Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Disaggregation of Revenue [Line Items]
Revenue	$ 1,025.6	$ 1,032.0	$ 3,109.7	$ 2,825.6
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	47.00%	54.00%	50.00%	52.00%
Precision and Engineered Strip [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	9.00%	12.00%	9.00%	14.00%
Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	18.00%	15.00%	17.00%	15.00%
Titanium and Titanium-based Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	19.00%	11.00%	16.00%	11.00%
Zirconium and Related Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	7.00%	8.00%	8.00%	8.00%
UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue	$ 556.8	$ 617.1	$ 1,713.4	$ 1,663.4
Asia [Member]
Disaggregation of Revenue [Line Items]
Revenue	141.2	169.6	449.2	483.8
Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	267.4	200.2	757.4	543.9
CANADA
Disaggregation of Revenue [Line Items]
Revenue	23.5	19.5	76.6	62.9
South America, Middle East, and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue	36.7	25.6	113.1	71.6
Aerospace and Defense Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	625.8	530.1	1,808.3	1,333.3
Jet Engines- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	329.4	312.6	981.2	757.9
Airframes- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	203.6	131.4	537.7	331.2
Defense [Member]
Disaggregation of Revenue [Line Items]
Revenue	92.8	86.1	289.4	244.2
Energy
Disaggregation of Revenue [Line Items]
Revenue	148.9	192.7	538.6	552.7
Oil & Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue	87.0	127.1	325.8	355.4
Specialty Energy [Member]
Disaggregation of Revenue [Line Items]
Revenue	61.9	65.6	212.8	197.3
Automotive Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	48.1	69.6	160.3	236.1
Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	44.8	48.5	115.2	149.5
Construction Mining Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	40.0	47.8	128.8	139.7
Food Equipment and Appliances Market
Disaggregation of Revenue [Line Items]
Revenue	16.2	45.2	58.6	141.9
Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	47.5	47.4	124.4	123.1
Other Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	54.3	50.7	175.5	149.3
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,129.9	1,151.8	3,461.5	3,131.1
Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue	$ 578.1	$ 505.0	$ 1,670.5	$ 1,325.6
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	42.00%	52.00%	45.00%	50.00%
Operating Segments | High Performance Materials & Components | Precision and Engineered Strip [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	1.00%	0.00%	1.00%	0.00%
Operating Segments | High Performance Materials & Components | Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	33.00%	32.00%	33.00%	34.00%
Operating Segments | High Performance Materials & Components | Titanium and Titanium-based Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	24.00%	16.00%	21.00%	16.00%
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	$ 242.6	$ 221.6	$ 661.1	$ 557.8
Operating Segments | High Performance Materials & Components | Asia [Member]
Disaggregation of Revenue [Line Items]
Revenue	37.8	60.7	132.8	155.4
Operating Segments | High Performance Materials & Components | Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	217.7	147.0	619.7	403.3
Operating Segments | High Performance Materials & Components | CANADA
Disaggregation of Revenue [Line Items]
Revenue	13.1	10.0	41.7	33.0
Operating Segments | High Performance Materials & Components | South America, Middle East, and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue	28.3	18.3	82.4	45.8
Operating Segments | High Performance Materials & Components | Aerospace and Defense Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	456.2	374.0	1,287.8	947.6
Operating Segments | High Performance Materials & Components | Jet Engines- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	312.5	285.7	914.2	696.1
Operating Segments | High Performance Materials & Components | Airframes- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	104.0	49.8	242.0	130.9
Operating Segments | High Performance Materials & Components | Defense [Member]
Disaggregation of Revenue [Line Items]
Revenue	39.7	38.5	131.6	120.6
Operating Segments | High Performance Materials & Components | Energy
Disaggregation of Revenue [Line Items]
Revenue	22.7	33.3	83.6	120.1
Operating Segments | High Performance Materials & Components | Oil & Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue	2.5	6.9	8.6	32.1
