Amortization of debt issuance costs 0.5 0.4 1.4 1.3 Total interest expense $ 3.0 $ 2.9 $ 9.0 $ 8.9

Derivative Financial Instrume_2

Derivative Financial Instruments and Hedging (Tables)	9 Months Ended
Derivative Financial Instruments and Hedging (Tables)	Sep. 30, 2023
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: Interest rate swap Prepaid expenses and other current assets $ 1.2 $ 1.4 Foreign exchange contracts Prepaid expenses and other current assets 0.2 — Nickel and other raw material contracts Prepaid expenses and other current assets 2.5 12.5 Natural gas contracts Prepaid expenses and other current assets 0.1 2.4 Interest rate swap Other assets — 0.5 Nickel and other raw material contracts Other assets — 0.5 Natural gas contracts Other assets 0.2 0.7 Total derivatives designated as hedging instruments $ 4.2 $ 18.0 Liability derivatives Balance sheet location Derivatives designated as hedging instruments: Natural gas contracts Other current liabilities 3.3 2.0 Nickel and other raw material contracts Other current liabilities 5.0 2.1 Natural gas contracts Other long-term liabilities 0.5 0.5 Nickel and other raw material contracts Other long-term liabilities 0.5 — Total derivatives designated as hedging instruments $ 9.3 $ 4.6
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, 2023 and 2022 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 2023 2022 2023 2022 Nickel and other raw material contracts $ (1.6) $ (0.7) $ (0.4) $ 0.8 Natural gas contracts (1.0) 5.4 (1.4) 4.1 Foreign exchange contracts 0.1 0.5 0.1 0.5 Interest rate swap 0.1 0.6 0.3 — Total $ (2.4) $ 5.8 $ (1.4) $ 5.4 Amount of Gain (Loss) Amount of Gain (Loss) Nine months ended September 30, Nine months ended September 30, Derivatives in Cash Flow Hedging Relationships 2023 2022 2023 2022 Nickel and other raw material contracts $ (8.5) $ 16.6 $ 3.5 $ 15.3 Natural gas contracts (7.8) 17.2 (4.6) 9.1 Foreign exchange contracts 0.3 0.9 0.2 0.6 Interest rate swap 0.3 2.1 0.8 (0.3) Total $ (15.7) $ 36.8 $ (0.1) $ 24.7

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	9 Months Ended
Fair Value of Financial Instruments (Tables)	Sep. 30, 2023
Fair Value Disclosures [Abstract]
Fair Value By Balance Sheet Grouping	The estimated fair value of financial instruments at September 30, 2023 was as follows: Fair Value Measurements at Reporting Date Using (In millions) Total Total Quoted Prices in Significant Cash and cash equivalents $ 432.9 $ 432.9 $ 432.9 $ — Derivative financial instruments: Assets 4.2 4.2 — 4.2 Liabilities 9.3 9.3 — 9.3 Debt (a) 2,205.9 2,359.3 2,044.8 314.5 The estimated fair value of financial instruments at December 31, 2022 was as follows: Fair Value Measurements at Reporting Date Using (In millions) Total Total Quoted Prices in Significant Cash and cash equivalents $ 584.0 $ 584.0 $ 584.0 $ — Derivative financial instruments: Assets 18.0 18.0 — 18.0 Liabilities 4.6 4.6 — 4.6 Debt (a) 1,765.2 1,964.5 1,665.7 298.8

Retirement Benefits (Tables)

Retirement Benefits (Tables)	9 Months Ended
Retirement Benefits (Tables)	Sep. 30, 2023
Retirement Benefits [Abstract]
Schedule Of Defined Benefit Plans Disclosures	For the three month periods ended September 30, 2023 and 2022, the components of pension and other postretirement benefit expense for the Company’s defined benefit plans included the following (in millions): Pension Benefits Other Postretirement Benefits Three months ended September 30, Three months ended September 30, 2023 2022 2023 2022 Service cost - benefits earned during the year $ 1.5 $ 3.0 $ 0.2 $ 0.3 Interest cost on benefits earned in prior years 24.0 17.2 2.7 1.9 Expected return on plan assets (25.7) (31.6) — — Amortization of prior service cost (credit) 0.1 0.1 (0.2) (0.2) Amortization of net actuarial loss 14.5 15.8 1.5 3.3 Total retirement benefit expense $ 14.4 $ 4.5 $ 4.2 $ 5.3 For the nine month periods ended September 30, 2023 and 2022, 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, 2023 2022 2023 2022 Service cost - benefits earned during the year $ 4.7 $ 9.0 $ 0.5 $ 0.8 Interest cost on benefits earned in prior years 72.0 52.5 8.2 5.8 Expected return on plan assets (77.0) (96.6) — — Amortization of prior service cost (credit) 0.3 0.3 (0.7) (0.7) Amortization of net actuarial loss 43.3 47.7 4.5 9.9 Settlement loss — 29.5 — — Total retirement benefit expense $ 43.3 $ 42.4 $ 12.5 $ 15.8

Business Segments (Tables)

Business Segments (Tables)	9 Months Ended
Business Segments (Tables)	Sep. 30, 2023
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, 2023 2022 2023 2022 Total sales: High Performance Materials & Components $ 578.1 $ 505.0 $ 1,670.5 $ 1,325.6 Advanced Alloys & Solutions 551.8 646.8 1,791.0 1,805.5 1,129.9 1,151.8 3,461.5 3,131.1 Intersegment sales: High Performance Materials & Components 38.6 47.4 132.8 130.3 Advanced Alloys & Solutions 65.7 72.4 219.0 175.2 104.3 119.8 351.8 305.5 Sales to external customers: High Performance Materials & Components 539.5 457.6 1,537.7 1,195.3 Advanced Alloys & Solutions 486.1 574.4 1,572.0 1,630.3 $ 1,025.6 $ 1,032.0 $ 3,109.7 $ 2,825.6 Three months ended September 30, Nine months ended September 30, 2023 2022 2023 2022 EBITDA: High Performance Materials & Components $ 115.7 $ 85.8 $ 303.9 $ 214.2 Advanced Alloys & Solutions 50.4 75.8 186.3 255.7 Total segment EBITDA 166.1 161.6 490.2 469.9 Corporate expenses (12.9) (14.2) (48.3) (47.9) Closed operations and other expense (5.1) (6.3) (11.3) (12.8) Depreciation & amortization (a) (35.6) (35.6) (106.6) (107.1) Interest expense, net (23.8) (20.8) (65.0) (67.8) Restructuring and other charges (4.2) (17.3) (14.6) (23.5) Loss on asset sales and sales of businesses, net — — (0.6) (134.2) Income before income taxes $ 84.5 $ 67.4 $ 243.8 $ 76.6 a) The following is depreciation & amortization by each business segment: Three months ended September 30, Nine months ended September 30, 2023 2022 2023 2022 High Performance Materials & Components $ 16.5 $ 16.7 $ 51.8 $ 51.5 Advanced Alloys & Solutions 17.3 17.1 49.6 50.0 Other 1.8 1.8 5.2 5.6 $ 35.6 $ 35.6 $ 106.6 $ 107.1

Per Share Information (Tables)

Per Share Information (Tables)	9 Months Ended
Per Share Information (Tables)	Sep. 30, 2023
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 per common share: (In millions, except per share amounts) Three months ended September 30, Nine months ended September 30, 2023 2022 2023 2022 Numerator: Numerator for basic income per common share – Net income attributable to ATI $ 75.7 $ 61.1 $ 221.8 $ 54.0 Effect of dilutive securities: 3.5% Convertible Senior Notes due 2025 2.7 2.8 7.9 — Numerator for diluted net income per common share – Net income attributable to ATI after assumed conversions $ 78.4 $ 63.9 $ 229.7 $ 54.0 Denominator: Denominator for basic net income per common share – weighted average shares 128.1 129.8 128.4 126.9 Effect of dilutive securities: Share-based compensation 3.3 2.2 2.9 2.0 3.5% Convertible Senior Notes due 2025 18.8 18.8 18.8 — Denominator for diluted net income per common share – adjusted weighted average shares and assumed conversions 150.2 150.8 150.1 128.9 Basic net income attributable to ATI per common share $ 0.59 $ 0.47 $ 1.73 $ 0.43 Diluted net income attributable to ATI per common share $ 0.52 $ 0.42 $ 1.53 $ 0.42

