Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue		$ 598.0	$ 1,018.7	$ 2,323.8	$ 3,103.9
Percent of revenue		100.00%	100.00%	100.00%	100.00%
Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of revenue		84.00%	85.00%	84.00%	85.00%
Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		28.00%	33.00%	30.00%	32.00%
Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue		11.00%	16.00%	14.00%	18.00%
Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		17.00%	19.00%	18.00%	18.00%
Precision rolled strip products
Disaggregation of Revenue [Line Items]
Percent of revenue		18.00%	12.00%	13.00%	11.00%
Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		10.00%	5.00%	9.00%	6.00%
Standard stainless products [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue		16.00%	15.00%	16.00%	15.00%
United States
Disaggregation of Revenue [Line Items]
Revenue		$ 348.6	$ 605.4	$ 1,431.9	$ 1,867.0
Europe
Disaggregation of Revenue [Line Items]
Revenue		94.8	186.3	390.3	604.5
Asia
Disaggregation of Revenue [Line Items]
Revenue		122.9	164.5	360.9	459.6
Canada
Disaggregation of Revenue [Line Items]
Revenue		12.9	25.6	51.1	81.4
South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		18.8	36.9	89.6	91.4
Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		246.1	503.5	1,104.2	1,601.3
Energy*
Disaggregation of Revenue [Line Items]
Revenue	[1]	130.4	217.2	468.5	599.1
Automotive
Disaggregation of Revenue [Line Items]
Revenue		59.4	73.6	186.7	224.1
Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue		45.9	44.5	118.6	116.7
Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		28.3	43.7	106.7	154.2
Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue		33.8	53.6	131.4	156.4
Medical
Disaggregation of Revenue [Line Items]
Revenue		27.3	41.0	95.8	129.5
Other
Disaggregation of Revenue [Line Items]
Revenue		26.8	41.6	111.9	122.6
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		623.4	1,097.8	2,482.4	3,351.9
Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue		$ 230.1	$ 471.6	$ 995.3	$ 1,540.7
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		38.00%	38.00%	38.00%	37.00%
Operating Segments | High Performance Materials & Components | Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue		30.00%	35.00%	35.00%	38.00%
Operating Segments | High Performance Materials & Components | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		32.00%	27.00%	27.00%	25.00%
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 | 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 [Member]
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		$ 129.3	$ 241.1	$ 522.8	$ 800.3
Operating Segments | High Performance Materials & Components | Europe
Disaggregation of Revenue [Line Items]
Revenue		60.7	134.5	286.2	454.0
Operating Segments | High Performance Materials & Components | Asia
Disaggregation of Revenue [Line Items]
Revenue		19.9	44.1	72.6	136.2
Operating Segments | High Performance Materials & Components | Canada
Disaggregation of Revenue [Line Items]
Revenue		4.4	12.6	21.1	38.9
Operating Segments | High Performance Materials & Components | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		7.0	19.5	39.6	52.1
Operating Segments | High Performance Materials & Components | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		171.8	377.7	782.9	1,223.1
Operating Segments | High Performance Materials & Components | Energy*
Disaggregation of Revenue [Line Items]
Revenue	[1]	24.4	36.2	71.3	97.7
Operating Segments | High Performance Materials & Components | Automotive
Disaggregation of Revenue [Line Items]
Revenue		1.5	2.2	4.5	8.9
Operating Segments | High Performance Materials & Components | Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue		0.2	0.1	0.7	0.4
Operating Segments | High Performance Materials & Components | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		4.0	5.4	14.3	37.3
Operating Segments | High Performance Materials & Components | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue		0.0	0.1	0.1	0.2
Operating Segments | High Performance Materials & Components | Medical
Disaggregation of Revenue [Line Items]
Revenue		10.6	18.9	37.9	64.7
Operating Segments | High Performance Materials & Components | Other
Disaggregation of Revenue [Line Items]
Revenue		8.8	11.2	30.6	49.2
Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue		$ 393.3	$ 626.2	$ 1,487.1	$ 1,811.2
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		74.00%	73.00%	71.00%	71.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		22.00%	29.00%	25.00%	26.00%
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		8.00%	13.00%	12.00%	11.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Precision rolled strip products
Disaggregation of Revenue [Line Items]
Percent of revenue		28.00%	22.00%	21.00%	22.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		16.00%	9.00%	13.00%	12.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Standard stainless products [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue		26.00%	27.00%	29.00%	29.00%
Operating Segments | Advanced Alloys & Solutions [Member] | United States
Disaggregation of Revenue [Line Items]
Revenue		$ 219.3	$ 364.3	$ 909.1	$ 1,066.7
Operating Segments | Advanced Alloys & Solutions [Member] | Europe
Disaggregation of Revenue [Line Items]
Revenue		34.1	51.8	104.1	150.5
Operating Segments | Advanced Alloys & Solutions [Member] | Asia
Disaggregation of Revenue [Line Items]
Revenue		103.0	120.4	288.3	323.4
Operating Segments | Advanced Alloys & Solutions [Member] | Canada
Disaggregation of Revenue [Line Items]
Revenue		8.5	13.0	30.0	42.5
Operating Segments | Advanced Alloys & Solutions [Member] | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		11.8	17.4	50.0	39.3
Operating Segments | Advanced Alloys & Solutions [Member] | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		74.3	125.8	321.3	378.2
Operating Segments | Advanced Alloys & Solutions [Member] | Energy*
Disaggregation of Revenue [Line Items]
Revenue	[1]	106.0	181.0	397.2	501.4
Operating Segments | Advanced Alloys & Solutions [Member] | Automotive
Disaggregation of Revenue [Line Items]
Revenue		57.9	71.4	182.2	215.2
Operating Segments | Advanced Alloys & Solutions [Member] | Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue		45.7	44.4	117.9	116.3
Operating Segments | Advanced Alloys & Solutions [Member] | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		24.3	38.3	92.4	116.9
Operating Segments | Advanced Alloys & Solutions [Member] | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue		33.8	53.5	131.3	156.2
Operating Segments | Advanced Alloys & Solutions [Member] | Medical
Disaggregation of Revenue [Line Items]
Revenue		16.7	22.1	57.9	64.8
Operating Segments | Advanced Alloys & Solutions [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue		18.0	30.4	81.3	73.4
External Customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		598.0	1,018.7	2,323.8	3,103.9
External Customers | Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue		221.3	451.8	942.3	1,481.5
External Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue		$ 376.7	$ 566.9	$ 1,381.5	$ 1,622.4

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