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Debt - Narrative (Details) |  | 3 Months Ended | 12 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 02, 2016 $ / shares | Dec. 31, 2019 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Sep. 30, 2024 USD ($) | Jun. 30, 2020 USD ($) | Dec. 31, 2016 USD ($) | May 24, 2016 USD ($) | Feb. 15, 2016 | Dec. 31, 2013
Interest Costs Incurred [Abstract] |  |  |  |  |  |  |  |  |  |  |
Interest expense |  |  | $ 104,900,000 | $ 102,100,000 | $ 134,900,000 |  |  |  |  |  |
Interest costs capitalized |  |  | 4,700,000 | 4,100,000 | 2,600,000 |  |  |  |  |  |
Interest costs paid |  |  | 105,700,000 | 102,600,000 | 133,800,000 |  |  |  |  |  |
Interest income |  |  | 5,900,000 | 1,100,000 | 1,100,000 |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |
2020 |  | $ 11,500,000 | 11,500,000 |  |  |  |  |  |  |  |
2021 |  | 4,800,000 | 4,800,000 |  |  |  |  |  |  |  |
2022 |  | 291,500,000 | 291,500,000 |  |  |  |  |  |  |  |
2023 |  | 502,700,000 | 502,700,000 |  |  |  |  |  |  |  |
2024 |  | 100,700,000 | 100,700,000 |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Payments of debt issuance costs |  |  | 5,500,000 | 0 | 800,000 |  |  |  |  |  |
Debt extinguishment charge |  | 21,600,000 | 21,600,000 | 0 | 37,000,000.0 |  |  |  |  |  |
Payment for Debt Extinguishment or Debt Prepayment Cost |  |  | 20,900,000 | 0 | 35,800,000 |  |  |  |  |  |
Write off of Deferred Debt Issuance Cost |  |  | $ 700,000 |  | $ 1,200,000 |  |  |  |  |  |
Allegheny Technologies $500 million 5.875% Senior Notes due 2023 [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument interest rate stated percentage |  |  |  |  |  |  |  |  |  | 7.875% | 5.875%
Increase in interest expense per downgrade notch, percent |  |  | 0.25% |  |  |  |  |  |  |  |
Maximum number of downgrade notches |  |  | 4 |  |  |  |  |  |  |  |
Maximum increase in interest rate expense, percent |  |  | 2.00% |  |  |  |  |  |  |  |
Debt instrument carrying amount |  | $ 500,000,000.0 | $ 500,000,000.0 | 500,000,000.0 |  |  |  |  |  |  |
Allegheny Technologies $500 million 5.875% Senior Notes due 2023 [Member] | Maximum |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument interest rate stated percentage |  | 7.875% | 7.875% |  |  |  |  |  |  |  |
Domestic Bank Group $500 million asset-based credit facility [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument carrying amount |  | $ 0 | $ 0 | 0 |  |  |  |  |  |  |
Line of credit facility amount outstanding |  | $ 35,300,000 | $ 35,300,000 |  |  |  |  |  |  |  |
Average outstanding amount |  |  |  | $ 43,000,000 |  |  |  |  |  |  |
Interest rate during period |  |  |  | 3.70% |  |  |  |  |  |  |
Term Loan due 2024 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument interest rate stated percentage |  | 4.21% | 4.21% |  |  |  |  |  |  |  |
Debt instrument carrying amount |  | $ 100,000,000.0 | $ 100,000,000.0 | $ 100,000,000.0 |  |  |  |  |  |  |
Debt Instrument, Prepayment Increments, Minimum |  |  | $ 25,000,000 |  |  |  |  |  |  |  |
Term Loan due 2024 | London Interbank Offered Rate (LIBOR) [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Variable rate spread |  |  | 2.00% |  |  |  |  |  |  |  |
Allegheny Technologies $500 million 5.95% Senior Notes due 2021 [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument interest rate stated percentage |  |  |  | 5.95% |  |  |  |  |  |  |
Debt instrument carrying amount |  | $ 0 | $ 0 | $ 500,000,000 |  |  |  |  |  |  |
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022 [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument interest rate stated percentage |  | 4.75% | 4.75% | 4.75% |  |  |  |  |  |  |
Debt instrument carrying amount |  | $ 287,500,000 | $ 287,500,000 | $ 287,500,000 |  |  |  |  |  |  |
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022 [Member] | Convertible Debt [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument interest rate stated percentage |  | 4.75% | 4.75% |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |  | $ 287,500,000 |  |
Convertible debt conversion rate in shares of stock | 0.0692042 |  |  |  |  |  |  |  |  |  |
Number of equity instruments issuable | 19,900,000 |  |  |  |  |  |  |  |  |  |
Conversion price of convertible notes | $ / shares | $ 14.45 |  |  |  |  |  |  |  |  |  |
Allegheny Technologies $350 million 9.375% Senior Notes due 2019 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument interest rate stated percentage |  |  |  |  |  |  |  | 9.375% |  |  |
