Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Disaggregation of Revenue [Line Items]
Revenue	$ 1,032.0	$ 725.7	$ 2,825.6	$ 2,034.4
Percent of revenue	100.00%	100.00%	100.00%	100.00%
Nickel-based Alloys and Specialty Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	54.00%	45.00%	52.00%	43.00%
Precision and Engineered Strip [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	12.00%	19.00%	14.00%	19.00%
Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	15.00%	15.00%	15.00%	16.00%
Titanium and Titanium-based Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	11.00%	12.00%	11.00%	12.00%
Zirconium and Related Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	8.00%	9.00%	8.00%	10.00%
UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue	$ 617.1	$ 397.3	$ 1,663.4	$ 1,126.9
Asia [Member]
Disaggregation of Revenue [Line Items]
Revenue	169.6	154.5	483.8	432.5
Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	200.2	111.3	543.9	337.8
CANADA
Disaggregation of Revenue [Line Items]
Revenue	19.5	18.4	62.9	55.7
South America, Middle East, and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue	25.6	44.2	71.6	81.5
Aerospace and Defense Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	530.1	282.9	1,333.3	818.3
Jet Engines- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	312.6	128.8	757.9	364.5
Airframes- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	131.4	71.3	331.2	183.9
Defense [Member]
Disaggregation of Revenue [Line Items]
Revenue	86.1	82.8	244.2	269.9
Energy
Disaggregation of Revenue [Line Items]
Revenue	192.7	166.2	552.7	433.6
Oil & Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue	127.1	92.4	355.4	231.0
Specialty Energy [Member]
Disaggregation of Revenue [Line Items]
Revenue	65.6	73.8	197.3	202.6
Automotive Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	69.6	78.3	236.1	237.8
Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	48.5	56.5	149.5	155.4
Construction Mining Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	47.8	25.3	139.7	89.7
Food Equipment and Appliances Market
Disaggregation of Revenue [Line Items]
Revenue	45.2	43.4	141.9	99.5
Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	47.4	34.3	123.1	95.3
Other Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	50.7	38.8	149.3	104.8
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,151.8	789.7	3,131.1	2,189.0
Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue	$ 505.0	$ 325.0	$ 1,325.6	$ 908.9
Percent of revenue	100.00%	100.00%	100.00%	100.00%
Operating Segments | High Performance Materials & Components | Nickel-based Alloys and Specialty Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	52.00%	46.00%	50.00%	43.00%
Operating Segments | High Performance Materials & Components | Precision and Engineered Strip [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	0.00%	0.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	32.00%	35.00%	34.00%	37.00%
Operating Segments | High Performance Materials & Components | Titanium and Titanium-based Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	16.00%	19.00%	16.00%	20.00%
Operating Segments | High Performance Materials & Components | Zirconium and Related Alloys [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	$ 221.6	$ 140.8	$ 557.8	$ 426.7
Operating Segments | High Performance Materials & Components | Asia [Member]
Disaggregation of Revenue [Line Items]
Revenue	60.7	58.3	155.4	119.0
Operating Segments | High Performance Materials & Components | Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	147.0	85.3	403.3	250.7
Operating Segments | High Performance Materials & Components | CANADA
Disaggregation of Revenue [Line Items]
Revenue	10.0	9.9	33.0	27.4
Operating Segments | High Performance Materials & Components | South America, Middle East, and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue	18.3	5.7	45.8	17.7
Operating Segments | High Performance Materials & Components | Aerospace and Defense Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	374.0	207.5	947.6	605.9
Operating Segments | High Performance Materials & Components | Jet Engines- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	285.7	120.4	696.1	338.3
Operating Segments | High Performance Materials & Components | Airframes- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	49.8	35.0	130.9	95.6
Operating Segments | High Performance Materials & Components | Defense [Member]
Disaggregation of Revenue [Line Items]
Revenue	38.5	52.1	120.6	172.0
Operating Segments | High Performance Materials & Components | Energy
Disaggregation of Revenue [Line Items]
Revenue	33.3	57.5	120.1	136.6
Operating Segments | High Performance Materials & Components | Oil & Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue	6.9	10.4	32.1	29.0
