Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue		$ 725.7	$ 598.0	$ 2,034.4	$ 2,323.8
Percent of revenue		100.00%	100.00%	100.00%	100.00%
Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of revenue		92.00%	88.00%	91.00%	88.00%
Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		37.00%	32.00%	34.00%	34.00%
Precision rolled strip products
Disaggregation of Revenue [Line Items]
Percent of revenue		19.00%	18.00%	19.00%	13.00%
Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue		15.00%	11.00%	16.00%	14.00%
Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		12.00%	17.00%	12.00%	18.00%
Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		9.00%	10.00%	10.00%	9.00%
Standard stainless products
Disaggregation of Revenue [Line Items]
Percent of revenue		8.00%	12.00%	9.00%	12.00%
United States
Disaggregation of Revenue [Line Items]
Revenue		$ 397.3	$ 348.6	$ 1,126.9	$ 1,431.9
Asia
Disaggregation of Revenue [Line Items]
Revenue		154.5	122.9	432.5	360.9
Europe
Disaggregation of Revenue [Line Items]
Revenue		111.3	94.8	337.8	390.3
Canada
Disaggregation of Revenue [Line Items]
Revenue		18.4	12.9	55.7	51.1
South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		44.2	18.8	81.5	89.6
Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		282.9	246.1	818.3	1,104.2
Energy
Disaggregation of Revenue [Line Items]
Revenue	[1]	166.2	130.4	433.6	468.5
Automotive
Disaggregation of Revenue [Line Items]
Revenue		91.8	59.4	251.3	186.7
Electronics
Disaggregation of Revenue [Line Items]
Revenue		34.8	45.9	133.7	118.6
Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		25.3	28.3	89.7	106.7
Food Equipment and Appliances Market
Disaggregation of Revenue [Line Items]
Revenue		51.6	33.8	107.7	131.4
Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue		34.3	27.3	95.3	95.8
Other
Disaggregation of Revenue [Line Items]
Revenue		38.8	26.8	104.8	111.9
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		789.7	623.4	2,189.0	2,482.4
Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue		$ 325.0	$ 230.1	$ 908.9	$ 995.3
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		46.00%	38.00%	43.00%	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		35.00%	30.00%	37.00%	34.00%
Operating Segments | High Performance Materials & Components | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		19.00%	32.00%	20.00%	28.00%
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		$ 140.8	$ 129.3	$ 426.7	$ 522.8
Operating Segments | High Performance Materials & Components | Asia
Disaggregation of Revenue [Line Items]
Revenue		58.3	19.9	119.0	72.6
Operating Segments | High Performance Materials & Components | Europe
Disaggregation of Revenue [Line Items]
Revenue		85.3	60.7	250.7	286.2
Operating Segments | High Performance Materials & Components | Canada
Disaggregation of Revenue [Line Items]
Revenue		9.9	4.4	27.4	21.1
Operating Segments | High Performance Materials & Components | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		5.7	7.0	17.7	39.6
Operating Segments | High Performance Materials & Components | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		207.5	171.8	605.9	782.9
Operating Segments | High Performance Materials & Components | Energy
Disaggregation of Revenue [Line Items]
Revenue	[1]	57.5	24.4	136.6	71.3
Operating Segments | High Performance Materials & Components | Automotive
Disaggregation of Revenue [Line Items]
Revenue		2.1	1.5	5.5	4.5
Operating Segments | High Performance Materials & Components | Electronics
Disaggregation of Revenue [Line Items]
Revenue		0.4	0.2	0.9	0.7
Operating Segments | High Performance Materials & Components | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		5.6	4.0	16.8	14.3
Operating Segments | High Performance Materials & Components | Food Equipment and Appliances Market
Disaggregation of Revenue [Line Items]
Revenue		0.0	0.0	0.1	0.1
Operating Segments | High Performance Materials & Components | Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue		16.8	10.6	42.6	37.9
Operating Segments | High Performance Materials & Components | Other
Disaggregation of Revenue [Line Items]
Revenue		10.1	8.8	33.1	30.6
Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue		$ 464.7	$ 393.3	$ 1,280.1	$ 1,487.1
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		86.00%	81.00%	85.00%	75.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		31.00%	29.00%	27.00%	29.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Precision rolled strip products
Disaggregation of Revenue [Line Items]
Percent of revenue		33.00%	28.00%	34.00%	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%	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%	8.00%	7.00%	10.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		15.00%	16.00%	17.00%	14.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Standard stainless products
Disaggregation of Revenue [Line Items]
Percent of revenue		14.00%	19.00%	15.00%	25.00%
Operating Segments | Advanced Alloys & Solutions [Member] | United States
Disaggregation of Revenue [Line Items]
Revenue		$ 256.5	$ 219.3	$ 700.2	$ 909.1
Operating Segments | Advanced Alloys & Solutions [Member] | Asia
Disaggregation of Revenue [Line Items]
Revenue		96.2	103.0	313.5	288.3
Operating Segments | Advanced Alloys & Solutions [Member] | Europe
Disaggregation of Revenue [Line Items]
Revenue		26.0	34.1	87.1	104.1
Operating Segments | Advanced Alloys & Solutions [Member] | Canada
Disaggregation of Revenue [Line Items]
Revenue		8.5	8.5	28.3	30.0
Operating Segments | Advanced Alloys & Solutions [Member] | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		38.5	11.8	63.8	50.0
Operating Segments | Advanced Alloys & Solutions [Member] | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		75.4	74.3	212.4	321.3
Operating Segments | Advanced Alloys & Solutions [Member] | Energy
Disaggregation of Revenue [Line Items]
Revenue	[1]	108.7	106.0	297.0	397.2
Operating Segments | Advanced Alloys & Solutions [Member] | Automotive
Disaggregation of Revenue [Line Items]
Revenue		89.7	57.9	245.8	182.2
Operating Segments | Advanced Alloys & Solutions [Member] | Electronics
Disaggregation of Revenue [Line Items]
Revenue		34.4	45.7	132.8	117.9
Operating Segments | Advanced Alloys & Solutions [Member] | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		19.7	24.3	72.9	92.4
Operating Segments | Advanced Alloys & Solutions [Member] | Food Equipment and Appliances Market
Disaggregation of Revenue [Line Items]
Revenue		51.6	33.8	107.6	131.3
Operating Segments | Advanced Alloys & Solutions [Member] | Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue		17.5	16.7	52.7	57.9
Operating Segments | Advanced Alloys & Solutions [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue		28.7	18.0	71.7	81.3
External Customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		725.7	598.0	2,034.4	2,323.8
External Customers | Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue		300.0	221.3	841.5	942.3
External Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue		$ 425.7	$ 376.7	$ 1,192.9	$ 1,381.5

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