Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 28, 2025	Sep. 29, 2024	Sep. 28, 2025	Sep. 29, 2024
Disaggregation of Revenue [Line Items]
Revenue	$ 1,125.5	$ 1,051.2	$ 3,410.3	$ 3,189.4
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	44.00%	46.00%	47.00%	45.00%
Precision and Engineered Strip [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	6.00%	9.00%	5.00%	9.00%
Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	22.00%	20.00%	21.00%	19.00%
Titanium and Titanium-based Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	19.00%	17.00%	18.00%	18.00%
Zirconium and Related Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	9.00%	8.00%	9.00%	9.00%
UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue	$ 680.3	$ 625.6	$ 1,974.5	$ 1,835.7
Asia [Member]
Disaggregation of Revenue [Line Items]
Revenue	124.6	123.3	388.1	365.0
Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	248.9	243.0	745.4	783.4
CANADA
Disaggregation of Revenue [Line Items]
Revenue	39.3	31.4	119.0	84.9
South America, Middle East, and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue	32.4	27.9	183.3	120.4
Aerospace and Defense Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	792.7	653.8	2,308.9	1,953.4
Jet Engines- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	433.6	365.9	1,302.8	1,029.9
Airframes- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	197.7	180.8	598.7	581.7
Defense [Member]
Disaggregation of Revenue [Line Items]
Revenue	161.4	107.1	407.4	341.8
Conventional energy
Disaggregation of Revenue [Line Items]
Revenue	56.8	72.6	271.5	241.2
Specialty Energy [Member]
Disaggregation of Revenue [Line Items]
Revenue	53.9	69.9	167.9	202.6
Automotive Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	61.3	63.8	186.7	190.6
Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	48.4	49.1	131.7	142.8
Construction Mining Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	36.8	41.8	103.0	113.2
Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	30.9	53.1	112.2	173.9
Other Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	44.7	47.1	128.4	171.7
Other Core Markets
Disaggregation of Revenue [Line Items]
Revenue	133.2	172.1	411.8	519.3
Core End Markets
Disaggregation of Revenue [Line Items]
Revenue	925.9	825.9	2,720.7	2,472.7
Industrial Markets
Disaggregation of Revenue [Line Items]
Revenue	199.6	225.3	689.6	716.7
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,231.6	1,201.3	3,733.9	3,582.3
Operating Segments \| HPMC
Disaggregation of Revenue [Line Items]
Revenue	$ 661.5	$ 612.9	$ 1,967.6	$ 1,828.6
Percent of revenue	100.00%	100.00%	100.00%	100.00%
Operating Segments \| HPMC \| Nickel-based Alloys and Specialty Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	44.00%	43.00%	43.00%	41.00%
Operating Segments \| HPMC \| Precision and Engineered Strip [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	0.00%	0.00%	0.00%	0.00%
Operating Segments \| HPMC \| Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	40.00%	36.00%	39.00%	36.00%
Operating Segments \| HPMC \| Titanium and Titanium-based Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	16.00%	21.00%	18.00%	23.00%
Operating Segments \| HPMC \| Zirconium and Related Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	0.00%	0.00%	0.00%	0.00%
Operating Segments \| HPMC \| UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue	$ 337.7	$ 285.8	$ 1,004.4	$ 797.9
Operating Segments \| HPMC \| Asia [Member]
Disaggregation of Revenue [Line Items]
Revenue	40.1	30.3	117.2	106.5
Operating Segments \| HPMC \| Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	177.4	201.4	537.4	636.9
Operating Segments \| HPMC \| CANADA
Disaggregation of Revenue [Line Items]
Revenue	22.3	16.4	59.4	45.4
Operating Segments \| HPMC \| South America, Middle East, and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue	25.4	18.5	77.4	57.6
Operating Segments \| HPMC \| Aerospace and Defense Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	554.1	474.9	1,650.3	1,395.0
Operating Segments \| HPMC \| Jet Engines- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	399.3	341.9	1,216.2	970.6
Operating Segments \| HPMC \| Airframes- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	71.0	84.1	230.1	263.5
Operating Segments \| HPMC \| Defense [Member]
Disaggregation of Revenue [Line Items]
Revenue	83.8	48.9	204.0	160.9
Operating Segments \| HPMC \| Conventional energy
Disaggregation of Revenue [Line Items]
Revenue	1.1	2.4	4.2	8.3
