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Business Segments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Sales	$ 658.3	$ 598.0	$ 770.3	$ 955.5	$ 1,018.6	$ 1,018.7	$ 1,080.4	$ 1,004.8	$ 2,982.1	$ 4,122.5	$ 4,046.6
Operating Segments
Segment Reporting Information [Line Items]
Sales									3,182.9	4,446.4	4,341.6
Operating Segments | Intersegment Sales [Member]
Segment Reporting Information [Line Items]
Sales									200.8	323.9	295.0
Operating Segments | External Sales [Member]
Segment Reporting Information [Line Items]
Sales									2,982.1	4,122.5	4,046.6
Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales									1,235.4	2,054.2	2,039.2
Operating Segments | High Performance Materials & Components | Intersegment Sales [Member]
Segment Reporting Information [Line Items]
Sales									70.8	75.7	76.1
Operating Segments | High Performance Materials & Components | External Sales [Member]
Segment Reporting Information [Line Items]
Sales									1,164.6	1,978.5	1,963.1
Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Sales									1,947.5	2,392.2	2,302.4
Operating Segments | Advanced Alloys & Solutions [Member] | Intersegment Sales [Member]
Segment Reporting Information [Line Items]
Sales									130.0	248.2	218.9
Operating Segments | Advanced Alloys & Solutions [Member] | External Sales [Member]
Segment Reporting Information [Line Items]
Sales									$ 1,817.5	$ 2,144.0	$ 2,083.5

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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.

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