Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue		$ 955.5	$ 1,004.8
Percent of revenue		100.00%	100.00%
Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of revenue		84.00%	85.00%
Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		32.00%	30.00%
Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue		16.00%	19.00%
Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		17.00%	19.00%
Precision and engineered strip
Disaggregation of Revenue [Line Items]
Percent of revenue		12.00%	11.00%
Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		7.00%	6.00%
Standard stainless products [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue		16.00%	15.00%
United States
Disaggregation of Revenue [Line Items]
Revenue		$ 606.1	$ 621.1
Europe
Disaggregation of Revenue [Line Items]
Revenue		171.1	211.0
Asia
Disaggregation of Revenue [Line Items]
Revenue		119.3	123.2
Canada
Disaggregation of Revenue [Line Items]
Revenue		22.0	26.4
South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		37.0	23.1
Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		492.5	525.6
Energy
Disaggregation of Revenue [Line Items]
Revenue	[1]	170.6	168.5
Automotive
Disaggregation of Revenue [Line Items]
Revenue		76.5	76.9
Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		43.6	57.9
Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue		50.4	53.2
Medical
Disaggregation of Revenue [Line Items]
Revenue		38.6	46.1
Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue		33.2	34.1
Other
Disaggregation of Revenue [Line Items]
Revenue		50.1	42.5
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		1,036.1	1,088.5
Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue		$ 448.5	$ 516.3
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		40.00%	37.00%
Operating Segments | High Performance Materials & Components | Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue		35.00%	38.00%
Operating Segments | High Performance Materials & Components | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		25.00%	25.00%
Operating Segments | High Performance Materials & Components | Precision and engineered strip
Disaggregation of Revenue [Line Items]
Percent of revenue		0.00%	0.00%
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 [Member]
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		$ 227.3	$ 269.0
Operating Segments | High Performance Materials & Components | Europe
Disaggregation of Revenue [Line Items]
Revenue		133.5	158.6
Operating Segments | High Performance Materials & Components | Asia
Disaggregation of Revenue [Line Items]
Revenue		25.1	42.7
Operating Segments | High Performance Materials & Components | Canada
Disaggregation of Revenue [Line Items]
Revenue		11.6	11.4
Operating Segments | High Performance Materials & Components | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		22.8	14.9
Operating Segments | High Performance Materials & Components | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		358.8	400.6
Operating Segments | High Performance Materials & Components | Energy
Disaggregation of Revenue [Line Items]
Revenue	[1]	24.1	28.1
Operating Segments | High Performance Materials & Components | Automotive
Disaggregation of Revenue [Line Items]
Revenue		2.0	3.6
Operating Segments | High Performance Materials & Components | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		5.3	18.5
Operating Segments | High Performance Materials & Components | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue		0.0	0.1
Operating Segments | High Performance Materials & Components | Medical
Disaggregation of Revenue [Line Items]
Revenue		17.0	24.7
Operating Segments | High Performance Materials & Components | Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue		0.3	0.0
Operating Segments | High Performance Materials & Components | Other
Disaggregation of Revenue [Line Items]
Revenue		12.8	21.0
Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue		$ 587.6	$ 572.2
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		70.00%	70.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		25.00%	23.00%
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		11.00%	12.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Precision and engineered strip
Disaggregation of Revenue [Line Items]
Percent of revenue		21.00%	23.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		13.00%	12.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Standard stainless products [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue		30.00%	30.00%
Operating Segments | Advanced Alloys & Solutions [Member] | United States
Disaggregation of Revenue [Line Items]
Revenue		$ 378.8	$ 352.1
Operating Segments | Advanced Alloys & Solutions [Member] | Europe
Disaggregation of Revenue [Line Items]
Revenue		37.6	52.4
Operating Segments | Advanced Alloys & Solutions [Member] | Asia
Disaggregation of Revenue [Line Items]
Revenue		94.2	80.5
Operating Segments | Advanced Alloys & Solutions [Member] | Canada
Disaggregation of Revenue [Line Items]
Revenue		10.4	15.0
Operating Segments | Advanced Alloys & Solutions [Member] | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		14.2	8.2
Operating Segments | Advanced Alloys & Solutions [Member] | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		133.7	125.0
Operating Segments | Advanced Alloys & Solutions [Member] | Energy
Disaggregation of Revenue [Line Items]
Revenue	[1]	146.5	140.4
Operating Segments | Advanced Alloys & Solutions [Member] | Automotive
Disaggregation of Revenue [Line Items]
Revenue		74.5	73.3
Operating Segments | Advanced Alloys & Solutions [Member] | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		38.3	39.4
Operating Segments | Advanced Alloys & Solutions [Member] | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue		50.4	53.1
Operating Segments | Advanced Alloys & Solutions [Member] | Medical
Disaggregation of Revenue [Line Items]
Revenue		21.6	21.4
Operating Segments | Advanced Alloys & Solutions [Member] | Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue		32.9	34.1
Operating Segments | Advanced Alloys & Solutions [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue		37.3	21.5
External Customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		955.5	1,004.8
External Customers | Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue		420.3	496.6
External Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue		$ 535.2	$ 508.2

[1] Includes the oil & gas, hydrocarbon and chemical processing, and electrical energy markets.