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Income (Loss) (Tables)	9 Months Ended
Accumulated Other Comprehensive Income (Loss) (Tables)	Sep. 30, 2023
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, 2023 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, June 30, 2023 $ (712.0) $ (73.6) $ (1.1) $ 70.4 $ (716.3) OCI before reclassifications — (7.1) (2.4) — (9.5) Amounts reclassified from AOCI (a) 12.1 (b) — (c) 1.4 (d) 3.4 16.9 Net current-period OCI 12.1 (7.1) (1.0) 3.4 7.4 Balance, September 30, 2023 $ (699.9) $ (80.7) $ (2.1) $ 73.8 $ (708.9) Attributable to noncontrolling interests: Balance, June 30, 2023 $ — $ 6.4 $ — $ — $ 6.4 OCI before reclassifications — (2.1) — — (2.1) Amounts reclassified from AOCI — (b) — — — — Net current-period OCI — (2.1) — — (2.1) Balance, September 30, 2023 $ — $ 4.3 $ — $ — $ 4.3 The changes in AOCI by component, net of tax, for the nine month period ended September 30, 2023 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, December 31, 2022 $ (736.1) $ (70.1) $ 13.5 $ 67.5 $ (725.2) OCI before reclassifications — (10.6) (15.7) — (26.3) Amounts reclassified from AOCI (a) 36.2 (b) — (c) 0.1 (d) 6.3 42.6 Net current-period OCI 36.2 (10.6) (15.6) 6.3 16.3 Balance, September 30, 2023 $ (699.9) $ (80.7) $ (2.1) $ 73.8 $ (708.9) Attributable to noncontrolling interests: Balance, December 31, 2022 $ — $ 7.7 $ — $ — $ 7.7 OCI before reclassifications — (3.4) — — (3.4) Amounts reclassified from AOCI — (b) — — — — Net current-period OCI — (3.4) — — (3.4) Balance, September 30, 2023 $ — $ 4.3 $ — $ — $ 4.3 (a) Amounts were included in net periodic benefit cost for pension and other postretirement benefit plans (see Note 11). (b) No amounts were reclassified to earnings. (c) Amounts related to derivatives are included in sales, cost of goods sold or interest expense in the period or periods the hedged item affects earnings (see Note 8). (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, 2022 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, June 30, 2022 $ (883.0) $ (65.1) $ 16.8 $ 22.4 $ (908.9) OCI before reclassifications — (20.6) 5.8 — (14.8) Amounts reclassified from AOCI (a) 14.5 (c) — (d) (5.4) (e) 4.7 13.8 Net current-period OCI 14.5 (20.6) 0.4 4.7 (1.0) Balance, September 30, 2022 $ (868.5) $ (85.7) $ 17.2 $ 27.1 $ (909.9) Attributable to noncontrolling interests: Balance, June 30, 2022 $ — $ 17.0 $ — $ — $ 17.0 OCI before reclassifications — (9.4) — — (9.4) Amounts reclassified from AOCI — (c) — — — — Net current-period OCI — (9.4) — — $ (9.4) Balance, September 30, 2022 $ — $ 7.6 $ — $ — $ 7.6 The changes in AOCI by component, net of tax, for the nine month period ended September 30, 2022 were as follows (in millions): Post- Currency Derivatives Deferred Tax Asset Valuation Allowance Total Attributable to ATI: Balance, December 31, 2021 $ (947.7) $ (64.9) $ 5.1 $ 15.8 $ (991.7) OCI before reclassifications — (40.8) 36.8 — (4.0) Amounts reclassified from AOCI (a) 79.2 (b) 20.0 (d) (24.7) (e) 11.3 85.8 Net current-period OCI 79.2 (20.8) 12.1 11.3 81.8 Balance, September 30, 2022 $ (868.5) $ (85.7) $ 17.2 $ 27.1 $ (909.9) Attributable to noncontrolling interests: Balance, December 31, 2021 $ — $ 26.0 $ — $ — $ 26.0 OCI before reclassifications — (18.4) — — (18.4) Amounts reclassified from AOCI — (c) — — — — Net current-period OCI — (18.4) — — $ (18.4) Balance, September 30, 2022 $ — $ 7.6 $ — $ — $ 7.6 (a) Amounts were included in net periodic benefit cost for pension and other postretirement benefit plans (see Note 11) and/or loss on asset sales and sales of businesses, net, as part of the loss on sale of the Sheffield, UK operations. (b) Amounts were included in loss on asset sales and sales of businesses, net, as part of the loss on sale of the Sheffield, UK operations. (c) No amounts were reclassified to earnings. (d) Amounts related to derivatives are included in sales, cost of goods sold or interest expense in the period or periods the hedged item affects earnings (see Note 8). (e) 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, 2023 and 2022 were as follows: Details about AOCI Components (In millions) Three months ended September 30, 2023 Three months ended September 30, 2022 Nine months ended September 30, 2023 Nine months ended September 30, 2022 Affected line item in the statements Postretirement benefit plans Prior service credit $ 0.1 0.1 $ 0.4 $ 0.4 (a) Actuarial losses (16.0) (19.1) (47.8) (57.6) (a) Settlement loss — — — (29.5) (b) (15.9) (19.0) (47.4) (86.7) (d) Total before tax (3.8) (4.5) (11.2) (7.5) Tax expense (benefit) (e) $ (12.1) $ (14.5) $ (36.2) $ (79.2) Net of tax Currency translation adjustment $ — $ — $ — $ (20.0) (b,d) Derivatives Nickel and other raw material contracts $ (0.5) $ 1.0 $ 4.6 $ 20.1 (c) Natural gas contracts (1.8) 5.4 (6.0) 12.0 (c) Foreign exchange contracts 0.2 0.7 0.3 0.8 (c) Interest rate swap 0.3 — 1.0 (0.4) (c) (1.8) 7.1 (0.1) 32.5 (d) Total before tax (0.4) 1.7 — 7.8 Tax expense (benefit) (e) $ (1.4) $ 5.4 $ (0.1) $ 24.7 Net of tax (a) Amounts are reported in nonoperating retirement benefit expense (see Note 11). (b) Amounts in 2022 were included in loss on asset sales and sales of businesses, net, as part of the loss on sale of the Sheffield UK operations. (c) Amounts related to derivatives, with the exception of the interest rate swap, are included in sales or cost of goods sold in the period or periods the hedged item affects earnings. Amounts related to the interest rate swap are included in interest expense in the same period as the interest expense on the Term Loan is recognized in earnings (see Note 8). (d) For pre-tax 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. (e) 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, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) segment	Sep. 30, 2022 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)	Jun. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)
Revenue recognition [Line Items]
Number of business segments \| segment			2
Impact of new accounting principle on prior period operating income	$ 125.2	$ 113.2	$ 358.1	$ 178.6
Unfavorable impact on pre-tax operating results	(84.5)	(67.4)	(243.8)	(76.6)
Equity adjustment for adoption	1,355.9	938.4	1,355.9	938.4	$ 1,309.2	$ 1,157.2	$ 893.9	$ 832.7
Additional Paid-in Capital [Member]
Revenue recognition [Line Items]
Equity adjustment for adoption	1,689.5	1,662.2	1,689.5	1,662.2	1,682	1,668.1	1,656.6	1,596.7
[RetainedEarningsMember]
Revenue recognition [Line Items]
Equity adjustment for adoption	$ 398.7	$ 100	$ 398.7	$ 100	$ 323	$ 176.9	$ 38.9	$ 72.7
ATI Inc. 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%
Allegheny & Tsingshan Stainless
Revenue recognition [Line Items]
Equity method investment ownership percentage	50%		50%
Uniti
Revenue recognition [Line Items]
Equity method investment ownership percentage	50%		50%