Debt instrument carrying amount |  |  |  |  |  |  |  | $ 350,000,000 |  |  |
Allegheny Technologies $350 million 5.875% Senior Notes due 2027 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument interest rate stated percentage |  | 5.875% | 5.875% |  |  |  |  |  |  |  |
Proceeds from Issuance of Debt |  |  | $ 344,500,000 |  |  |  |  |  |  |  |
Debt instrument carrying amount |  | $ 350,000,000 | 350,000,000 | 0 |  |  |  |  |  |  |
STAL Revolving Credit Facility [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Line of credit maximum borrowing capacity |  | 20,000,000 | 20,000,000 |  |  |  |  |  |  |  |
Revolving Credit Facility [Member] | Domestic Bank Group $500 million asset-based credit facility [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility, Increase to Maximum Borrowing Capacity |  | 100,000,000 | 100,000,000 |  |  |  |  |  |  |  |
Line of credit maximum borrowing capacity |  | $ 500,000,000 | 500,000,000 |  |  |  |  |  |  |  |
Payments of debt issuance costs |  |  | $ 2,200,000 | $ 2,100,000 |  |  |  |  |  |  |
Fixed charge coverage ratio |  | 1.00 | 1.00 |  |  |  |  |  |  |  |
Remaining borrowing capacity, percent of minimum borrowing capacity |  | 12.50% | 12.50% |  |  |  |  |  |  |  |
Minimum remaining borrowing capacity |  | $ 62,500,000 | $ 62,500,000 |  |  |  |  |  |  |  |
Revolving Credit Facility [Member] | Domestic Bank Group $500 million asset-based credit facility [Member] | Minimum | London Interbank Offered Rate (LIBOR) [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Variable rate spread |  |  | 1.25% |  |  |  |  |  |  |  |
Revolving Credit Facility [Member] | Domestic Bank Group $500 million asset-based credit facility [Member] | Minimum | Base Rate [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Variable rate spread |  |  | 0.25% |  |  |  |  |  |  |  |
Revolving Credit Facility [Member] | Domestic Bank Group $500 million asset-based credit facility [Member] | Maximum | London Interbank Offered Rate (LIBOR) [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Variable rate spread |  |  | 1.75% |  |  |  |  |  |  |  |
Revolving Credit Facility [Member] | Domestic Bank Group $500 million asset-based credit facility [Member] | Maximum | Base Rate [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Variable rate spread |  |  | 0.75% |  |  |  |  |  |  |  |
Letter of Credit [Member] | Domestic Bank Group $500 million asset-based credit facility [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Line of credit maximum borrowing capacity |  | 200,000,000 | $ 200,000,000 |  |  |  |  |  |  |  |
Interest Rate Swap [Member] | Term Loan due 2024 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Derivative notional amount |  | $ 50,000,000 | $ 50,000,000 |  |  |  |  |  |  |  |
Forecast [Member] | Delayed-Draw Term Loan [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Line of credit maximum borrowing capacity |  |  |  |  |  |  | $ 100,000,000 |  |  |  |
Forecast [Member] | Revolving Credit Facility [Member] | Domestic Bank Group $500 million asset-based credit facility [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility, Increase to Maximum Borrowing Capacity |  |  |  |  |  | $ 200,000,000 |  |  |  |  |
Allegheny Technologies Inc | STAL Revolving Credit Facility [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Joint venture ownership percentage |  | 60.00% | 60.00% |  |  |  |  |  |  |  |

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- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910 + Details
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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(16)) -URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04.(a)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
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- Definition Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04.(a)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
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- Definition Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04.(a)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
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- Definition Amount of cash outflow for cost from early extinguishment and prepayment of debt. Includes, but is not limited to, third-party cost, premium paid, and other fee paid to lender directly for debt extinguishment or debt prepayment. Excludes accrued interest. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3291-108585 + Details
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