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Business Segments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Sales	$ 1,037.9	$ 1,020.2	$ 1,009.5	$ 979.0	$ 909.9	$ 869.1	$ 880.2	$ 865.9	$ 4,046.6	$ 3,525.1	$ 3,134.6
Operating Segments
Segment Reporting Information [Line Items]
Sales									4,212.6	3,654.5	3,240.3
Operating Segments | Intersegment Sales [Member]
Segment Reporting Information [Line Items]
Sales									166.0	129.4	105.7
Operating Segments | External Sales [Member]
Segment Reporting Information [Line Items]
Sales									4,046.6	3,525.1	3,134.6
Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales									2,413.6	2,127.0	1,979.5
Operating Segments | High Performance Materials & Components | Intersegment Sales [Member]
Segment Reporting Information [Line Items]
Sales									79.4	59.6	49.1
Operating Segments | High Performance Materials & Components | External Sales [Member]
Segment Reporting Information [Line Items]
Sales									2,334.2	2,067.4	1,930.4
Operating Segments | Flat Rolled Products
Segment Reporting Information [Line Items]
Sales									1,799.0	1,527.5	1,260.8
Operating Segments | Flat Rolled Products | Intersegment Sales [Member]
Segment Reporting Information [Line Items]
Sales									86.6	69.8	56.6
Operating Segments | Flat Rolled Products | External Sales [Member]
Segment Reporting Information [Line Items]
Sales									$ 1,712.4	$ 1,457.7	$ 1,204.2

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Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130545-203045

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