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Business Segments - Narrative (Details) $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2019 USD ($) | Sep. 30, 2019 USD ($) | Jun. 30, 2019 USD ($) | Dec. 31, 2019 USD ($) employee segment | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Gain on asset sales, net |  |  |  | $ 89.8 | $ 0.0 | $ 0.0 |
Restructuring and other charges | $ 4.5 |  |  | $ 4.5 | 0.0 | 0.0 |
Number of Reportable Segments | segment |  |  |  | 2,000,000 |  |  |
Restructuring charges |  |  |  | $ 4.5 | 0.0 | 0.0 |
Income (loss) from equity method investments |  |  |  | (10.7) | (1.0) | 0.0 |
Total international sales |  |  |  | 1,667.9 | 1,698.4 | 1,454.5 |
Sales by U.S. operations to foreign countries |  |  |  | 1,262.6 | 1,303.8 | 1,078.6 |
LIFO-related net realizable value charge | (33.6) |  |  | (33.6) | (2.9) |  |
Inventory valuation reserves | $ 104.1 |  |  | 104.1 | 88.5 |  |
Closed operations and other expenses |  |  |  | 25.5 | 21.6 | 34.0 |
Gain (Loss) on Disposition of Oil and Gas and Timber Property |  | $ 62.4 | $ 29.3 | 91.7 |  |  |
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal |  |  |  | $ (1.8) | 0.0 | 0.0 |
Flat Rolled Products |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Restructuring and Related Cost, Expected Number of Positions Eliminated | employee |  |  |  | 70 |  |  |
Allegheny & Tsingshan Stainless |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  |  | 50.00% |  |  |
Revenue from related parties |  |  |  | $ 14.6 | 4.1 |  |
Income (loss) from equity method investments |  |  |  | (19.3) | (3.9) |  |
Allegheny & Tsingshan Stainless | Flat Rolled Products |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Income (loss) from equity method investments |  |  |  | $ (12.2) |  |  |
Uniti [Member] |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  |  | 50.00% |  |  |
Revenue from related parties |  |  |  | $ 31.3 | 49.4 | 38.6 |
Uniti [Member] | Flat Rolled Products |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Income (loss) from equity method investments |  |  |  | $ 1.5 | $ 2.9 | $ 0.6 |
Aerospace & Defense Market Concentration Risk [Member] | Revenue Benchmark [Member] | High Performance Materials & Components |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Percentage of sales |  |  |  | 75.00% |  |  |
Oil & Gas and Aerospace & Defense Market Concentration [Member] | Revenue Benchmark [Member] | Flat Rolled Products |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Percentage of sales |  |  |  | 40.00% |  |  |
Allegheny Technologies $350 million 9.375% Senior Notes due 2019 |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  |  |  | 9.375%
Cast Products |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal |  | $ 6.2 |  | $ 6.2 |  |  |
Forged Products |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
LIFO-related net realizable value charge | $ (5.2) |  |  | (5.2) |  |  |
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal |  |  | $ (7.7) | $ (8.1) |  |  |
Allegheny Technologies Inc | STAL Precision Stainless Steel Company Limited [Member] |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Joint venture ownership percentage | 60.00% |  |  | 60.00% |  |  |
Allegheny Technologies Inc | Uniti [Member] | VSMPO [Member] |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Joint venture ownership percentage by unaffiliated entity | 50.00% |  |  | 50.00% |  |  |

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