Revenue from Contracts with C_3

Revenue from Contracts with Customers - Narrative (Details)	3 Months Ended		9 Months Ended
Revenue from Contracts with Customers - Narrative (Details)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) segment	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Revenue recognition [Line Items]
Inventories, net	$ 1,353,900,000		$ 1,353,900,000		$ 1,195,700,000
Sales	1,025,600,000	$ 1,032,000,000	3,109,700,000	$ 2,825,600,000
Cost of sales	831,000,000	848,200,000	2,512,800,000	2,297,100,000
Short-term contract assets	56,600,000	70,200,000	56,600,000	70,200,000	64,100,000	$ 53,900,000
Other current liabilities	246,600,000		246,600,000		219,800,000
Other long-term liabilities	185,300,000		185,300,000		207,700,000
Short-term contract liabilities	110,200,000	120,700,000	110,200,000	120,700,000	149,100,000	116,200,000
Long-term contract liabilities	40,400,000	72,000,000	$ 40,400,000	72,000,000	66,800,000	$ 84,400,000
Number of business segments \| segment			2
Confirmed order backlog	3,600,000,000	2,700,000,000	$ 3,600,000,000	2,700,000,000
Confirmed orders with current performance obligations	0.70		0.70
Accounts receivable with customers	686,700,000		686,700,000		586,900,000
Contract costs for obtaining and fulfilling contracts	7,700,000		7,700,000		$ 7,300,000
Amortization of contract costs	$ 200,000	$ 200,000	$ 900,000	$ 700,000

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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Disaggregation of Revenue [Line Items]
Revenue	$ 1,025.6	$ 1,032	$ 3,109.7	$ 2,825.6
Percent of revenue	100%	100%	100%	100%
Nickel-based Alloys and Specialty Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	47%	54%	50%	52%
Precision and Engineered Strip [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	9%	12%	9%	14%
Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	18%	15%	17%	15%
Titanium and Titanium-based Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	19%	11%	16%	11%
Zirconium and Related Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	7%	8%	8%	8%
UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue	$ 556.8	$ 617.1	$ 1,713.4	$ 1,663.4
Asia [Member]
Disaggregation of Revenue [Line Items]
Revenue	141.2	169.6	449.2	483.8
Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	267.4	200.2	757.4	543.9
CANADA
Disaggregation of Revenue [Line Items]
Revenue	23.5	19.5	76.6	62.9
South America, Middle East, and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue	36.7	25.6	113.1	71.6
Aerospace and Defense Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	625.8	530.1	1,808.3	1,333.3
Jet Engines- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	329.4	312.6	981.2	757.9
Airframes- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	203.6	131.4	537.7	331.2
Defense [Member]
Disaggregation of Revenue [Line Items]
Revenue	92.8	86.1	289.4	244.2
Energy
Disaggregation of Revenue [Line Items]
Revenue	148.9	192.7	538.6	552.7
Oil & Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue	87	127.1	325.8	355.4
Specialty Energy [Member]
Disaggregation of Revenue [Line Items]
Revenue	61.9	65.6	212.8	197.3
Automotive Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	48.1	69.6	160.3	236.1
Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	44.8	48.5	115.2	149.5
Construction Mining Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	40	47.8	128.8	139.7
Food Equipment and Appliances Market
Disaggregation of Revenue [Line Items]
Revenue	16.2	45.2	58.6	141.9
Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	47.5	47.4	124.4	123.1
Other Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	54.3	50.7	175.5	149.3
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,129.9	1,151.8	3,461.5	3,131.1
Operating Segments \| High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue	$ 578.1	$ 505	$ 1,670.5	$ 1,325.6
Percent of revenue	100%	100%	100%	100%
Operating Segments \| High Performance Materials & Components \| Nickel-based Alloys and Specialty Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	42%	52%	45%	50%
Operating Segments \| High Performance Materials & Components \| Precision and Engineered Strip [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	1%	0%	1%	0%
Operating Segments \| High Performance Materials & Components \| Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	33%	32%	33%	34%
Operating Segments \| High Performance Materials & Components \| Titanium and Titanium-based Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	24%	16%	21%	16%
Operating Segments \| High Performance Materials & Components \| Zirconium and Related Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	0%	0%	0%	0%
Operating Segments \| High Performance Materials & Components \| UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue	$ 242.6	$ 221.6	$ 661.1	$ 557.8
Operating Segments \| High Performance Materials & Components \| Asia [Member]
Disaggregation of Revenue [Line Items]
Revenue	37.8	60.7	132.8	155.4
Operating Segments \| High Performance Materials & Components \| Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	217.7	147	619.7	403.3
Operating Segments \| High Performance Materials & Components \| CANADA
Disaggregation of Revenue [Line Items]
Revenue	13.1	10	41.7	33
Operating Segments \| High Performance Materials & Components \| South America, Middle East, and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue	28.3	18.3	82.4	45.8
Operating Segments \| High Performance Materials & Components \| Aerospace and Defense Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	456.2	374	1,287.8	947.6
Operating Segments \| High Performance Materials & Components \| Jet Engines- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	312.5	285.7	914.2	696.1
Operating Segments \| High Performance Materials & Components \| Airframes- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	104	49.8	242	130.9
Operating Segments \| High Performance Materials & Components \| Defense [Member]
Disaggregation of Revenue [Line Items]
Revenue	39.7	38.5	131.6	120.6
Operating Segments \| High Performance Materials & Components \| Energy
Disaggregation of Revenue [Line Items]
Revenue	22.7	33.3	83.6	120.1
Operating Segments \| High Performance Materials & Components \| Oil & Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue	2.5	6.9	8.6	32.1
Operating Segments \| High Performance Materials & Components \| Specialty Energy [Member]
Disaggregation of Revenue [Line Items]
Revenue	20.2	26.4	75	88
Operating Segments \| High Performance Materials & Components \| Automotive Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	7.2	3.5	19.5	8.7
Operating Segments \| High Performance Materials & Components \| Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	0.6	0.7	1.8	1.9
Operating Segments \| High Performance Materials & Components \| Construction Mining Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	7.7	9.5	26.9	25.8
Operating Segments \| High Performance Materials & Components \| Food Equipment and Appliances Market
Disaggregation of Revenue [Line Items]
Revenue	0	0.2	0	0.2
Operating Segments \| High Performance Materials & Components \| Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	28.9	22.3	70.7	52.3
Operating Segments \| High Performance Materials & Components \| Other Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	16.2	14.1	47.4	38.7
Operating Segments \| Advanced Alloys & Solutions
Disaggregation of Revenue [Line Items]
Revenue	$ 551.8	$ 646.8	$ 1,791	$ 1,805.5
Percent of revenue	100%	100%	100%	100%
Operating Segments \| Advanced Alloys & Solutions \| Nickel-based Alloys and Specialty Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	52%	57%	56%	55%
Operating Segments \| Advanced Alloys & Solutions \| Precision and Engineered Strip [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	19%	22%	18%	25%
Operating Segments \| Advanced Alloys & Solutions \| Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	0%	0%	0%	0%
Operating Segments \| Advanced Alloys & Solutions \| Titanium and Titanium-based Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	13%	7%	11%	6%
Operating Segments \| Advanced Alloys & Solutions \| Zirconium and Related Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	16%	14%	15%	14%
Operating Segments \| Advanced Alloys & Solutions \| UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue	$ 314.2	$ 395.5	$ 1,052.3	$ 1,105.6
Operating Segments \| Advanced Alloys & Solutions \| Asia [Member]
Disaggregation of Revenue [Line Items]
Revenue	103.4	108.9	316.4	328.4
Operating Segments \| Advanced Alloys & Solutions \| Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	49.7	53.2	137.7	140.6
Operating Segments \| Advanced Alloys & Solutions \| CANADA
Disaggregation of Revenue [Line Items]
Revenue	10.4	9.5	34.9	29.9
Operating Segments \| Advanced Alloys & Solutions \| South America, Middle East, and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue	8.4	7.3	30.7	25.8
Operating Segments \| Advanced Alloys & Solutions \| Aerospace and Defense Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	169.6	156.1	520.5	385.7
Operating Segments \| Advanced Alloys & Solutions \| Jet Engines- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	16.9	26.9	67	61.8
Operating Segments \| Advanced Alloys & Solutions \| Airframes- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	99.6	81.6	295.7	200.3
Operating Segments \| Advanced Alloys & Solutions \| Defense [Member]
Disaggregation of Revenue [Line Items]
Revenue	53.1	47.6	157.8	123.6
Operating Segments \| Advanced Alloys & Solutions \| Energy
Disaggregation of Revenue [Line Items]
Revenue	126.2	159.4	455	432.6
Operating Segments \| Advanced Alloys & Solutions \| Oil & Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue	84.5	120.2	317.2	323.3
Operating Segments \| Advanced Alloys & Solutions \| Specialty Energy [Member]
Disaggregation of Revenue [Line Items]
Revenue	41.7	39.2	137.8	109.3
Operating Segments \| Advanced Alloys & Solutions \| Automotive Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	40.9	66.1	140.8	227.4
Operating Segments \| Advanced Alloys & Solutions \| Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	44.2	47.8	113.4	147.6
Operating Segments \| Advanced Alloys & Solutions \| Construction Mining Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	32.3	38.3	101.9	113.9
Operating Segments \| Advanced Alloys & Solutions \| Food Equipment and Appliances Market
Disaggregation of Revenue [Line Items]
Revenue	16.2	45	58.6	141.7
Operating Segments \| Advanced Alloys & Solutions \| Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	18.6	25.1	53.7	70.8
Operating Segments \| Advanced Alloys & Solutions \| Other Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	38.1	36.6	128.1	110.6
External Customers \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,025.6	1,032	3,109.7	2,825.6
External Customers \| Operating Segments \| High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue	539.5	457.6	1,537.7	1,195.3
External Customers \| Operating Segments \| Advanced Alloys & Solutions
Disaggregation of Revenue [Line Items]
Revenue	$ 486.1	$ 574.4	$ 1,572	$ 1,630.3

