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Debt - Convertible Notes Narrative (Details) | 3 Months Ended | 9 Months Ended |  |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2021 USD ($) | Sep. 30, 2021 USD ($) tradingDays $ / shares | Sep. 30, 2020 USD ($) | Sep. 30, 2021 USD ($) tradingDays $ / shares | Sep. 30, 2020 USD ($) | Jun. 30, 2021 USD ($) | Jan. 01, 2021 USD ($) | Dec. 31, 2020 USD ($) | Jul. 02, 2020 USD ($) | Jun. 30, 2020 USD ($) | Dec. 31, 2019 USD ($) | Feb. 15, 2016 | Dec. 31, 2013
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Payment for early debt extinguishment |  |  |  | $ 0 | $ 19,100,000 |  |  |  |  |  |  |  |
Debt issuance costs |  |  |  | 9,300,000 | 9,100,000 |  |  |  |  |  |  |  |
Equity adjustment for adoption |  | $ 680,200,000 | $ 1,849,800,000 | 680,200,000 | 1,849,800,000 | $ 606,100,000 |  | $ 641,400,000 |  | $ 1,842,300,000 | $ 2,193,200,000 |  |
Debt issuance costs |  | 22,800,000 |  | 22,800,000 |  |  |  | 14,500,000 |  |  |  |  |
Interest expense, net |  | (25,100,000) | (25,100,000) | (72,200,000) | (68,700,000) |  |  |  |  |  |  |  |
Debt extinguishment charge |  | 0 | 0 | 0 | 21,500,000 |  |  |  |  |  |  |  |
Estimate Of Fair Value Fair Value Disclosure |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt |  | 2,558,700,000 |  | 2,558,700,000 |  |  |  | 1,847,700,000 |  |  |  |  |
Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt |  | 2,296,000,000 |  | 2,296,000,000 |  |  |  | 1,594,200,000 |  |  |  |  |
Forecast |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Payment for early debt extinguishment | $ 64,500,000 |  |  |  |  |  |  |  |  |  |  |  |
Debt extinguishment charge | 65,500,000 |  |  |  |  |  |  |  |  |  |  |  |
Additional Paid-in Capital [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Equity adjustment for adoption |  | 1,592,100,000 | $ 1,622,200,000 | 1,592,100,000 | $ 1,622,200,000 | $ 1,587,500,000 |  | 1,625,500,000 |  | $ 1,617,800,000 | $ 1,618,000,000 |  |
Additional Paid-in Capital [Member] | Accounting Standards Update 2020-06 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Equity adjustment for adoption |  |  |  |  |  |  | $ (49,800,000) |  |  |  |  |  |
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt, Gross |  | $ 291,400,000 |  | $ 291,400,000 |  |  |  | $ 291,400,000 |  |  |  |  |
Interest rate |  | 3.50% |  | 3.50% |  |  |  | 3.50% |  |  |  |  |
Debt instrument, convertible, redemption price as a percent of principal amount |  |  |  | 100.00% |  |  |  |  |  |  |  |  |
Debt instrument, convertible, threshold percentage of stock price trigger |  |  |  | 130.00% |  |  |  |  |  |  |  |  |
Debt instrument, convertible, threshold trading days | tradingDays |  |  |  | 20 |  |  |  |  |  |  |  |  |
Debt instrument, convertible, threshold consecutive trading days | tradingDays |  |  |  | 30 |  |  |  |  |  |  |  |  |
Debt instrument, convertible, conversion ratio |  |  |  | 0.0645745 |  |  |  |  |  |  |  |  |
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares |  | $ 15.49 |  | $ 15.49 |  |  |  |  |  |  |  |  |
Debt instrument, convertible, number of equity instruments (in shares) |  |  |  | 18,800,000 |  |  |  |  |  |  |  |  |
Equity component of convertible debt |  | $ 0 |  | $ 0 |  |  |  | $ 46,800,000 | $ 51,400,000 |  |  |  |
Debt issuance costs |  | $ 7,000,000 |  | $ 7,000,000 |  |  |  | 8,200,000 |  |  |  |  |
Debt instrument, interest rate, effective percentage |  | 4.20% | 8.40% | 4.20% | 8.40% |  |  |  |  |  |  |  |
Cap price (in dollars per share) | $ / shares |  | $ 19.76 |  | $ 19.76 |  |  |  |  |  |  |  |  |
Interest Expense, Debt, Excluding Amortization |  | $ 2,500,000 | $ 2,400,000 | $ 7,600,000 | $ 2,400,000 |  |  |  |  |  |  |  |
Amortization of Debt Issuance Costs |  | 500,000 | 400,000 | 1,300,000 | 400,000 |  |  |  |  |  |  |  |
Interest expense, net |  | 3,000,000.0 | $ 2,800,000 | 8,900,000 | $ 2,800,000 |  |  |  |  |  |  |  |
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt | Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt |  | 402,000,000 |  | 402,000,000 |  |  |  |  |  |  |  |  |
