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Selected Quarterly Financial Data (Unaudited) - Narrative (Details) - USD ($) shares in Millions, $ in Millions |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Nov. 07, 2017 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Unusual Or Infrequent Item [Line Items] |  |  |  |  |  |  |  |
Gain on joint venture deconsolidation |  | $ 15.9 |  |  |  | $ 15.9 | $ 0.0 | $ 0.0
Deconsolidation, Gain (Loss), Amount, Net of tax |  | $ 14.7 |  |  |  |  |  |
NRV charge |  |  |  |  |  | (27.9) | (54.0) | (39.9)
Impairment of goodwill |  |  |  | $ 114.4 |  | 0.0 | 114.4 | 0.0
Impairment of goodwill net of tax |  |  |  | $ 113.6 |  |  |  |
Debt extinguishment charge |  |  | $ 37.0 |  |  | $ 0.0 | $ 37.0 | 0.0
Impact of U.S. tax reform |  |  | $ (4.1) |  |  |  |  |
Shares issued | 17 |  |  |  |  |  |  |
SEC Schedule, 12-09, Reserve, Inventory | Titanium sponge inventory |  |  |  |  |  |  |  |
Unusual Or Infrequent Item [Line Items] |  |  |  |  |  |  |  |
Inventory valuation charges |  |  |  |  | $ 11.3 |  |  |
Allegheny Technologies $350 million 9.375% Senior Notes due 2019 [Member] |  |  |  |  |  |  |  |
Unusual Or Infrequent Item [Line Items] |  |  |  |  |  |  |  |
Debt instrument carrying amount |  |  |  |  |  |  |  | $ 350.0
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  |  |  |  | 9.375%
Allegheny & Tsingshan Stainless [Member] | Tsingshan Group [Member] |  |  |  |  |  |  |  |
Unusual Or Infrequent Item [Line Items] |  |  |  |  |  |  |  |
Joint venture ownership percentage by unaffiliated entity |  |  |  |  |  | 50.00% |  |

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