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, 2023	Sep. 30, 2022
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of beginning of fiscal year	$ 7.7	$ 3.8
Expense to increase the reserve	0.2	0.7
Write-off of uncollectible accounts	(4.2)	(0.6)
Balance as of period end	$ 3.7	$ 3.9

Revenue from Contracts with C_6

Revenue from Contracts with Customers - Contract Assets and Liabilities (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Contract Assets, Current [Roll Forward]
Balance as of beginning of fiscal year	$ 64.1	$ 53.9
Recognized in current year	66.6	74.1
Reclassified to accounts receivable	(74.1)	(57.9)
Contract with Customer, Asset, Current, Reclassification	0	0.1
Balance as of period end	56.6	70.2
Contract Liabilities, Current [Roll Forward]
Balance as of beginning of fiscal year	149.1	116.2
Recognized in current year	61.4	105.3
Amounts in beginning balance reclassified to revenue	(86.7)	(80.8)
Current year amounts reclassified to revenue	(40.9)	(42.9)
Other	(0.1)	0.8
Reclassification to/from long-term and contract asset	27.4	22.1
Balance as of period end	110.2	120.7
Contract Liabilities, Noncurrent [Roll Forward]
Balance as of beginning of fiscal year	66.8	84.4
Recognized in current year	1	9.6
Reclassification to/from short-term	(27.4)	(22)
Balance as of period end	$ 40.4	$ 72

Inventories (Details)

Inventories (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Inventory [Line Items]
Raw materials and supplies	$ 255.4	$ 213.6
Work-in-process	1,056	941.1
Finished goods	127.6	111.9
Inventory, Gross, Total	1,439	1,266.6
Inventory valuation reserves	(85.1)	(70.9)
Total inventories, net	$ 1,353.9	$ 1,195.7

Property, Plant and Equipment_2

Property, Plant and Equipment (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
Property, Plant and Equipment [Abstract]
Land	$ 32.2	$ 31.5
Buildings and leasehold improvements	655.9	601.6
Equipment	2,984.7	2,895.5
Property Plant And Equipment, gross	3,672.8	3,528.6
Accumulated depreciation and amortization	(2,046.5)	(1,979.5)
Total property, plant and equipment, net	1,626.3	$ 1,549.1
Construction in progress	267.1
Capital Expenditures Incurred but Not yet Paid	$ 28.9

Joint Ventures (Details)

Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		10 Months Ended	58 Months Ended
Joint Ventures (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2018	Dec. 31, 2022
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 432.9	$ 584		$ 432.9			$ 584
Sale of noncontrolling interests				0	$ 0.9
Income (loss) from equity investments	(0.7)		$ 0.8	(1)	11.2
Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Tariff refunds receivable			19.7		19.7
Allegheny & Tsingshan Stainless \| A&T Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments					9.9
Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 1			$ 1
Sale of noncontrolling interests					0.9
Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50%			50%
Allegheny & Tsingshan Stainless \| Tsingshan Group
Schedule of Equity Method Investments [Line Items]
Sale of noncontrolling interests		5.5				$ 12	17.5
Uniti
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50%			50%
Advanced Alloys & Solutions \| Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 85			$ 85
Advanced Alloys & Solutions \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	(0.5)		(0.5)	(1.3)	8.7
Advanced Alloys & Solutions \| Uniti
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	(0.2)		$ 1.3	0.3	$ 2.5
Advanced Alloys & Solutions \| Related Party \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Other receivables	1.9	3.2		1.9			3.2
Prepaid expenses and other current assets \| Advanced Alloys & Solutions \| Related Party \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Other receivables	0.5	0.4		0.5			0.4
Other Noncurrent Assets [Member] \| Advanced Alloys & Solutions \| Related Party \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Other receivables	$ 1.4	$ 2.8		$ 1.4			$ 2.8
China Baowu Steel Group Corporation Limited \| Advanced Alloys & Solutions \| 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%			40%
Tsingshan Group \| Tsingshan Group \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50%			50%
VSMPO \| VSMPO \| Uniti
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50%			50%
ATI Inc. \| Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	51%			51%
ATI Inc. \| Advanced Alloys & Solutions \| Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	60%			60%

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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Disaggregation of Revenue [Line Items]
Rent and royalty income	$ 0.7	$ 0.5	$ 2	$ 1.7
Gains from disposal of property, plant and equipment, net	0	0.1	0.3	0.3
Income (loss) from equity investments	(0.7)	0.8	(1)	11.2
Gain from sale of business			(0.6)	(141)
Litigation Reserve	0	(19.9)	0	(28.5)
Total other income (expense), net	$ 0	$ (18.5)	$ 1.3	$ (15.3)

Supplemental Financial Statem_4

Supplemental Financial Statement Information - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023 USD ($) segment	Sep. 30, 2022 USD ($) segment	Sep. 30, 2023 USD ($) segment	Sep. 30, 2022 USD ($) segment	Dec. 31, 2022 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring charges (credits)	$ (0.5)	$ (2.6)	$ 2.2	$ (5)
Number of positions with decreased severance-related reserve \| segment	(10)	(60)		(110)
Restructuring reserve	$ 10.9		10.9		$ 9.8
Supplier finance program, obligation	14		$ 14		$ 23.7
Restructuring and Related Cost, Expected Number of Positions Eliminated \| segment			40
Other Current Liabilities
Restructuring Cost and Reserve [Line Items]
Restructuring reserve	6.7		$ 6.7
Other Noncurrent Liabilities
Restructuring Cost and Reserve [Line Items]
Restructuring reserve	$ 4.2		$ 4.2

