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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

X

- Definition

Aggregate revenue during the period from the sale of goods in the normal course of business, after deducting returns, allowances and discounts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(a))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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- Definition

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