Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 29, 2025	Jun. 30, 2024	Jun. 29, 2025	Jun. 30, 2024
Disaggregation of Revenue [Line Items]
Revenue	$ 1,140.4	$ 1,095.3	$ 2,284.8	$ 2,138.2
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	48.00%	44.00%	48.00%	44.00%
Precision and Engineered Strip [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	5.00%	9.00%	5.00%	9.00%
Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	21.00%	19.00%	21.00%	19.00%
Titanium and Titanium-based Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	17.00%	20.00%	18.00%	19.00%
Zirconium and Related Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	9.00%	8.00%	8.00%	9.00%
UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue	$ 650.4	$ 638.5	$ 1,294.2	$ 1,210.1
Asia [Member]
Disaggregation of Revenue [Line Items]
Revenue	149.2	125.5	263.5	241.7
Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	235.8	278.2	496.5	540.4
CANADA
Disaggregation of Revenue [Line Items]
Revenue	39.9	25.9	79.7	53.5
South America, Middle East, and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue	65.1	27.2	150.9	92.5
Aerospace and Defense Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	761.8	683.9	1,516.2	1,299.6
Jet Engines- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	447.8	352.8	869.2	664.0
Airframes- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	195.2	210.8	401.0	400.9
Defense [Member]
Disaggregation of Revenue [Line Items]
Revenue	118.8	120.3	246.0	234.7
Conventional energy
Disaggregation of Revenue [Line Items]
Revenue	92.9	66.1	214.7	168.6
Specialty Energy [Member]
Disaggregation of Revenue [Line Items]
Revenue	63.5	76.6	114.0	132.7
Automotive Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	64.8	70.8	125.4	126.8
Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	43.7	40.8	83.3	93.7
Construction Mining Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	33.3	44.2	66.2	71.4
Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	38.9	61.7	81.3	120.8
Other Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	41.5	51.2	83.7	124.6
Other Core Markets
Disaggregation of Revenue [Line Items]
Revenue	146.1	179.1	278.6	347.2
Core End Markets
Disaggregation of Revenue [Line Items]
Revenue	907.9	863.0	1,794.8	1,646.8
Industrial Markets
Disaggregation of Revenue [Line Items]
Revenue	232.5	232.3	490.0	491.4
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,240.6	1,248.4	2,502.3	2,381.0
Operating Segments | HPMC
Disaggregation of Revenue [Line Items]
Revenue	$ 662.1	$ 643.8	$ 1,306.1	$ 1,215.7
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%	39.00%	43.00%	39.00%
Operating Segments | HPMC | Precision and Engineered Strip [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	0.00%	0.00%	0.00%	1.00%
Operating Segments | HPMC | Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	39.00%	37.00%	39.00%	36.00%
Operating Segments | HPMC | Titanium and Titanium-based Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	17.00%	24.00%	18.00%	24.00%
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	$ 350.3	$ 271.4	$ 666.7	$ 512.1
Operating Segments | HPMC | Asia [Member]
Disaggregation of Revenue [Line Items]
Revenue	43.8	37.3	77.1	76.2
Operating Segments | HPMC | Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	168.7	224.0	360.0	435.5
Operating Segments | HPMC | CANADA
Disaggregation of Revenue [Line Items]
Revenue	17.0	13.7	37.1	29.0
Operating Segments | HPMC | South America, Middle East, and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue	29.0	15.6	52.0	39.1
Operating Segments | HPMC | Aerospace and Defense Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	558.7	477.5	1,096.2	920.1
Operating Segments | HPMC | Jet Engines- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	419.6	331.8	816.9	628.7
Operating Segments | HPMC | Airframes- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	77.3	93.7	159.1	179.4
Operating Segments | HPMC | Defense [Member]
Disaggregation of Revenue [Line Items]
Revenue	61.8	52.0	120.2	112.0
Operating Segments | HPMC | Conventional energy
Disaggregation of Revenue [Line Items]
Revenue	1.4	2.4	3.1	5.9