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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Restructuring Reserve [Roll Forward]
Balance at December 31, 2022			$ 9.8
Restructuring charges (credits)	$ (0.5)	$ (2.6)	2.2	$ (5)
Payments			(1.1)
Balance at September 30, 2023	$ 10.9		$ 10.9

Debt Schedule of Debt (Details)

Debt Schedule of Debt (Details) - USD ($)	9 Months Ended	12 Months Ended
Debt Schedule of Debt (Details) - USD ($)	Sep. 30, 2023	Dec. 31, 2022	Aug. 11, 2023
Debt Instrument [Line Items]
Debt Issuance Costs, Net	$ (20,700,000)	$ (17,200,000)
Debt	2,185,200,000	1,748,000,000
Short-term debt and current portion of long-term debt	37,500,000	41,700,000
Long-term debt	2,147,700,000	1,706,300,000
ATI Inc. 5.875% Notes due 2027
Debt Instrument [Line Items]
Debt, Gross	$ 350,000,000	$ 350,000,000
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Interest rate	5.875%	5.875%
Debt Instrument, Maturity Date	Dec. 01, 2027	Dec. 01, 2027
ATI Inc. 3.5% Convertible Senior Notes due 2025 \| Convertible Debt
Debt Instrument [Line Items]
Debt, Gross	$ 291,400,000	$ 291,400,000
Debt Issuance Costs, Net	$ (3,400,000)	$ (4,800,000)
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Interest rate	3.50%	3.50%
Debt Instrument, Maturity Date	Jun. 15, 2025	Jun. 15, 2025
Allegheny Ludlum 6.95% Debentures due 2025 (a)
Debt Instrument [Line Items]
Debt, Gross	$ 150,000,000	$ 150,000,000
Debt Instrument, Issuer	Allegheny Ludlum	Allegheny Ludlum
Interest rate	6.95%	6.95%
Debt Instrument, Maturity Date	Dec. 15, 2025	Dec. 15, 2025
Foreign credit facilities
Debt Instrument [Line Items]
Short-term Debt	$ 11,200,000	$ 19,400,000
Finance leases and other
Debt Instrument [Line Items]
Debt, Gross	103,300,000	79,400,000
ATI 2031 Notes
Debt Instrument [Line Items]
Debt, Gross	$ 350,000,000	$ 350,000,000
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Interest rate	5.125%	5.125%
Debt Instrument, Maturity Date	Oct. 01, 2031	Oct. 01, 2031
ATI 2029 Notes
Debt Instrument [Line Items]
Debt, Gross	$ 325,000,000	$ 325,000,000
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Interest rate	4.875%	4.875%
Debt Instrument, Maturity Date	Oct. 01, 2029	Oct. 01, 2029
Domestic Bank Group $600 million asset-based credit facility
Debt Instrument [Line Items]
Short-term Debt	$ 0	$ 0
2027 Term Loan
Debt Instrument [Line Items]
Debt, Gross	$ 200,000,000	$ 200,000,000
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Interest rate	4.21%
Debt Instrument, Maturity Date	Sep. 09, 2027	Sep. 09, 2027
ATI 2030 Notes
Debt Instrument [Line Items]
Debt, Gross	$ 425,000,000	$ 0
Debt Issuance Costs, Net			$ (6,200,000)
Debt Instrument, Issuer	ATI Inc.
Interest rate	7.25%
Debt Instrument, Maturity Date	Aug. 15, 2030

Debt - Revolving Credit Facilit

Debt - Revolving Credit Facility Narrative (Details)		3 Months Ended	9 Months Ended
Debt - Revolving Credit Facility Narrative (Details)	Aug. 11, 2023 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2023 USD ($) segment	Sep. 30, 2022 USD ($)	Sep. 09, 2027 USD ($)	Dec. 31, 2022 USD ($)
Debt Instrument [Line Items]
Debt issuance costs		$ 20,700,000	$ 20,700,000			$ 17,200,000
Pension Plans Defined Benefit
Debt Instrument [Line Items]
Contributions by employer		222,000,000	272,000,000
Foreign credit facilities
Debt Instrument [Line Items]
Maximum borrowing capacity		57,000,000	57,000,000
Short-term Debt		11,200,000	11,200,000			19,400,000
Domestic Bank Group $600 million asset-based credit facility
Debt Instrument [Line Items]
Bridge loan		60,000,000	60,000,000
Amount utilized to support the issuance of letters of credit		31,700,000	31,700,000
Average borrowings during period			17,000,000	$ 0
Short-term Debt		0	$ 0			0
Interest rate during period			6.50%
Domestic Bank Group $600 million asset-based credit facility \| Revolving credit facility
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 600,000,000	$ 600,000,000
Minimum fixed charge coverage ratio allowed in event of default		1	1
Percentage of borrowing capacity unavailable due to fixed charge coverage ratio covenant		10%	10%
Borrowing capacity unavailable due to fixed charge coverage ratio covenant, minimum		$ 60,000,000	$ 60,000,000
Minimum required liquidity number of days prior to maturity of senior notes			90 days
Debt covenant, undrawn availability requirement, amount		$ 120,000,000	$ 120,000,000
Debt covenant, undrawn availability requirement, percent of total available liquidity		20%	20%
Domestic Bank Group $600 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 $600 million asset-based credit facility \| Revolving credit facility \| Minimum \| Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Variable interest rate (percentage)			1.25%
Domestic Bank Group $600 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 $600 million asset-based credit facility \| Revolving credit facility \| Maximum \| Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Variable interest rate (percentage)			1.75%
Domestic Bank Group $600 million asset-based credit facility \| Letter of credit sub-facility
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 200,000,000	$ 200,000,000
2027 Term Loan
Debt Instrument [Line Items]
Outstanding borrowings		$ 200,000,000	200,000,000			200,000,000
Minimum prepayment increments allowed			$ 25,000,000
Interest rate		4.21%	4.21%
2027 Term Loan \| Interest Rate Swap
Debt Instrument [Line Items]
Notional amount of derivative		$ 50,000,000	$ 50,000,000
2027 Term Loan \| Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Variable interest rate (percentage)			2%
Allegheny Ludlum 6.95% Debentures due 2025 (a)
Debt Instrument [Line Items]
Outstanding borrowings		$ 150,000,000	$ 150,000,000			$ 150,000,000
Interest rate		6.95%	6.95%			6.95%
ATI 2030 Notes
Debt Instrument [Line Items]
Outstanding borrowings		$ 425,000,000	$ 425,000,000			$ 0
Interest rate		7.25%	7.25%
Debt issuance costs	$ 6,200,000
Debt instrument, amortization period			7 years
Debt instrument, redemption price, percentage			101%
Proceeds from Debt, Net of Issuance Costs	$ 418,800,000
ATI 2030 Notes \| Minimum
Debt Instrument [Line Items]
Debt Instrument, Notice Period Required for Redemption \| segment			15
ATI 2030 Notes \| Maximum
Debt Instrument [Line Items]
Debt Instrument, Notice Period Required for Redemption \| segment			60
Forecast \| Domestic Bank Group $600 million asset-based credit facility \| Revolving credit facility
Debt Instrument [Line Items]
Line of credit facility, increase to maximum borrowing capacity					$ 300,000,000

