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Business Segments (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Sales | $ 909.9 | $ 869.1 | $ 880.2 | $ 865.9 | $ 796.1 | $ 770.5 | $ 810.5 | $ 757.5 | $ 3,525.1 | $ 3,134.6 | $ 3,719.6
External Sales [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Sales |  |  |  |  |  |  |  |  | 3,525.1 | 3,134.6 | 3,719.6
Operating Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Sales |  |  |  |  |  |  |  |  | 3,654.5 | 3,240.3 | 3,870.6
Operating Segments [Member] | Intersegment Sales [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Sales |  |  |  |  |  |  |  |  | 129.4 | 105.7 | 151.0
Operating Segments [Member] | External Sales [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Sales |  |  |  |  |  |  |  |  | 3,525.1 | 3,134.6 | 3,719.6
Operating Segments [Member] | High Performance Materials & Components [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Sales |  |  |  |  |  |  |  |  | 2,127.0 | 1,979.5 | 2,062.7
Operating Segments [Member] | High Performance Materials & Components [Member] | Intersegment Sales [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Sales |  |  |  |  |  |  |  |  | 59.6 | 49.1 | 76.8
Operating Segments [Member] | High Performance Materials & Components [Member] | External Sales [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Sales |  |  |  |  |  |  |  |  | 2,067.4 | 1,930.4 | 1,985.9
Operating Segments [Member] | Flat Rolled Products [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Sales |  |  |  |  |  |  |  |  | 1,527.5 | 1,260.8 | 1,807.9
Operating Segments [Member] | Flat Rolled Products [Member] | Intersegment Sales [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Sales |  |  |  |  |  |  |  |  | 69.8 | 56.6 | 74.2
Operating Segments [Member] | Flat Rolled Products [Member] | External Sales [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Sales |  |  |  |  |  |  |  |  | $ 1,457.7 | $ 1,204.2 | $ 1,733.7

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