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Joint Ventures (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Cash and cash equivalents | $ 109.9 | $ 159.8 |  |  | $ 141.6 | $ 229.6
Sales to noncontrolling interests | 7.4 | 0.0 |  |  |  |
Gain on joint venture deconsolidation (See Note 5) | $ 15.9 | 0.0 |  |  |  |
Next Gen Alloys LLC [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Joint venture ownership percentage | 51.00% |  |  |  |  |
Cash and cash equivalents | $ 12.0 |  |  |  |  |
Sales to noncontrolling interests | 2.7 |  |  |  |  |
Allegheny & Tsingshan Stainless [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Sales to noncontrolling interests | $ 5.0 |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  |  |  |  |
Uniti [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  |  |  |  |
Tsingshan Group [Member] | Allegheny & Tsingshan Stainless [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Joint venture ownership percentage by unaffiliated entity | 50.00% |  |  |  |  |
VSMPO [Member] | Uniti [Member] |  |  |  |  |  |
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 [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Joint venture ownership percentage | 60.00% |  |  |  |  |
Cash and cash equivalents | $ 40.8 |  |  |  |  |
Flat Rolled Products | Allegheny & Tsingshan Stainless [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Income (loss) from equity method investments | (0.6) |  |  |  |  |
Flat Rolled Products | Uniti [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Income (loss) from equity method investments | $ 0.5 | $ 0.2 |  |  |  |
Flat Rolled Products | 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% |  |  |  |  |
Forecast | Allegheny & Tsingshan Stainless [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Sales to noncontrolling interests |  |  | $ 12.5 | $ 17.5 |  |

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