Debt - Convertible Notes Narrat

Debt - Convertible Notes Narrative (Details)	3 Months Ended		9 Months Ended
Debt - Convertible Notes Narrative (Details)	Sep. 30, 2023 USD ($) $ / shares	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) tradingDays $ / shares	Sep. 30, 2022 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)	Jun. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)
Debt Instrument [Line Items]
Equity adjustment for adoption	$ 1,355,900,000	$ 938,400,000	$ 1,355,900,000	$ 938,400,000	$ 1,309,200,000	$ 1,157,200,000	$ 893,900,000	$ 832,700,000
Debt issuance costs	20,700,000		20,700,000			17,200,000
Interest expense, net	23,800,000	$ 20,800,000	65,000,000	$ 67,800,000
Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt	2,359,300,000		2,359,300,000			1,964,500,000
Fair Value Inputs Level 1 \| Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt	2,044,800,000		2,044,800,000			1,665,700,000
ATI Inc. 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%
Debt instrument, convertible, threshold percentage of stock price trigger			130%
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 issuance costs	$ 3,400,000		$ 3,400,000			$ 4,800,000
Debt instrument, interest rate, effective percentage	4.20%	4.20%	4.20%	4.20%
Cap price (in dollars per share) \| $ / shares	$ 19.76		$ 19.76
Interest Expense, Debt, Excluding Amortization	$ 2,500,000	$ 2,500,000	$ 7,600,000	$ 7,600,000
Amortization of Debt Issuance Costs	500,000	400,000	1,400,000	1,300,000
Interest expense, net	3,000,000	$ 2,900,000	$ 9,000,000	$ 8,900,000
Debt instrument, convertible, number of equity instruments (in shares)			18,800,000
ATI Inc. 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	$ 786,000,000		$ 786,000,000			$ 590,000,000

Derivative Financial Instrume_3

Derivative Financial Instruments and Hedging (Details) € in Millions, lb in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) lb	Sep. 30, 2022 USD ($)	Sep. 30, 2023 EUR (€)	Sep. 30, 2023 USD ($)
Derivative Instruments Gain Loss [Line Items]
Percentage of estimated annual nickel requirements			9%
Percentage Of Forecasted Natural Gas Usage Hedged for 2023			80%
Percentage Of Forecasted Natural Gas Usage Hedged for 2024			55%
Percentage Of Forecasted Natural Gas Usage Hedged for 2025			15%
Reclassification of Cash Flow Hedge Gain (Loss) [Abstract]
Cash Flow Hedge Gain (Loss) To Be Reclassified Within Twelve Months			$ (4.3)
Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	$ (1.4)	$ 5.4	(0.1)	$ 24.7
Cash Flow Hedging \| Other Comprehensive Income (Loss) [Member]
Derivative Instruments Gain Loss [Line Items]
Net derivative gain (loss) on hedge transactions	(2.4)	5.8	$ (15.7)	36.8
Nickel
Derivative Instruments Gain Loss [Line Items]
Notional amount of nickel hedge (in pounds of nickel) \| lb			6
Interest Rate Swap \| Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	0.3	0	$ 0.8	(0.3)
Interest Rate Swap \| Cash Flow Hedging \| Other Comprehensive Income (Loss) [Member]
Derivative Instruments Gain Loss [Line Items]
Net derivative gain (loss) on hedge transactions	0.1	0.6	0.3	2.1
Nickel and other raw material contracts \| Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	(0.4)	0.8	3.5	15.3
Nickel and other raw material contracts \| Cash Flow Hedging \| Other Comprehensive Income (Loss) [Member]
Derivative Instruments Gain Loss [Line Items]
Net derivative gain (loss) on hedge transactions	(1.6)	(0.7)	(8.5)	16.6
Natural gas contracts \| Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	(1.4)	4.1	(4.6)	9.1
Natural gas contracts \| Cash Flow Hedging \| Other Comprehensive Income (Loss) [Member]
Derivative Instruments Gain Loss [Line Items]
Net derivative gain (loss) on hedge transactions	(1)	5.4	(7.8)	17.2
Foreign exchange contracts \| Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	0.1	0.5	0.2	0.6
Foreign exchange contracts \| Cash Flow Hedging \| Other Comprehensive Income (Loss) [Member]
Derivative Instruments Gain Loss [Line Items]
Net derivative gain (loss) on hedge transactions	$ 0.1	$ 0.5	$ 0.3	$ 0.9
Designated as Hedging Instrument \| Foreign exchange forward \| Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Notional amount of derivative \| €					€ 0
2027 Term Loan
Derivative Instruments Gain Loss [Line Items]
Debt Instrument, Interest Rate, Stated Percentage					4.21%	4.21%
2027 Term Loan \| Interest Rate Swap
Derivative Instruments Gain Loss [Line Items]
Notional amount of derivative						$ 50

Derivative Financial Instrume_4

Derivative Financial Instruments and Hedging (Details2) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Cash Flow Hedging
Derivatives Fair Value [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	$ (1.4)	$ 5.4	$ (0.1)	$ 24.7
Cash Flow Hedging \| Other Comprehensive Income (Loss) [Member]
Derivatives Fair Value [Line Items]
Net derivative gain (loss) on hedge transactions	(2.4)	5.8	(15.7)	36.8
Nickel and other raw material contracts \| Cash Flow Hedging
Derivatives Fair Value [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	(0.4)	0.8	3.5	15.3
Nickel and other raw material contracts \| Cash Flow Hedging \| Other Comprehensive Income (Loss) [Member]
Derivatives Fair Value [Line Items]
Net derivative gain (loss) on hedge transactions	(1.6)	(0.7)	(8.5)	16.6
Interest Rate Swap \| Cash Flow Hedging
Derivatives Fair Value [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	0.3	0	0.8	(0.3)
Interest Rate Swap \| Cash Flow Hedging \| Other Comprehensive Income (Loss) [Member]
Derivatives Fair Value [Line Items]
Net derivative gain (loss) on hedge transactions	0.1	0.6	0.3	2.1
Natural gas contracts \| Cash Flow Hedging
Derivatives Fair Value [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	(1.4)	4.1	(4.6)	9.1
Natural gas contracts \| Cash Flow Hedging \| Other Comprehensive Income (Loss) [Member]
Derivatives Fair Value [Line Items]
Net derivative gain (loss) on hedge transactions	(1)	5.4	(7.8)	17.2
Foreign exchange contracts \| Cash Flow Hedging
Derivatives Fair Value [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	0.1	0.5	0.2	0.6
Foreign exchange contracts \| Cash Flow Hedging \| Other Comprehensive Income (Loss) [Member]
Derivatives Fair Value [Line Items]
Net derivative gain (loss) on hedge transactions	0.1	$ 0.5	0.3	$ 0.9
Designated as Hedging Instrument
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Asset	4.2		4.2		$ 18
Derivative Fair Value Of Derivative Liability	9.3		9.3		4.6
Designated as Hedging Instrument \| Nickel and other raw material contracts \| Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Asset	2.5		2.5		12.5
Designated as Hedging Instrument \| Nickel and other raw material contracts \| Other assets
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Asset	0		0		0.5
Designated as Hedging Instrument \| Nickel and other raw material contracts \| Accrued liabilities
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Liability	5		5		2.1
Designated as Hedging Instrument \| Nickel and other raw material contracts \| Other long-term liabilities
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Liability	0.5		0.5		0
Designated as Hedging Instrument \| Interest Rate Swap \| Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Asset	1.2		1.2		1.4
Designated as Hedging Instrument \| Interest Rate Swap \| Other assets
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Asset	0		0		0.5
Designated as Hedging Instrument \| Natural gas contracts \| Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Asset	0.1		0.1		2.4
Designated as Hedging Instrument \| Natural gas contracts \| Other assets
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Asset	0.2		0.2		0.7
Designated as Hedging Instrument \| Natural gas contracts \| Accrued liabilities
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Liability	3.3		3.3		2
Designated as Hedging Instrument \| Natural gas contracts \| Other long-term liabilities
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Liability	0.5		0.5		0.5
Designated as Hedging Instrument \| Foreign exchange contracts \| Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Derivative Fair Value Of Derivative Asset	$ 0.2		$ 0.2		$ 0

