Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenue		$ 692.5	$ 955.5
Percent of revenue		100.00%	100.00%
Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of revenue		88.00%	88.00%
Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		31.00%	35.00%
Precision rolled strip products
Disaggregation of Revenue [Line Items]
Percent of revenue		20.00%	12.00%
Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue		15.00%	16.00%
Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		12.00%	18.00%
Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		10.00%	7.00%
Standard stainless products
Disaggregation of Revenue [Line Items]
Percent of revenue		12.00%	12.00%
United States
Disaggregation of Revenue [Line Items]
Revenue		$ 399.9	$ 606.1
Asia
Disaggregation of Revenue [Line Items]
Revenue		142.7	119.3
Europe
Disaggregation of Revenue [Line Items]
Revenue		113.1	171.1
Canada
Disaggregation of Revenue [Line Items]
Revenue		17.3	22.0
South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		19.5	37.0
Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		254.2	492.5
Energy
Disaggregation of Revenue [Line Items]
Revenue	[1]	149.1	170.6
Automotive
Disaggregation of Revenue [Line Items]
Revenue		91.5	76.5
Electronics
Disaggregation of Revenue [Line Items]
Revenue		55.6	33.2
Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		42.5	43.6
Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue		35.4	50.4
Medical
Disaggregation of Revenue [Line Items]
Revenue		29.0	38.6
Other
Disaggregation of Revenue [Line Items]
Revenue		35.2	50.1
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		741.2	1,036.1
Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue		$ 265.2	$ 448.5
Percent of revenue		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%
Operating Segments | High Performance Materials & Components | Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		36.00%	40.00%
Operating Segments | High Performance Materials & Components | Precision rolled strip products
Disaggregation of Revenue [Line Items]
Percent of revenue		0.00%	0.00%
Operating Segments | High Performance Materials & Components | Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue		43.00%	35.00%
Operating Segments | High Performance Materials & Components | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		21.00%	25.00%
Operating Segments | High Performance Materials & Components | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		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%
Operating Segments | High Performance Materials & Components | United States
Disaggregation of Revenue [Line Items]
Revenue		$ 128.3	$ 227.3
Operating Segments | High Performance Materials & Components | Asia
Disaggregation of Revenue [Line Items]
Revenue		20.1	25.1
Operating Segments | High Performance Materials & Components | Europe
Disaggregation of Revenue [Line Items]
Revenue		79.1	133.5
Operating Segments | High Performance Materials & Components | Canada
Disaggregation of Revenue [Line Items]
Revenue		8.2	11.6
Operating Segments | High Performance Materials & Components | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		5.2	22.8
Operating Segments | High Performance Materials & Components | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		179.8	358.8
Operating Segments | High Performance Materials & Components | Energy
Disaggregation of Revenue [Line Items]
Revenue	[1]	30.5	24.1
Operating Segments | High Performance Materials & Components | Automotive
Disaggregation of Revenue [Line Items]
Revenue		2.0	2.0
Operating Segments | High Performance Materials & Components | Electronics
Disaggregation of Revenue [Line Items]
Revenue		0.3	0.3
Operating Segments | High Performance Materials & Components | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		5.1	5.3
Operating Segments | High Performance Materials & Components | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue		0.0	0.0
Operating Segments | High Performance Materials & Components | Medical
Disaggregation of Revenue [Line Items]
Revenue		11.2	17.0
Operating Segments | High Performance Materials & Components | Other
Disaggregation of Revenue [Line Items]
Revenue		12.0	12.8
Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue		$ 476.0	$ 587.6
Percent of revenue		100.00%	100.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of revenue		81.00%	77.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		28.00%	31.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Precision rolled strip products
Disaggregation of Revenue [Line Items]
Percent of revenue		31.00%	22.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue		0.00%	0.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		7.00%	11.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		15.00%	13.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Standard stainless products
Disaggregation of Revenue [Line Items]
Percent of revenue		19.00%	23.00%
Operating Segments | Advanced Alloys & Solutions [Member] | United States
Disaggregation of Revenue [Line Items]
Revenue		$ 271.6	$ 378.8
Operating Segments | Advanced Alloys & Solutions [Member] | Asia
Disaggregation of Revenue [Line Items]
Revenue		122.6	94.2
Operating Segments | Advanced Alloys & Solutions [Member] | Europe
Disaggregation of Revenue [Line Items]
Revenue		34.0	37.6
Operating Segments | Advanced Alloys & Solutions [Member] | Canada
Disaggregation of Revenue [Line Items]
Revenue		9.1	10.4
Operating Segments | Advanced Alloys & Solutions [Member] | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		14.3	14.2
Operating Segments | Advanced Alloys & Solutions [Member] | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		74.4	133.7
Operating Segments | Advanced Alloys & Solutions [Member] | Energy
Disaggregation of Revenue [Line Items]
Revenue	[1]	118.6	146.5
Operating Segments | Advanced Alloys & Solutions [Member] | Automotive
Disaggregation of Revenue [Line Items]
Revenue		89.5	74.5
Operating Segments | Advanced Alloys & Solutions [Member] | Electronics
Disaggregation of Revenue [Line Items]
Revenue		55.3	32.9
Operating Segments | Advanced Alloys & Solutions [Member] | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		37.4	38.3
Operating Segments | Advanced Alloys & Solutions [Member] | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue		35.4	50.4
Operating Segments | Advanced Alloys & Solutions [Member] | Medical
Disaggregation of Revenue [Line Items]
Revenue		17.8	21.6
Operating Segments | Advanced Alloys & Solutions [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue		23.2	37.3
External Customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		692.5	955.5
External Customers | Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue		240.9	420.3
External Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue		$ 451.6	$ 535.2

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