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Selected Quarterly Financial Data (Unaudited) - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Unusual Or Infrequent Item [Line Items] |  |  |
Gain (Loss) on Disposition of Oil and Gas and Timber Property |  |  | $ 91.7
Net gain (loss) on sale of businesses | $ 13.8 | $ 0.0 | (7.1)
Restructuring and other credits (charges) | 10.5 | (1,132.1) | (4.5)
Joint venture restructuring and impairment charges | 0.0 | (2.4) | (11.4)
Valuation allowance increase (decrease) due to change in judgment on the realizability of deferred tax assets |  |  | (45.1)
Impairment of goodwill | 0.0 | (287.0) | 0.0
Severance Costs |  | (60.5) |
Termination benefits |  | (17.4) |
Other Asset Impairment Charges | 0.0 | (1,041.5) | 0.0
Other Postretirement Benefits |  |  |
Unusual Or Infrequent Item [Line Items] |  |  |
Termination benefits | $ 0.0 | $ (6.7) | 0.0
Allegheny & Tsingshan Stainless |  |  |
Unusual Or Infrequent Item [Line Items] |  |  |
Joint venture restructuring and impairment charges |  |  | (11.4)
Equity method investment ownership percentage | 50.00% |  |
Forged Products |  |  |
Unusual Or Infrequent Item [Line Items] |  |  |
Net gain (loss) on sale of businesses |  |  | (13.3)
Cast Products |  |  |
Unusual Or Infrequent Item [Line Items] |  |  |
Net gain (loss) on sale of businesses |  |  | 6.2
Other Asset Impairment Charges |  |  | $ (10.2)
Tsingshan Group | Allegheny & Tsingshan Stainless |  |  |
Unusual Or Infrequent Item [Line Items] |  |  |
Equity method investment ownership percentage | 50.00% |  |

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