Operating Segments | HPMC | Specialty Energy [Member]
Disaggregation of Revenue [Line Items]
Revenue	14.7	22.5	27.1	40.7
Operating Segments | HPMC | Automotive Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	2.8	3.8	4.2	8.8
Operating Segments | HPMC | Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	0.0	2.0	0.0	3.0
Operating Segments | HPMC | Construction Mining Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	8.1	8.3	15.2	15.0
Operating Segments | HPMC | Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	15.4	33.0	31.2	68.9
Operating Segments | HPMC | Other Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	7.7	12.5	15.9	29.5
Operating Segments | HPMC | Other Core Markets
Disaggregation of Revenue [Line Items]
Revenue	30.1	57.5	58.3	112.6
Operating Segments | HPMC | Core End Markets
Disaggregation of Revenue [Line Items]
Revenue	588.8	535.0	1,154.5	1,032.7
Operating Segments | AA&S
Disaggregation of Revenue [Line Items]
Revenue	$ 578.5	$ 604.6	$ 1,196.2	$ 1,165.3
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	53.00%	49.00%	54.00%	50.00%
Operating Segments | AA&S | Precision and Engineered Strip [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	11.00%	18.00%	11.00%	18.00%
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	17.00%	15.00%	17.00%	13.00%
Operating Segments | AA&S | Zirconium and Related Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue	19.00%	18.00%	18.00%	19.00%
Operating Segments | AA&S | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue	$ 300.1	$ 367.1	$ 627.5	$ 698.0
Operating Segments | AA&S | Asia [Member]
Disaggregation of Revenue [Line Items]
Revenue	105.4	88.2	186.4	165.5
Operating Segments | AA&S | Europe [Member]
Disaggregation of Revenue [Line Items]
Revenue	67.1	54.2	136.5	104.9
Operating Segments | AA&S | CANADA
Disaggregation of Revenue [Line Items]
Revenue	22.9	12.2	42.6	24.5
Operating Segments | AA&S | South America, Middle East, and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue	36.1	11.6	98.9	53.4
Operating Segments | AA&S | Aerospace and Defense Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	203.1	206.4	420.0	379.5
Operating Segments | AA&S | Jet Engines- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	28.2	21.0	52.3	35.3
Operating Segments | AA&S | Airframes- Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue	117.9	117.1	241.9	221.5
Operating Segments | AA&S | Defense [Member]
Disaggregation of Revenue [Line Items]
Revenue	57.0	68.3	125.8	122.7
Operating Segments | AA&S | Conventional energy
Disaggregation of Revenue [Line Items]
Revenue	91.5	63.7	211.6	162.7
Operating Segments | AA&S | Specialty Energy [Member]
Disaggregation of Revenue [Line Items]
Revenue	48.8	54.1	86.9	92.0
Operating Segments | AA&S | Automotive Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	62.0	67.0	121.2	118.0
Operating Segments | AA&S | Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	43.7	38.8	83.3	90.7
Operating Segments | AA&S | Construction Mining Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	25.2	35.9	51.0	56.4
Operating Segments | AA&S | Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	23.5	28.7	50.1	51.9
Operating Segments | AA&S | Other Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	33.8	38.7	67.8	95.1
Operating Segments | AA&S | Other Core Markets
Disaggregation of Revenue [Line Items]
Revenue	116.0	121.6	220.3	234.6
Operating Segments | AA&S | Core End Markets
Disaggregation of Revenue [Line Items]
Revenue	319.1	328.0	640.3	614.1
Sales to external customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,140.4	1,095.3	2,284.8	2,138.2
Sales to external customers | Operating Segments | HPMC
Disaggregation of Revenue [Line Items]
Revenue	608.8	562.0	1,192.9	1,091.9
Sales to external customers | Operating Segments | AA&S
Disaggregation of Revenue [Line Items]
Revenue	531.6	533.3	1,091.9	1,046.3
Industrial Markets | Operating Segments | HPMC
Disaggregation of Revenue [Line Items]
Revenue	20.0	27.0	38.4	59.2
Industrial Markets | Operating Segments | AA&S
Disaggregation of Revenue [Line Items]
Revenue	$ 212.5	$ 205.3	$ 451.6	$ 432.2