Fair Value of Financial Instr_3

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Carrying Value Reported Amount Fair Value Disclosure
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 432.9	$ 584
Derivative financial instruments, Assets	4.2	18
Derivative Financial Instruments, liabilities	9.3	4.6
Debt	2,205.9	1,765.2
Estimate Of Fair Value Fair Value Disclosure
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	432.9	584
Derivative financial instruments, Assets	4.2	18
Derivative Financial Instruments, liabilities	9.3	4.6
Debt	2,359.3	1,964.5
Estimate Of Fair Value Fair Value Disclosure \| Fair Value Inputs Level 1
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	432.9	584
Derivative financial instruments, Assets	0	0
Derivative Financial Instruments, liabilities	0	0
Debt	2,044.8	1,665.7
Estimate Of Fair Value Fair Value Disclosure \| Fair Value Inputs Level 2
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Derivative financial instruments, Assets	4.2	18
Derivative Financial Instruments, liabilities	9.3	4.6
Debt	314.5	298.8
Convertible Debt \| ATI Inc. 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	$ 786	$ 590

Retirement Benefits (Details1)

Retirement Benefits (Details1) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
Retirement Benefits (Details1) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022	Jun. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Sheffield, UK Operations
Defined Benefit Plan Disclosure [Line Items]
Portion of loss in AOCI			$ 29.5
Pension Plans Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the year	$ 1.5	$ 3		$ 4.7	$ 9
Interest cost on benefits earned in prior years	24	17.2		72	52.5
Expected return on plan assets	(25.7)	(31.6)		(77)	(96.6)
Amortization of prior service cost (credit)	0.1	0.1		0.3	0.3
Amortization of net actuarial loss	14.5	15.8		43.3	47.7
Settlement loss (gain)				0	29.5
Total retirement benefit expense (income)	14.4	4.5		43.3	42.4
Contributions by employer	222			272
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the year	0.2	0.3		0.5	0.8
Interest cost on benefits earned in prior years	2.7	1.9		8.2	5.8
Expected return on plan assets	0	0		0	0
Amortization of prior service cost (credit)	(0.2)	(0.2)		(0.7)	(0.7)
Amortization of net actuarial loss	1.5	3.3		4.5	9.9
Settlement loss (gain)				0	0
Total retirement benefit expense (income)	$ 4.2	$ 5.3		$ 12.5	$ 15.8

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
Income Taxes - Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Income Tax Disclosure [Abstract]
Income tax (provision) benefit	$ (4.9)	$ (3)	$ (12.9)	$ (11.3)

Business Segments (Details)

Business Segments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
Business Segments (Details) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Sales	$ 1,025.6	$ 1,032	$ 3,109.7	$ 2,825.6
Segment EBITDA	166.1	161.6	490.2	469.9
Corporate expenses	(12.9)	(14.2)	(48.3)	(47.9)
Closed operations and other expense	(5.1)	(6.3)	(11.3)	(12.8)
Depreciation & amortization	(35.6)	(35.6)	(106.6)	(107.1)
Interest expense, net	(23.8)	(20.8)	(65)	(67.8)
Income (loss) from equity investments	(0.7)	0.8	(1)	11.2
Restructuring and other charges	(4.2)	(17.3)	(14.6)	(23.5)
Loss on asset sales and sales of businesses, net		0		(134.2)
Income before income taxes	84.5	67.4	243.8	76.6
Operating Segments
Segment Reporting Information [Line Items]
Sales	1,129.9	1,151.8	3,461.5	3,131.1
Depreciation & amortization	(35.6)	(35.6)	(106.6)	(107.1)
Operating Segments \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	578.1	505	1,670.5	1,325.6
Segment EBITDA	115.7	85.8	303.9	214.2
Depreciation & amortization	(16.5)	(16.7)	(51.8)	(51.5)
Operating Segments \| Advanced Alloys & Solutions
Segment Reporting Information [Line Items]
Sales	551.8	646.8	1,791	1,805.5
Segment EBITDA	50.4	75.8	186.3	255.7
Depreciation & amortization	(17.3)	(17.1)	(49.6)	(50)
Operating Segments \| Corporate Segment
Segment Reporting Information [Line Items]
Depreciation & amortization	(1.8)	(1.8)	(5.2)	(5.6)
Internal Customers \| Operating Segments
Segment Reporting Information [Line Items]
Sales	104.3	119.8	351.8	305.5
Internal Customers \| Operating Segments \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	38.6	47.4	132.8	130.3
Internal Customers \| Operating Segments \| Advanced Alloys & Solutions
Segment Reporting Information [Line Items]
Sales	65.7	72.4	219	175.2
External Customers \| Operating Segments
Segment Reporting Information [Line Items]
Sales	1,025.6	1,032	3,109.7	2,825.6
External Customers \| Operating Segments \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	539.5	457.6	1,537.7	1,195.3
External Customers \| Operating Segments \| Advanced Alloys & Solutions
Segment Reporting Information [Line Items]
Sales	$ 486.1	$ 574.4	$ 1,572	$ 1,630.3

Business Segments - Additional

Business Segments - Additional Information (Details)	3 Months Ended		9 Months Ended
Business Segments - Additional Information (Details)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) segment	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)
Restructuring Cost and Reserve [Line Items]
Number of business segments \| segment			2
Government sponsored COVID relief income				$ 34,300,000
Gain (Loss) on sale of business	$ (100,000)	$ 0	$ (800,000)	(134,200,000)
Litigation Reserve	0	19,900,000	0	28,500,000
Restructuring charges (credits)	(500,000)	(2,600,000)	2,200,000	(5,000,000)
Loss on asset sales and sales of businesses, net		0		(134,200,000)
Restructuring and other charges	(4,200,000)	$ (17,300,000)	(14,600,000)	(23,500,000)
Transaction costs from sales of businesses, net of proceeds			(300,000)	(2,800,000)
Start Up Costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges (credits)	2,800,000		8,500,000
Unplanned Outage Costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges (credits)	1,900,000		1,900,000
Asset Write-offs
Restructuring Cost and Reserve [Line Items]
Restructuring charges (credits)			2,000,000
Facility Closing
Restructuring Cost and Reserve [Line Items]
Depreciation			800,000
ATI Inc. 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
Sheffield, UK Operations
Restructuring Cost and Reserve [Line Items]
Gain (Loss) on sale of business				(141,000,000)
Pico Rivera, CA Operations
Restructuring Cost and Reserve [Line Items]
Gain (Loss) on sale of assets				6,800,000
Northbrook, IL Operations
Restructuring Cost and Reserve [Line Items]
Loss on asset sales and sales of businesses, net	$ 0		$ (600,000)
High Performance Materials & Components
Restructuring Cost and Reserve [Line Items]
Employee retention credits and Aviation Manufacturing Jobs Protection Program				27,500,000
Advanced Alloys & Solutions
Restructuring Cost and Reserve [Line Items]
Employee retention credits				$ 6,800,000

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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Apr. 28, 2023	Dec. 31, 2022	Feb. 02, 2022
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income attributable to ATI	$ 75.7	$ 61.1	$ 221.8	$ 54
Net income attributable to ATI after assumed conversions	$ 78.4	$ 63.9	$ 229.7	$ 54
Denominator for basic net income per common share – weighted average shares	128.1	129.8	128.4	126.9
Share-based compensation	3.3	2.2	2.9	2
Denominator for diluted net income per common share – adjusted weighted average shares and assumed conversions	150.2	150.8	150.1	128.9
Basic net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.59	$ 0.47	$ 1.73	$ 0.43
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.52	$ 0.42	$ 1.53	$ 0.42
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	0	0	0	22.5
Purchase of treasury stock	$ (45)	$ (15)	$ (55.1)	$ (104.9)
Stock repurchase plan, authorized amount					$ 75		$ 150
Treasury Stock, Shares, Acquired	1	0.5	1.2	4
Purchase of treasury stock	$ (45.4)	$ (15)	$ (55.5)	$ (104.9)
Treasury Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Purchase of treasury stock	(45.4)	(15)	(55.5)	(104.9)
ATI Inc. 3.5% Convertible Senior Notes due 2025
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Convertible Senior Notes- Effect on numerator	$ 2.7	$ 2.8	$ 7.9	$ 0
Convertible Senior Notes- Effect on denominator	18.8	18.8	18.8	0
ATI Inc. 3.5% Convertible Senior Notes due 2025 \| Convertible Debt
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Interest rate	3.50%		3.50%			3.50%