Operating Segments | High Performance Materials & Components | Specialty Energy [Member]
Disaggregation of Revenue [Line Items]
Revenue	20.2	26.4	75.0	88.0
Operating Segments | High Performance Materials & Components | Automotive Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	7.2	3.5	19.5	8.7
Operating Segments | High Performance Materials & Components | Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	0.6	0.7	1.8	1.9
Operating Segments | High Performance Materials & Components | Construction Mining Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	7.7	9.5	26.9	25.8
Operating Segments | High Performance Materials & Components | Food Equipment and Appliances Market
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.2	0.0	0.2
Operating Segments | High Performance Materials & Components | Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	28.9	22.3	70.7	52.3
Operating Segments | High Performance Materials & Components | Other Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	16.2	14.1	47.4	38.7
Operating Segments | Advanced Alloys & Solutions
Disaggregation of Revenue [Line Items]
Revenue	$ 551.8	$ 646.8	$ 1,791.0	$ 1,805.5
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	52.00%	57.00%	56.00%	55.00%
Operating Segments | Advanced Alloys & Solutions | Precision and Engineered Strip [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	19.00%	22.00%	18.00%	25.00%
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	13.00%	7.00%	11.00%	6.00%
Operating Segments | Advanced Alloys & Solutions | Zirconium and Related Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	16.00%	14.00%	15.00%	14.00%
Operating Segments | Advanced Alloys & Solutions | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue	$ 314.2	$ 395.5	$ 1,052.3	$ 1,105.6
Operating Segments | Advanced Alloys & Solutions | Asia [Member]
Disaggregation of Revenue [Line Items]
Revenue	103.4	108.9	316.4	328.4
Operating Segments | Advanced Alloys & Solutions | Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	49.7	53.2	137.7	140.6
Operating Segments | Advanced Alloys & Solutions | CANADA
Disaggregation of Revenue [Line Items]
Revenue	10.4	9.5	34.9	29.9
Operating Segments | Advanced Alloys & Solutions | South America, Middle East, and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue	8.4	7.3	30.7	25.8
Operating Segments | Advanced Alloys & Solutions | Aerospace and Defense Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	169.6	156.1	520.5	385.7
Operating Segments | Advanced Alloys & Solutions | Jet Engines- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	16.9	26.9	67.0	61.8
Operating Segments | Advanced Alloys & Solutions | Airframes- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	99.6	81.6	295.7	200.3
Operating Segments | Advanced Alloys & Solutions | Defense [Member]
Disaggregation of Revenue [Line Items]
Revenue	53.1	47.6	157.8	123.6
Operating Segments | Advanced Alloys & Solutions | Energy
Disaggregation of Revenue [Line Items]
Revenue	126.2	159.4	455.0	432.6
Operating Segments | Advanced Alloys & Solutions | Oil & Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue	84.5	120.2	317.2	323.3
Operating Segments | Advanced Alloys & Solutions | Specialty Energy [Member]
Disaggregation of Revenue [Line Items]
Revenue	41.7	39.2	137.8	109.3
Operating Segments | Advanced Alloys & Solutions | Automotive Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	40.9	66.1	140.8	227.4
Operating Segments | Advanced Alloys & Solutions | Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	44.2	47.8	113.4	147.6
Operating Segments | Advanced Alloys & Solutions | Construction Mining Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	32.3	38.3	101.9	113.9
Operating Segments | Advanced Alloys & Solutions | Food Equipment and Appliances Market
Disaggregation of Revenue [Line Items]
Revenue	16.2	45.0	58.6	141.7
Operating Segments | Advanced Alloys & Solutions | Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	18.6	25.1	53.7	70.8
Operating Segments | Advanced Alloys & Solutions | Other Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	38.1	36.6	128.1	110.6
External Customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,025.6	1,032.0	3,109.7	2,825.6
External Customers | Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue	539.5	457.6	1,537.7	1,195.3
External Customers | Operating Segments | Advanced Alloys & Solutions
Disaggregation of Revenue [Line Items]
Revenue	$ 486.1	$ 574.4	$ 1,572.0	$ 1,630.3