Operating Segments | High Performance Materials & Components | Specialty Energy [Member]
Disaggregation of Revenue [Line Items]
Revenue	26.4	47.1	88.0	107.6
Operating Segments | High Performance Materials & Components | Automotive Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	3.5	2.1	8.7	5.5
Operating Segments | High Performance Materials & Components | Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	0.7	0.4	1.9	0.9
Operating Segments | High Performance Materials & Components | Construction Mining Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	9.5	5.6	25.8	16.8
Operating Segments | High Performance Materials & Components | Food Equipment and Appliances Market
Disaggregation of Revenue [Line Items]
Revenue	0.2	0.0	0.2	0.1
Operating Segments | High Performance Materials & Components | Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	22.3	16.8	52.3	42.6
Operating Segments | High Performance Materials & Components | Other Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	14.1	10.1	38.7	33.1
Operating Segments | Advanced Alloys & Solutions
Disaggregation of Revenue [Line Items]
Revenue	$ 646.8	$ 464.7	$ 1,805.5	$ 1,280.1
Percent of revenue	100.00%	100.00%	100.00%	100.00%
Operating Segments | Advanced Alloys & Solutions | Nickel-based Alloys and Specialty Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	57.00%	45.00%	55.00%	42.00%
Operating Segments | Advanced Alloys & Solutions | Precision and Engineered Strip [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	22.00%	33.00%	25.00%	34.00%
Operating Segments | Advanced Alloys & Solutions | Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	0.00%	0.00%	0.00%	0.00%
Operating Segments | Advanced Alloys & Solutions | Titanium and Titanium-based Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	7.00%	7.00%	6.00%	7.00%
Operating Segments | Advanced Alloys & Solutions | Zirconium and Related Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	14.00%	15.00%	14.00%	17.00%
Operating Segments | Advanced Alloys & Solutions | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue	$ 395.5	$ 256.5	$ 1,105.6	$ 700.2
Operating Segments | Advanced Alloys & Solutions | Asia [Member]
Disaggregation of Revenue [Line Items]
Revenue	108.9	96.2	328.4	313.5
Operating Segments | Advanced Alloys & Solutions | Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	53.2	26.0	140.6	87.1
Operating Segments | Advanced Alloys & Solutions | CANADA
Disaggregation of Revenue [Line Items]
Revenue	9.5	8.5	29.9	28.3
Operating Segments | Advanced Alloys & Solutions | South America, Middle East, and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue	7.3	38.5	25.8	63.8
Operating Segments | Advanced Alloys & Solutions | Aerospace and Defense Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	156.1	75.4	385.7	212.4
Operating Segments | Advanced Alloys & Solutions | Jet Engines- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	26.9	8.4	61.8	26.2
Operating Segments | Advanced Alloys & Solutions | Airframes- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	81.6	36.3	200.3	88.3
Operating Segments | Advanced Alloys & Solutions | Defense [Member]
Disaggregation of Revenue [Line Items]
Revenue	47.6	30.7	123.6	97.9
Operating Segments | Advanced Alloys & Solutions | Energy
Disaggregation of Revenue [Line Items]
Revenue	159.4	108.7	432.6	297.0
Operating Segments | Advanced Alloys & Solutions | Oil & Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue	120.2	82.0	323.3	202.0
Operating Segments | Advanced Alloys & Solutions | Specialty Energy [Member]
Disaggregation of Revenue [Line Items]
Revenue	39.2	26.7	109.3	95.0
Operating Segments | Advanced Alloys & Solutions | Automotive Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	66.1	76.2	227.4	232.3
Operating Segments | Advanced Alloys & Solutions | Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	47.8	56.1	147.6	154.5
Operating Segments | Advanced Alloys & Solutions | Construction Mining Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	38.3	19.7	113.9	72.9
Operating Segments | Advanced Alloys & Solutions | Food Equipment and Appliances Market
Disaggregation of Revenue [Line Items]
Revenue	45.0	43.4	141.7	99.4
Operating Segments | Advanced Alloys & Solutions | Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	25.1	17.5	70.8	52.7
Operating Segments | Advanced Alloys & Solutions | Other Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	36.6	28.7	110.6	71.7
External Customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,032.0	725.7	2,825.6	2,034.4
External Customers | Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue	457.6	300.0	1,195.3	841.5
External Customers | Operating Segments | Advanced Alloys & Solutions
Disaggregation of Revenue [Line Items]
Revenue	$ 574.4	$ 425.7	$ 1,630.3	$ 1,192.9