Accumulated Other Comprehensi_3

Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2023		Sep. 30, 2022		Sep. 30, 2023		Sep. 30, 2022
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ (716.3)		$ (908.9)		$ (725.2)		$ (991.7)
OCI before reclassifications	(9.5)		(14.8)		(26.3)		(4)
Amounts reclassified from AOCI	16.9		13.8		42.6		85.8
Net current-period OCI	7.4		(1)		16.3		81.8
Ending balance	(708.9)		(909.9)		(708.9)		(909.9)
Increase (Decrease) in Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Beginning balance	6.4		17		7.7		26
OCI before reclassifications	(2.1)		(9.4)		(3.4)		(18.4)
Amounts reclassified from AOCI	0		0		0		0
Net current-period OCI	(2.1)		(9.4)		(3.4)		(18.4)
Ending balance	4.3		7.6		4.3		7.6
Post- retirement benefit plans [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(712)		(883)		(736.1)		(947.7)
OCI before reclassifications	0		0		0		0
Amounts reclassified from AOCI	12.1	[1]	14.5	[2]	36.2	[1]	79.2	[2]
Net current-period OCI	12.1		14.5		36.2		79.2
Ending balance	(699.9)		(868.5)		(699.9)		(868.5)
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	(73.6)		(65.1)		(70.1)		(64.9)
OCI before reclassifications	(7.1)		(20.6)		(10.6)		(40.8)
Amounts reclassified from AOCI	0	[3]	0	[3]	0	[3]	20	[4]
Net current-period OCI	(7.1)		(20.6)		(10.6)		(20.8)
Ending balance	(80.7)		(85.7)		(80.7)		(85.7)
Increase (Decrease) in Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Beginning balance	6.4		17		7.7		26
OCI before reclassifications	(2.1)		(9.4)		(3.4)		(18.4)
Amounts reclassified from AOCI	0	[3]	0	[5]	0	[3]	0	[5]
Net current-period OCI	(2.1)		(9.4)		(3.4)		(18.4)
Ending balance	4.3		7.6		4.3		7.6
Derivatives [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(1.1)		16.8		13.5		5.1
OCI before reclassifications	(2.4)		5.8		(15.7)		36.8
Amounts reclassified from AOCI	1.4	[6]	(5.4)	[7]	0.1	[6]	(24.7)	[7]
Net current-period OCI	(1)		0.4		(15.6)		12.1
Ending balance	(2.1)		17.2		(2.1)		17.2
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
Accumulated Deferred Tax Asset Valuation Allowance [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	70.4		22.4		67.5		15.8
OCI before reclassifications	0		0		0		0
Amounts reclassified from AOCI	3.4	[8]	4.7	[9]	6.3	[8]	11.3	[9]
Net current-period OCI	3.4		4.7		6.3		11.3
Ending balance	73.8		27.1		73.8		27.1
Increase (Decrease) in Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Beginning balance	0		0		0		0
OCI before reclassifications	0		0		0		0
Amounts reclassified from AOCI	0		0		0		0
Net current-period OCI	0		0		0		0
Ending balance	$ 0		$ 0		$ 0		$ 0

[1]Amounts were included in net periodic benefit cost for pension and other postretirement benefit plans (see Note 11).[2]Amounts were included in net periodic benefit cost for pension and other postretirement benefit plans (see Note 11) and/or loss on asset sales and sales of businesses, net, as part of the loss on sale of the Sheffield, UK operations.[3]No amounts were reclassified to earnings.[4]Amounts were included in loss on asset sales and sales of businesses, net, as part of the loss on sale of the Sheffield, UK operations.[5]No amounts were reclassified to earnings.[6]Amounts related to derivatives are included in sales, cost of goods sold or interest expense in the period or periods the hedged item affects earnings (see Note 8).[7]Amounts related to derivatives are included in sales, cost of goods sold or interest expense in the period or periods the hedged item affects earnings (see Note 8).[8]Represents the net change in deferred tax asset valuation allowances on changes in AOCI balances between the balance sheet dates.[9]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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization to net income of net prior service credits		$ (0.1)	$ (0.1)	$ (0.4)	$ (0.4)
Amount reclassified from AOCI, Postretirement benefit plans, Actuarial losses		16	19.1	47.8	57.6
Settlement loss included in net income		0	0	0	29.5
Income before income taxes		84.5	67.4	243.8	76.6
Income tax provision		4.9	3	12.9	11.3
Reclassification adjustment included in net income		0	0	0	20
Sales		1,025.6	1,032	3,109.7	2,825.6
Cost of sales		831	848.2	2,512.8	2,297.1
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 of net prior service credits	[1]	0.1	0.1	0.4	0.4
Amount reclassified from AOCI, Postretirement benefit plans, Actuarial losses	[1]	(16)	(19.1)	(47.8)	(57.6)
Settlement loss included in net income	[2]	0	0	0	(29.5)
Income before income taxes	[3]	(15.9)	(19)	(47.4)	(86.7)
Income tax provision	[4]	(3.8)	(4.5)	(11.2)	(7.5)
Net income (loss)		(12.1)	(14.5)	(36.2)	(79.2)
Reclassification out of Accumulated Other Comprehensive Income [Member] \| Currency translation adjustment [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification adjustment included in net income	[2],[3]	0	0	0	(20)
Reclassification out of Accumulated Other Comprehensive Income [Member] \| Derivatives [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	[3]	(1.8)	7.1	(0.1)	32.5
Income tax provision	[4]	(0.4)	1.7	0	7.8
Net income (loss)		(1.4)	5.4	(0.1)	24.7
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]
Sales	[5]	(0.5)	1	4.6	20.1
Cost of sales	[5]	(0.5)	1	4.6	20.1
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	[5]	(1.8)	5.4	(6)	12
Reclassification out of Accumulated Other Comprehensive Income [Member] \| Derivatives [Member] \| Foreign Exchange
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of sales	[5]	0.2	0.7	0.3	0.8
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	[5]	$ 0.3	$ 0	$ 1	$ (0.4)

[1]Amounts are reported in nonoperating retirement benefit expense (see Note 11).[2]Amounts in 2022 were included in loss on asset sales and sales of businesses, net, as part of the loss on sale of the Sheffield UK operations.[3]For pre-tax 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.[4]These amounts exclude the impact of any deferred tax asset valuation allowances, when applicable.[5]Amounts related to derivatives, with the exception of the interest rate swap, are included in sales or cost of goods sold in the period or periods the hedged item affects earnings. Amounts related to the interest rate swap are included in interest expense in the same period as the interest expense on the Term Loan is recognized in earnings (see Note 8).

Commitments and Contingencies (

Commitments and Contingencies (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Components of Environmental Loss Accrual [Abstract]
Accrual For Environmental Loss Contingencies	$ 13		$ 13
Accrued Environmental Loss Contingencies Current	6		6
Federal Superfund and comparable state-managed sites	3		3
Formerly owned or operated sites	7		7
Owned or controlled sites at which Company operations have been or plan to be discontinued	3		3
Loss Contingency, Estimate [Abstract]
Litigation Reserve	0	$ (19.9)	0	$ (28.5)
Maximum
Loss Contingency, Estimate [Abstract]
Loss contingency maximum possible loss	$ 16		$ 16

Subsequent Events (Details)

Subsequent Events (Details) $ in Millions	3 Months Ended	9 Months Ended
Subsequent Events (Details) $ in Millions	Sep. 30, 2023 USD ($)	Sep. 30, 2023 USD ($)	Oct. 18, 2023 uSRetireeAndBeneficiary
Subsequent Event
Subsequent Event [Line Items]
Pension plan transferred			85%
Number of participants transferred			8,200
Number of participants			1,900
Pension Plans Defined Benefit
Subsequent Event [Line Items]
Contributions by employer \| $	$ 222	$ 272