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Business Segments - Narrative (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2016 USD ($) | Dec. 31, 2018 USD ($) segment | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items] |  |  |  |
Number of Reportable Segments | segment |  | 2 |  |
Restructuring charges |  | $ 0.0 | $ 0.0 | $ 523.8
Income (loss) from equity method investments |  | (1.0) | 0.0 | 0.0
Total international sales |  | 1,698.4 | 1,454.5 | 1,277.1
Sales by U.S. operations to foreign countries |  | 1,303.8 | 1,078.6 | 971.4
LIFO-related net realizable value charge |  | (2.9) | (43.1) |
Inventory valuation reserves |  | 88.5 | 121.5 |
Closed operations and other expenses |  | $ 21.6 | 34.0 | 34.6
STAL Precision Stainless Steel Company Limited [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Joint venture ownership percentage |  | 60.00% |  |
Allegheny & Tsingshan Stainless [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Equity method investment ownership percentage |  | 50.00% |  |
Revenue from related parties |  | $ 4.1 |  |
Allegheny & Tsingshan Stainless [Member] | Flat Rolled Products |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Income (loss) from equity method investments |  | $ (3.9) |  |
Uniti [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Equity method investment ownership percentage |  | 50.00% |  |
Revenue from related parties |  | $ 49.4 | 38.6 | 20.3
Uniti [Member] | Flat Rolled Products |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Income (loss) from equity method investments |  | $ 2.9 | $ 0.6 | 0.5
Uniti [Member] | VSMPO [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Joint venture ownership percentage by unaffiliated entity |  | 50.00% |  |
SEC Schedule, 12-09, Reserve, Inventory | Titanium sponge inventory |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Inventory valuation charges | $ 11.3 |  |  |
Employee benefit costs [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Restructuring charges |  |  |  | 20.8
Employee benefit costs [Member] | Flat Rolled Products |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Restructuring charges |  |  |  | 4.9
Employee benefit costs [Member] | High Performance Materials & Components |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Restructuring charges |  |  |  | 7.5
Long-lived asset impairment [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Restructuring charges |  |  |  | 471.3
Facility Closing [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Restructuring charges |  |  |  | 31.7
Facility Closing [Member] | Flat Rolled Products |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Restructuring charges |  |  |  | 8.4
Facility Closing [Member] | High Performance Materials & Components |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Restructuring charges |  |  |  | 23.8
Employee benefit costs including special termination benefits [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Restructuring charges |  |  |  | $ 24.2
Aerospace & Defense Market Concentration Risk [Member] | Sales Revenue, Net [Member] | High Performance Materials & Components |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Percentage of sales |  | 75.00% |  |
Oil & Gas and Automotive Market Concentration [Member] | Sales Revenue, Net [Member] | Flat Rolled Products |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Percentage of sales |  | 45.00% |  |
Allegheny Technologies $350 million 9.375% Senior Notes due 2019 [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  | 9.375%

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