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Business Segments - Narrative (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2017 USD ($) segment | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items] |  |  |  |  |
Number of Reportable Segments | segment |  |  | 2 |  |
Restructuring charges |  |  | $ 0.0 | $ 527.2 | $ 64.3
Total international sales |  |  | 1,454.5 | 1,277.1 | 1,577.0
Sales by U.S. operations to foreign countries |  |  | 1,078.6 | 971.4 | 1,215.8
LIFO-related net realizable value charge |  |  | (43.1) | (97.3) |
Inventory valuation reserves |  |  | 121.5 | 169.0 |
Closed operations and other expenses |  |  | 34.0 | 34.6 | 22.1
Closed operations environmental costs |  |  | 2.9 | 5.8 | 4.5
Decrease in employee benefit liabilities |  |  | (3.7) |  |
Other corporate expense |  |  | 22.7 | 23.8 | 15.3
Foreign currency remeasurement impacts |  |  | 7.3 |  |
Flat Rolled Products [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Increase (Decrease) in Assets |  |  |  | (32.0) |
Income (loss) from equity method investments |  |  | $ 0.6 | 0.5 | (0.1)
High Performance Materials & Components [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Increase (Decrease) in Assets |  |  |  | (521.0) |
STAL Precision Stainless Steel Company Limited [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Joint venture ownership percentage |  |  | 60.00% |  |
Uniti [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Equity method investment ownership percentage |  |  | 50.00% |  |
Revenue from related parties |  |  | $ 38.6 | 20.3 | 55.4
Uniti [Member] | VSMPO [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Joint venture ownership percentage by unaffiliated entity |  |  | 50.00% |  |
Corporate, Non-Segment [Member] | Closed Company and Other [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Retirement Benefit Expense |  |  | $ 4.8 | 5.0 | 2.3
Inventory Valuation Reserve [Member] | Titanium sponge inventory [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Inventory valuation charges | $ 11.3 |  |  |  |
Inventory Valuation Reserve [Member] | Non-PQ grade titanium sponge [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Inventory valuation charges |  | $ 25.4 |  |  |
Employee benefit costs [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Restructuring charges |  |  |  | 24.2 |
Employee benefit costs [Member] | Flat Rolled Products [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Restructuring charges |  |  |  | 4.9 |
Employee benefit costs [Member] | High Performance Materials & Components [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Restructuring charges |  |  |  | 7.5 |
Long-lived asset impairment [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Restructuring charges |  |  |  | 471.3 |
Asset Impairment Charges |  |  |  |  | $ 54.5
Facility Closing [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Restructuring charges |  | $ 3.5 |  | 31.7 |
Facility Closing [Member] | Flat Rolled Products [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Restructuring charges |  |  |  | 8.4 |
Facility Closing [Member] | High Performance Materials & Components [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Restructuring charges |  |  |  | $ 23.8 |
Aerospace & Defense Market Concentration Risk [Member] | Sales Revenue, Net [Member] | High Performance Materials & Components [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Percentage of sales |  |  | 75.00% |  |
Oil & Gas and Automotive Market Concentration [Member] | Sales Revenue, Net [Member] | Flat Rolled Products [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Percentage of sales |  |  | 40.00% |  |

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