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Revenue from Contracts with Customers - Disaggregation of Revenue (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
Disaggregation of Revenue [Line Items]
Revenue	$ 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
Percent of total revenue									100.00%	100.00%	100.00%
Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of total revenue									88.00%	89.00%	87.00%
Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									33.00%	35.00%	33.00%
Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of total revenue									14.00%	18.00%	20.00%
Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									17.00%	18.00%	17.00%
Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									9.00%	6.00%	5.00%
Precision rolled strip
Disaggregation of Revenue [Line Items]
Percent of total revenue									15.00%	12.00%	12.00%
Standard stainless products
Disaggregation of Revenue [Line Items]
Percent of total revenue									12.00%	11.00%	13.00%
United States
Disaggregation of Revenue [Line Items]
Revenue									$ 1,809.1	$ 2,454.6	$ 2,348.1
China
Disaggregation of Revenue [Line Items]
Revenue									315.5	355.6	320.0
Germany
Disaggregation of Revenue [Line Items]
Revenue									125.4	219.3	247.2
UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenue									126.5	173.8	242.1
France
Disaggregation of Revenue [Line Items]
Revenue									85.0	155.5	183.6
JAPAN
Disaggregation of Revenue [Line Items]
Revenue									86.2	147.7	214.9
Rest of World
Disaggregation of Revenue [Line Items]
Revenue									434.4	616.0	490.7
Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue									1,360.0	2,130.4	1,965.5
Energy*
Disaggregation of Revenue [Line Items]
Revenue									618.9	796.9	780.7
Automotive
Disaggregation of Revenue [Line Items]
Revenue									263.2	296.6	323.4
Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue									159.2	205.8	244.9
Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue									142.0	195.0	226.0
Medical
Disaggregation of Revenue [Line Items]
Revenue									119.1	172.4	183.1
Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue									177.7	163.2	156.9
Other
Disaggregation of Revenue [Line Items]
Revenue									142.0	162.2	166.1
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue									3,182.9	4,446.4	4,341.6
Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue									$ 1,235.4	$ 2,054.2	$ 2,039.2
Percent of total revenue									100.00%	100.00%	100.00%
Operating Segments | High Performance Materials & Components | Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of total revenue									100.00%	100.00%	100.00%
Operating Segments | High Performance Materials & Components | Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									38.00%	38.00%	37.00%
Operating Segments | High Performance Materials & Components | Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of total revenue									34.00%	36.00%	40.00%
Operating Segments | High Performance Materials & Components | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									28.00%	26.00%	23.00%
Operating Segments | High Performance Materials & Components | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									0.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | Precision rolled strip
Disaggregation of Revenue [Line Items]
Percent of total revenue									0.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | Standard stainless products
Disaggregation of Revenue [Line Items]
Percent of total revenue									0.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | United States
Disaggregation of Revenue [Line Items]
Revenue									$ 641.0	$ 1,042.6	$ 983.6
Operating Segments | High Performance Materials & Components | China
Disaggregation of Revenue [Line Items]
Revenue									31.0	93.9	71.2
Operating Segments | High Performance Materials & Components | Germany
Disaggregation of Revenue [Line Items]
Revenue									76.6	147.2	159.0
Operating Segments | High Performance Materials & Components | UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenue									101.9	156.7	225.6
Operating Segments | High Performance Materials & Components | France
Disaggregation of Revenue [Line Items]
Revenue									66.6	132.7	146.3
Operating Segments | High Performance Materials & Components | JAPAN
Disaggregation of Revenue [Line Items]
Revenue									44.7	95.0	128.1
Operating Segments | High Performance Materials & Components | Rest of World
Disaggregation of Revenue [Line Items]
Revenue									202.8	310.4	249.3
Operating Segments | High Performance Materials & Components | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue									946.9	1,624.1	1,562.9
Operating Segments | High Performance Materials & Components | Energy*
Disaggregation of Revenue [Line Items]
Revenue									106.2	153.6	133.9
Operating Segments | High Performance Materials & Components | Automotive
Disaggregation of Revenue [Line Items]
Revenue									5.5	10.5	9.5
Operating Segments | High Performance Materials & Components | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue									0.0	0.3	0.4
Operating Segments | High Performance Materials & Components | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue									18.6	42.5	72.7
Operating Segments | High Performance Materials & Components | Medical
Disaggregation of Revenue [Line Items]
Revenue									47.7	85.4	106.0
Operating Segments | High Performance Materials & Components | Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue									0.9	0.5	1.5
Operating Segments | High Performance Materials & Components | Other
Disaggregation of Revenue [Line Items]
Revenue									38.8	61.6	76.2
Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue									$ 1,947.5	$ 2,392.2	$ 2,302.4
Percent of total revenue									100.00%	100.00%	100.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of total revenue									80.00%	78.00%	75.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									29.00%	33.00%	31.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of total revenue									0.00%	0.00%	0.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									11.00%	11.00%	10.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									15.00%	11.00%	11.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Precision rolled strip
Disaggregation of Revenue [Line Items]
Percent of total revenue									25.00%	23.00%	23.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Standard stainless products
Disaggregation of Revenue [Line Items]
Percent of total revenue									20.00%	22.00%	25.00%
Operating Segments | Advanced Alloys & Solutions [Member] | United States
Disaggregation of Revenue [Line Items]
Revenue									$ 1,168.1	$ 1,412.0	$ 1,364.5
Operating Segments | Advanced Alloys & Solutions [Member] | China
Disaggregation of Revenue [Line Items]
Revenue									284.5	261.7	248.8
Operating Segments | Advanced Alloys & Solutions [Member] | Germany
Disaggregation of Revenue [Line Items]
Revenue									48.8	72.1	88.2
Operating Segments | Advanced Alloys & Solutions [Member] | UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenue									24.6	17.1	16.5
Operating Segments | Advanced Alloys & Solutions [Member] | France
Disaggregation of Revenue [Line Items]
Revenue									18.4	22.8	37.3
Operating Segments | Advanced Alloys & Solutions [Member] | JAPAN
Disaggregation of Revenue [Line Items]
Revenue									41.5	52.7	86.8
Operating Segments | Advanced Alloys & Solutions [Member] | Rest of World
Disaggregation of Revenue [Line Items]
Revenue									231.6	305.6	241.4
Operating Segments | Advanced Alloys & Solutions [Member] | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue									413.1	506.3	402.6
Operating Segments | Advanced Alloys & Solutions [Member] | Energy*
Disaggregation of Revenue [Line Items]
Revenue									512.7	643.3	646.8
Operating Segments | Advanced Alloys & Solutions [Member] | Automotive
Disaggregation of Revenue [Line Items]
Revenue									257.7	286.1	313.9
Operating Segments | Advanced Alloys & Solutions [Member] | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue									159.2	205.5	244.5
Operating Segments | Advanced Alloys & Solutions [Member] | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue									123.4	152.5	153.3
Operating Segments | Advanced Alloys & Solutions [Member] | Medical
Disaggregation of Revenue [Line Items]
Revenue									71.4	87.0	77.1
Operating Segments | Advanced Alloys & Solutions [Member] | Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue									176.8	162.7	155.4
Operating Segments | Advanced Alloys & Solutions [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue									103.2	100.6	89.9
External Customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue									2,982.1	4,122.5	4,046.6
External Customers | Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue									1,164.6	1,978.5	1,963.1
External Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue									$ 1,817.5	$ 2,144.0	$ 2,083.5

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