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Joint Ventures (Details) - USD ($) $ in Millions | 3 Months Ended | 10 Months Ended | 12 Months Ended | 34 Months Ended |
--- | --- | --- | --- | --- | ---
Dec. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2020 | Dec. 31, 2016
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Sale to noncontrolling interest |  |  |  | $ 0.0 | $ 14.4 | $ 3.7 |  |
Gain on joint venture deconsolidation |  | $ 15.9 |  | 0.0 | 15.9 | 0.0 |  |
Cash and cash equivalents | $ 490.8 |  | $ 382.0 | 490.8 | 382.0 | 141.6 |  | $ 229.6
Income (loss) from equity method investments |  |  |  | (10.7) | (1.0) | 0.0 |  |
Intercompany notes receivable | 0.0 |  | 0.0 | 0.0 | 0.0 |  |  |
Undistributed earnings of investees accounted for under equity method | 14.5 |  |  | 14.5 |  |  |  |
Non-cash joint venture impairment charge | 11.4 |  |  | 11.4 | 0.0 | 0.0 |  |
Shanghai STAL Precision Stainless Steel Co Ltd [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Cash and cash equivalents | 22.9 |  |  | 22.9 |  |  |  |
Next Gen Alloys LLC [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Sale to noncontrolling interest |  |  |  |  | 2.7 |  |  |
Cash and cash equivalents | $ 9.9 |  |  | $ 9.9 |  |  |  |
Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  |  | 50.00% |  |  |  |
Impairment of long-lived assets held-for-use |  |  |  | $ 14.2 |  |  |  |
Income (loss) from equity method investments |  |  |  | (19.3) | (3.9) |  |  |
Revenue from related parties |  |  |  | 14.6 | 4.1 |  |  |
Related party accounts receivable | $ 0.1 |  | 0.6 | 0.1 | 0.6 |  |  |
Due from affiliates, allowance for credit loss | $ 4.3 |  |  | 4.3 |  |  |  |
Non-cash joint venture impairment charge |  |  |  | $ 11.4 |  |  |  |
Uniti [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  |  | 50.00% |  |  |  |
Revenue from related parties |  |  |  | $ 31.3 | 49.4 | 38.6 |  |
Related party accounts receivable | $ 0.2 |  | 1.8 | 0.2 | 1.8 |  |  |
Flat Rolled Products | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Income (loss) from equity method investments |  |  |  | (12.2) |  |  |  |
Due from affiliates | 36.8 |  | 10.5 | 36.8 | 10.5 |  |  |
Flat Rolled Products | Uniti [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Income (loss) from equity method investments |  |  |  | 1.5 | 2.9 | $ 0.6 |  |
Prepaid expenses and other current assets [Member] | Flat Rolled Products | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Due from affiliates | 8.3 |  |  | 8.3 |  |  |  |
Other Noncurrent Assets [Member] | Flat Rolled Products | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Due from affiliates | $ 28.5 |  | 10.5 | $ 28.5 | $ 10.5 |  |  |
Allegheny Technologies Inc | Shanghai STAL Precision Stainless Steel Co Ltd [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Joint venture ownership percentage | 60.00% |  |  | 60.00% |  |  |  |
Allegheny Technologies Inc | Next Gen Alloys LLC [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Joint venture ownership percentage | 51.00% |  |  | 51.00% |  |  |  |
Allegheny Technologies Inc | China Baowu Steel Group Corporation Limited [Member] | Shanghai STAL Precision Stainless Steel Co Ltd [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Joint venture ownership percentage by unaffiliated entity | 40.00% |  |  | 40.00% |  |  |  |
Allegheny Technologies Inc | VSMPO [Member] | Uniti [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Joint venture ownership percentage by unaffiliated entity | 50.00% |  |  | 50.00% |  |  |  |
Property, Plant and Equipment | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Income (loss) from equity method investments |  |  |  | $ (7.1) |  |  |  |
Tsingshan Group | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Sale to noncontrolling interest |  |  | $ 12.0 |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  |  | 50.00% |  |  |  |
Forecast [Member] | Tsingshan Group | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Sale to noncontrolling interest |  |  |  |  |  |  | $ 17.5 |

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