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Summary of Significant Accounting Policies - Principles of Consolidation (Details) - USD ($) $ in Millions | Dec. 31, 2021 | Dec. 31, 2020
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Uniti [Member] |  |
Noncontrolling Interest [Line Items] |  |
Equity method investment ownership percentage | 50.00% |
Related party accounts receivable | $ 6.1 | $ 1.4
Allegheny & Tsingshan Stainless |  |
Noncontrolling Interest [Line Items] |  |
Equity method investment ownership percentage | 50.00% |
Related party accounts receivable | $ 0.0 | $ 0.0
Shanghai STAL Precision Stainless Steel Co Ltd [Member] | Allegheny Technologies Inc |  |
Noncontrolling Interest [Line Items] |  |
Joint venture ownership percentage | 60.00% |

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