Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue		$ 616.2	$ 770.3	$ 1,308.7	$ 1,725.8
Percent of revenue		100.00%	100.00%	100.00%	100.00%
Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of revenue		95.00%	87.00%	91.00%	87.00%
Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		33.00%	34.00%	32.00%	34.00%
Precision rolled strip products
Disaggregation of Revenue [Line Items]
Percent of revenue		19.00%	12.00%	20.00%	12.00%
Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue		18.00%	14.00%	17.00%	15.00%
Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		13.00%	19.00%	12.00%	18.00%
Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		12.00%	8.00%	10.00%	8.00%
Standard stainless products
Disaggregation of Revenue [Line Items]
Percent of revenue		5.00%	13.00%	9.00%	13.00%
United States
Disaggregation of Revenue [Line Items]
Revenue		$ 329.7	$ 477.2	$ 729.6	$ 1,083.3
Asia
Disaggregation of Revenue [Line Items]
Revenue		135.3	118.7	278.0	238.0
Europe
Disaggregation of Revenue [Line Items]
Revenue		113.4	124.4	226.5	295.5
Canada
Disaggregation of Revenue [Line Items]
Revenue		20.0	16.2	37.3	38.2
South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		17.8	33.8	37.3	70.8
Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		281.2	365.6	535.4	858.1
Energy
Disaggregation of Revenue [Line Items]
Revenue	[1]	118.3	167.5	267.4	338.1
Automotive
Disaggregation of Revenue [Line Items]
Revenue		68.0	50.8	159.5	127.3
Electronics
Disaggregation of Revenue [Line Items]
Revenue		43.3	39.5	98.9	72.7
Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		21.9	34.8	64.4	78.4
Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue		20.7	47.2	56.1	97.6
Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue		32.0	29.9	61.0	68.5
Other
Disaggregation of Revenue [Line Items]
Revenue		30.8	35.0	66.0	85.1
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		658.1	822.9	1,399.3	1,859.0
Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue		$ 318.7	$ 316.7	$ 583.9	$ 765.2
Percent of revenue		100.00%	100.00%	100.00%	100.00%
Operating Segments | High Performance Materials & Components | Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of revenue		100.00%	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 revenue		45.00%	36.00%	41.00%	38.00%
Operating Segments | High Performance Materials & Components | Precision rolled strip products
Disaggregation of Revenue [Line Items]
Percent of revenue		0.00%	0.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue		36.00%	35.00%	39.00%	35.00%
Operating Segments | High Performance Materials & Components | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		19.00%	29.00%	20.00%	27.00%
Operating Segments | High Performance Materials & Components | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		0.00%	0.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | Standard stainless products
Disaggregation of Revenue [Line Items]
Percent of revenue		0.00%	0.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | United States
Disaggregation of Revenue [Line Items]
Revenue		$ 157.6	$ 166.2	$ 285.9	$ 393.5
Operating Segments | High Performance Materials & Components | Asia
Disaggregation of Revenue [Line Items]
Revenue		40.6	27.6	60.7	52.7
Operating Segments | High Performance Materials & Components | Europe
Disaggregation of Revenue [Line Items]
Revenue		86.3	92.0	165.4	225.5
Operating Segments | High Performance Materials & Components | Canada
Disaggregation of Revenue [Line Items]
Revenue		9.3	5.1	17.5	16.7
Operating Segments | High Performance Materials & Components | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		6.8	9.8	12.0	32.6
Operating Segments | High Performance Materials & Components | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		218.6	252.3	398.4	611.1
Operating Segments | High Performance Materials & Components | Energy
Disaggregation of Revenue [Line Items]
Revenue	[1]	48.6	22.8	79.1	46.9
Operating Segments | High Performance Materials & Components | Automotive
Disaggregation of Revenue [Line Items]
Revenue		1.4	1.0	3.4	3.0
Operating Segments | High Performance Materials & Components | Electronics
Disaggregation of Revenue [Line Items]
Revenue		0.2	0.2	0.5	0.5
Operating Segments | High Performance Materials & Components | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		6.1	5.0	11.2	10.3
Operating Segments | High Performance Materials & Components | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue		0.1	0.1	0.1	0.1
Operating Segments | High Performance Materials & Components | Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue		14.6	10.3	25.8	27.3
Operating Segments | High Performance Materials & Components | Other
Disaggregation of Revenue [Line Items]
Revenue		11.0	9.0	23.0	21.8
Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue		$ 339.4	$ 506.2	$ 815.4	$ 1,093.8
Percent of revenue		100.00%	100.00%	100.00%	100.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of revenue		90.00%	79.00%	85.00%	68.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		21.00%	32.00%	25.00%	28.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Precision rolled strip products
Disaggregation of Revenue [Line Items]
Percent of revenue		39.00%	20.00%	35.00%	18.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue		0.00%	0.00%	0.00%	0.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		6.00%	13.00%	6.00%	10.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		24.00%	14.00%	19.00%	12.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Standard stainless products
Disaggregation of Revenue [Line Items]
Percent of revenue		10.00%	21.00%	15.00%	32.00%
Operating Segments | Advanced Alloys & Solutions [Member] | United States
Disaggregation of Revenue [Line Items]
Revenue		$ 172.1	$ 311.0	$ 443.7	$ 689.8
Operating Segments | Advanced Alloys & Solutions [Member] | Asia
Disaggregation of Revenue [Line Items]
Revenue		94.7	91.1	217.3	185.3
Operating Segments | Advanced Alloys & Solutions [Member] | Europe
Disaggregation of Revenue [Line Items]
Revenue		27.1	32.4	61.1	70.0
Operating Segments | Advanced Alloys & Solutions [Member] | Canada
Disaggregation of Revenue [Line Items]
Revenue		10.7	11.1	19.8	21.5
Operating Segments | Advanced Alloys & Solutions [Member] | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		11.0	24.0	25.3	38.2
Operating Segments | Advanced Alloys & Solutions [Member] | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		62.6	113.3	137.0	247.0
Operating Segments | Advanced Alloys & Solutions [Member] | Energy
Disaggregation of Revenue [Line Items]
Revenue	[1]	69.7	144.7	188.3	291.2
Operating Segments | Advanced Alloys & Solutions [Member] | Automotive
Disaggregation of Revenue [Line Items]
Revenue		66.6	49.8	156.1	124.3
Operating Segments | Advanced Alloys & Solutions [Member] | Electronics
Disaggregation of Revenue [Line Items]
Revenue		43.1	39.3	98.4	72.2
Operating Segments | Advanced Alloys & Solutions [Member] | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		15.8	29.8	53.2	68.1
Operating Segments | Advanced Alloys & Solutions [Member] | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue		20.6	47.1	56.0	97.5
Operating Segments | Advanced Alloys & Solutions [Member] | Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue		17.4	19.6	35.2	41.2
Operating Segments | Advanced Alloys & Solutions [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue		19.8	26.0	43.0	63.3
External Customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		616.2	770.3	1,308.7	1,725.8
External Customers | Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue		300.6	300.7	541.5	721.0
External Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue		$ 315.6	$ 469.6	$ 767.2	$ 1,004.8

[1] Includes the oil & gas, downstream processing, and specialty energy markets.