Operating Segments \| HPMC \| Specialty Energy [Member]
Disaggregation of Revenue [Line Items]
Revenue	19.4	26.3	46.5	67.0
Operating Segments \| HPMC \| Automotive Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	1.7	4.6	5.9	13.4
Operating Segments \| HPMC \| Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0	0.0	3.0
Operating Segments \| HPMC \| Construction Mining Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	5.3	4.9	20.5	19.9
Operating Segments \| HPMC \| Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	11.8	28.6	43.0	97.5
Operating Segments \| HPMC \| Other Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	9.5	10.7	25.4	40.2
Operating Segments \| HPMC \| Other Core Markets
Disaggregation of Revenue [Line Items]
Revenue	31.2	54.9	89.5	167.5
Operating Segments \| HPMC \| Core End Markets
Disaggregation of Revenue [Line Items]
Revenue	585.3	529.8	1,739.8	1,562.5
Operating Segments \| AA&S
Disaggregation of Revenue [Line Items]
Revenue	$ 570.1	$ 588.4	$ 1,766.3	$ 1,753.7
Percent of revenue	100.00%	100.00%	100.00%	100.00%
Operating Segments \| AA&S \| Nickel-based Alloys and Specialty Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	45.00%	50.00%	51.00%	50.00%
Operating Segments \| AA&S \| Precision and Engineered Strip [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	13.00%	21.00%	11.00%	19.00%
Operating Segments \| AA&S \| Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	0.00%	0.00%	0.00%	0.00%
Operating Segments \| AA&S \| Titanium and Titanium-based Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	22.00%	12.00%	19.00%	13.00%
Operating Segments \| AA&S \| Zirconium and Related Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	20.00%	17.00%	19.00%	18.00%
Operating Segments \| AA&S \| UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue	$ 342.6	$ 339.8	$ 970.1	$ 1,037.8
Operating Segments \| AA&S \| Asia [Member]
Disaggregation of Revenue [Line Items]
Revenue	84.5	93.0	270.9	258.5
Operating Segments \| AA&S \| Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	71.5	41.6	208.0	146.5
Operating Segments \| AA&S \| CANADA
Disaggregation of Revenue [Line Items]
Revenue	17.0	15.0	59.6	39.5
Operating Segments \| AA&S \| South America, Middle East, and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue	7.0	9.4	105.9	62.8
Operating Segments \| AA&S \| Aerospace and Defense Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	238.6	178.9	658.6	558.4
Operating Segments \| AA&S \| Jet Engines- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	34.3	24.0	86.6	59.3
Operating Segments \| AA&S \| Airframes- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	126.7	96.7	368.6	318.2
Operating Segments \| AA&S \| Defense [Member]
Disaggregation of Revenue [Line Items]
Revenue	77.6	58.2	203.4	180.9
Operating Segments \| AA&S \| Conventional energy
Disaggregation of Revenue [Line Items]
Revenue	55.7	70.2	267.3	232.9
Operating Segments \| AA&S \| Specialty Energy [Member]
Disaggregation of Revenue [Line Items]
Revenue	34.5	43.6	121.4	135.6
Operating Segments \| AA&S \| Automotive Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	59.6	59.2	180.8	177.2
Operating Segments \| AA&S \| Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	48.4	49.1	131.7	139.8
Operating Segments \| AA&S \| Construction Mining Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	31.5	36.9	82.5	93.3
Operating Segments \| AA&S \| Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	19.1	24.5	69.2	76.4
Operating Segments \| AA&S \| Other Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	35.2	36.4	103.0	131.5
Operating Segments \| AA&S \| Other Core Markets
Disaggregation of Revenue [Line Items]
Revenue	102.0	117.2	322.3	351.8
Operating Segments \| AA&S \| Core End Markets
Disaggregation of Revenue [Line Items]
Revenue	340.6	296.1	980.9	910.2
Sales to external customers \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,125.5	1,051.2	3,410.3	3,189.4
Sales to external customers \| Operating Segments \| HPMC
Disaggregation of Revenue [Line Items]
Revenue	602.9	552.4	1,795.8	1,644.3
Sales to external customers \| Operating Segments \| AA&S
Disaggregation of Revenue [Line Items]
Revenue	522.6	498.8	1,614.5	1,545.1
Industrial Markets \| Operating Segments \| HPMC
Disaggregation of Revenue [Line Items]
Revenue	17.6	22.6	56.0	81.8
Industrial Markets \| Operating Segments \| AA&S
Disaggregation of Revenue [Line Items]
Revenue	$ 182.0	$ 202.7	$ 633.6	$ 634.9