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Joint Ventures (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended | 24 Months Ended |
--- | --- | --- | --- | ---
Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2017
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Cash and cash equivalents | $ 217.0 | $ 109.9 | $ 382.0 |  | $ 141.6
Sales to noncontrolling interests | 0.0 | 7.4 |  |  |
Gain on joint venture deconsolidation | 0.0 | 15.9 |  |  |
Investment in subsidiaries | $ 0.0 |  | 0.0 |  |
Next Gen Alloys LLC |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Joint venture ownership percentage | 51.00% |  |  |  |
Cash and cash equivalents | $ 9.0 |  |  |  |
Sales to noncontrolling interests |  | 2.7 |  |  |
Allegheny & Tsingshan Stainless |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Sales to noncontrolling interests |  | 5.0 | $ 12.0 |  |
Equity method investment ownership percentage | 50.00% |  |  |  |
Uniti |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  |  |  |
Tsingshan Group | Allegheny & Tsingshan Stainless |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Joint venture ownership percentage by unaffiliated entity | 50.00% |  |  |  |
VSMPO | Uniti |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Joint venture ownership percentage by unaffiliated entity | 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% |  |  |  |
Cash and cash equivalents | $ 39.4 |  |  |  |
Flat Rolled Products | Allegheny & Tsingshan Stainless |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Income (loss) from equity method investments | (3.3) | (0.6) |  |  |
Investment in subsidiaries | 7.8 |  |  |  |
Flat Rolled Products | Uniti |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Income (loss) from equity method investments | $ 0.5 | $ 0.5 |  |  |
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% |  |  |  |
Forecast | Allegheny & Tsingshan Stainless |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Sales to noncontrolling interests |  |  |  | $ 17.5 |

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