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Debt - Schedule of Debt (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Feb. 15, 2016 | [1] | Dec. 31, 2013
Debt Instrument [Line Items] |  |  |  |  |  |
Debt Issuance Costs, Net |  |  | $ 13.1 | $ 17.1 |  |
Total short-term and long-term debt |  |  | 1,540.7 | 1,877.0 |  |
Short-term debt and current portion of long-term debt |  |  | 10.1 | 105.1 |  |
Total long-term debt |  |  | $ 1,530.6 | $ 1,771.9 |  |
Allegheny Technologies $500 million 5.875% Senior Notes due 2023 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Debt instrument issuer | [1] |  | Allegheny Technologies | Allegheny Technologies |  |
Debt instrument interest rate stated percentage |  |  |  |  | 7.875% | 5.875%
Debt instrument carrying amount | [1] |  | $ 500.0 | $ 500.0 |  |
Allegheny Technologies $500 million 5.95% Senior Notes due 2021 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Debt instrument issuer |  |  | Allegheny Technologies | Allegheny Technologies |  |
Debt instrument interest rate stated percentage |  |  | 5.95% | 5.95% |  |
Debt instrument carrying amount |  |  | $ 500.0 | $ 500.0 |  |
Allegheny Technologies $350 million 9.375% Senior Notes due 2019 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Debt instrument issuer |  |  |  | Allegheny Technologies |  |
Debt instrument interest rate stated percentage |  |  |  | 9.375% |  |
Debt instrument carrying amount |  |  | $ 0.0 | $ 350.0 |  |
Allegheny Technologies, $287.5 million Convertible Senior Notes, 4.75%, Due 2022 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Debt instrument issuer |  |  | Allegheny Technologies | Allegheny Technologies |  |
Debt instrument interest rate stated percentage |  |  | 4.75% | 4.75% |  |
Debt instrument carrying amount |  |  | $ 287.5 | $ 287.5 |  |
2022 Term Loan [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Debt instrument issuer |  |  | Allegheny Technologies |  |  |
Debt instrument carrying amount |  |  | $ 100.0 | $ 100.0 |  |
Allegheny Ludlum 6.95% Debentures due 2025 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Debt instrument issuer |  |  | Allegheny Ludlum | Allegheny Ludlum |  |
Debt instrument interest rate stated percentage |  |  | 6.95% | 6.95% |  |
Debt instrument carrying amount |  |  | $ 150.0 | $ 150.0 |  |
Domestic Bank Group $400 million asset-based credit facility [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Debt instrument carrying amount |  |  | 0.0 | 0.0 |  |
Other [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Debt instrument carrying amount |  |  | 10.0 | 2.2 |  |
Foreign credit agreements [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Debt instrument carrying amount |  |  | $ 6.3 | $ 4.4 |  |
Maximum [Member] | Allegheny Technologies $500 million 5.875% Senior Notes due 2023 [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Debt instrument interest rate stated percentage |  |  | 7.875% |  |  |
London Interbank Offered Rate (LIBOR) [Member] | 2022 Term Loan [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Variable rate spread |  | 3.00% |  | 3.50% |  |
Revolving Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member] | Domestic Bank Group $400 million asset-based credit facility [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Variable rate spread |  |  | 1.75% | 2.00% |  |
Revolving Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member] | Domestic Bank Group $400 million asset-based credit facility [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Variable rate spread |  |  | 2.25% | 2.50% |  |
[1] | Bearing interest at 7.875% effective February 15, 2016.

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