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v3.20.1

Selected Quarterly Financial Data (Unaudited) - Narrative (Details) - USD ($) shares in Millions, $ in Millions |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Nov. 07, 2017 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Unusual Or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |
Gain (Loss) on Disposition of Oil and Gas and Timber Property |  |  | $ 62.4 | $ 29.3 |  | $ 91.7 |  |  |
Gain (Loss) On Disposition Of Oil And Gas And Timber Property, Net Of Tax |  |  | 60.5 | 27.3 |  |  |  |  |
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal |  |  |  |  |  | (1.8) | $ 0.0 | $ 0.0 |
Restructuring and other charges |  | $ (4.5) |  |  |  | (4.5) | 0.0 | 0.0 |
Restructuring And Other Charges, Net Of Tax |  | (4.3) |  |  |  |  |  |  |
Gain on joint venture deconsolidation |  |  |  |  | $ 15.9 | 0.0 | 15.9 | 0.0 |
Deconsolidation, Gain (Loss), Amount, Net of tax |  |  |  |  | $ 14.7 |  |  |  |
NRV charge |  |  |  |  |  | 25.6 | (27.9) | (54.0) |
Impairment of goodwill |  |  |  |  |  | 0.0 | 0.0 | 114.4 |
Debt extinguishment charge |  | (21.6) |  |  |  | (21.6) | 0.0 | (37.0) |
Debt extinguishment charge, net of tax |  | (20.5) |  |  |  |  |  |  |
Non-cash joint venture impairment charge |  | (11.4) |  |  |  | (11.4) | $ 0.0 | $ 0.0 |
Non-cash joint venture impairment charge, net of tax |  | (10.8) |  |  |  |  |  |  |
Valuation allowance decrease due to change in judgment on the realizability of deferred tax assets |  | $ 41.9 |  |  |  | 45.1 |  |  |
Shares issued | 17 |  |  |  |  |  |  |  |
Allegheny Technologies $350 million 9.375% Senior Notes due 2019 |  |  |  |  |  |  |  |  |
Unusual Or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument carrying amount |  |  |  |  |  |  |  |  | $ 350.0
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  |  |  |  |  | 9.375%
Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |  |
Unusual Or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |
Non-cash joint venture impairment charge |  |  |  |  |  | $ (11.4) |  |  |
Equity method investment ownership percentage |  | 50.00% |  |  |  | 50.00% |  |  |
Forged Products |  |  |  |  |  |  |  |  |
Unusual Or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal |  |  |  | (7.7) |  | $ (8.1) |  |  |
Disposal Group, Not Discontinued Operation, Gain (Loss) On Disposal, Net Of Tax |  |  |  | $ (7.2) |  |  |  |  |
Cast Products |  |  |  |  |  |  |  |  |
Unusual Or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal |  |  | 6.2 |  |  | $ 6.2 |  |  |
Disposal Group, Not Discontinued Operation, Gain (Loss) On Disposal, Net Of Tax |  |  | $ 6.0 |  |  |  |  |  |
Tsingshan Group | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |  |
Unusual Or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |
Equity method investment ownership percentage |  | 50.00% |  |  |  | 50.00% |  |  |

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