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Joint Ventures (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 17 Months Ended |  |
--- | --- | --- | --- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Mar. 31, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Aug. 06, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Cash and cash equivalents | $ 281.2 | $ 122.4 |  | $ 281.2 | $ 122.4 |  | $ 382.0 | $ 141.6
Sales to noncontrolling interests |  |  |  | 0.0 | 14.4 |  |  |
Gain on joint venture deconsolidation | 0.0 | 0.0 |  | 0.0 | 15.9 |  |  |
Income (loss) from equity method investments | (3.5) | 2.3 |  | (6.3) | 2.4 |  |  |
Investment in subsidiaries | $ 0.0 |  |  | $ 0.0 |  |  | $ 0.0 |
Next Gen Alloys LLC |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Joint venture ownership percentage | 51.00% |  |  | 51.00% |  |  |  |
Cash and cash equivalents | $ 6.9 |  |  | $ 6.9 |  |  |  |
Sales to noncontrolling interests |  |  | $ 2.7 |  |  |  |  |
Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Sales to noncontrolling interests |  |  |  |  | 12.0 |  |  |
Equity method investment ownership percentage | 50.00% |  |  | 50.00% |  |  |  |
Uniti |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  |  | 50.00% |  |  |  |
Tsingshan Group | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Joint venture ownership percentage by unaffiliated entity | 50.00% |  |  | 50.00% |  |  |  |
VSMPO | Uniti |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Joint venture ownership percentage by unaffiliated entity | 50.00% |  |  | 50.00% |  |  |  |
Flat Rolled Products | Shanghai STAL Precision Stainless Steel Co Ltd |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Joint venture ownership percentage | 60.00% |  |  | 60.00% |  |  |  |
Cash and cash equivalents | $ 45.5 |  |  | $ 45.5 |  |  |  |
Flat Rolled Products | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Income (loss) from equity method investments | (4.0) | 1.1 |  | (7.3) | 0.5 |  |  |
Investment in subsidiaries | 3.7 |  |  | 3.7 |  |  |  |
Due from Affiliates | 43.8 |  |  | 43.8 |  |  |  |
Flat Rolled Products | Uniti |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Income (loss) from equity method investments | $ 0.5 | $ 1.3 |  | $ 1.0 | $ 1.8 |  |  |
Flat Rolled Products | China Baowu Steel Group Corporation Limited | Shanghai STAL Precision Stainless Steel Co Ltd |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Joint venture ownership percentage by unaffiliated entity | 40.00% |  |  | 40.00% |  |  |  |
Forecast | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Sales to noncontrolling interests |  |  |  |  |  | $ 17.5 |  |

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