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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 | [1] | 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 | [1] |  | $ 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 | $ 100,000,000 | $ 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 |  |  | $ 350,000,000.0 | 350,000,000.0 | 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% |  |  |  |  |  |  |  |
[1] | Bearing interest at 7.875% effective February 15, 2016.

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