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt | Accounting Standards Update 2020-06 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Equity adjustment for adoption |  |  |  |  |  |  | $ (45,400,000) |  |  |  |  |  |
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt | Additional Paid-in Capital [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt issuance costs |  |  |  |  |  |  |  | 1,600,000 |  |  |  |  |
Allegheny Technologies 4.75% Convertible Senior Notes due 2022 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt, Gross |  | $ 84,200,000 |  | $ 84,200,000 |  |  |  | $ 84,200,000 |  |  |  |  |
Interest rate |  | 4.75% |  | 4.75% |  |  |  | 4.75% |  |  |  |  |
Allegheny Technologies 4.75% Convertible Senior Notes due 2022 | Convertible Debt |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, convertible, redemption price as a percent of principal amount |  |  |  | 100.00% |  |  |  |  |  |  |  |  |
Debt instrument, convertible, conversion ratio |  |  |  | 0.0692042 |  |  |  |  |  |  |  |  |
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares |  | $ 14.45 |  | $ 14.45 |  |  |  |  |  |  |  |  |
Debt instrument, convertible, number of equity instruments (in shares) |  |  |  | 5,800,000 |  |  |  |  |  |  |  |  |
Debt issuance costs |  | $ 400,000 |  | $ 400,000 |  |  |  | $ 700,000 |  |  |  |  |
Debt instrument, interest rate, effective percentage |  | 5.40% | 5.40% | 5.40% | 5.40% |  |  |  |  |  |  |  |
Interest Expense, Debt, Excluding Amortization |  | $ 1,000,000.0 | $ 1,000,000.0 | $ 3,000,000.0 | $ 7,600,000 |  |  |  |  |  |  |  |
Amortization of Debt Issuance Costs |  | 200,000 | 100,000 | 400,000 | 900,000 |  |  |  |  |  |  |  |
Interest expense, net |  | 1,200,000 | $ 1,100,000 | 3,400,000 | $ 8,500,000 |  |  |  |  |  |  |  |
Debt instrument, repurchased face amount |  |  |  |  |  |  |  | 203,200,000 |  |  |  |  |
Allegheny Technologies 4.75% Convertible Senior Notes due 2022 | Convertible Debt | Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt |  | 115,000,000 |  | 115,000,000 |  |  |  |  |  |  |  |  |
Allegheny Technologies 5.875% Notes due 2023 (a) |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt, Gross |  | $ 500,000,000 |  | $ 500,000,000 |  |  |  | $ 500,000,000.0 |  |  |  |  |
Interest rate |  | 7.875% |  | 7.875% |  |  |  | 7.875% |  |  |  | 7.875% | 5.875%
Debt instrument, notice period required for redemption | tradingDays |  | 30 |  |  |  |  |  |  |  |  |  |  |
Allegheny Technologies 5.875% Notes due 2023 (a) | Forecast |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Write off of deferred debt issue costs | $ 1,000,000 |  |  |  |  |  |  |  |  |  |  |  |
ATI 2029 and 2031 Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt issuance costs |  | $ 4,700,000 |  |  |  |  |  |  |  |  |  |  |
Debt instrument, redemption price, percentage |  | 101.00% |  |  |  |  |  |  |  |  |  |  |
Proceeds from Debt, Net of Issuance Costs |  | $ 665,700,000 |  |  |  |  |  |  |  |  |  |  |
ATI 2029 and 2031 Notes | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, notice period required for redemption | tradingDays |  | 15 |  |  |  |  |  |  |  |  |  |  |
ATI 2029 and 2031 Notes | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, notice period required for redemption | tradingDays |  | 60 |  |  |  |  |  |  |  |  |  |  |
ATI 2029 Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt, Gross |  | $ 325,000,000.0 |  | $ 325,000,000.0 |  |  |  | $ 0 |  |  |  |  |
Interest rate |  | 4.875% |  | 4.875% |  |  |  |  |  |  |  |  |
Debt instrument, amortization period |  | 8 years |  |  |  |  |  |  |  |  |  |  |
ATI 2031 Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt, Gross |  | $ 350,000,000 |  | $ 350,000,000 |  |  |  | $ 0 |  |  |  |  |
Interest rate |  | 5.125% |  | 5.125% |  |  |  |  |  |  |  |  |
Debt instrument, amortization period |  | 10 years |  |  |  |  |  |  |  |  |  |  |

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