Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 29, 2024	Dec. 31, 2023	Jan. 01, 2023
Disaggregation of Revenue [Line Items]
Revenue	$ 4,362.1	$ 4,173.7	$ 3,836.0
Percent of total revenue	100.00%	100.00%	100.00%
Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue	45.00%	49.00%	52.00%
Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of total revenue	20.00%	17.00%	15.00%
Titanium and Titanium-based Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of total revenue	18.00%	17.00%	11.00%
Zirconium and Related Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of total revenue	9.00%	7.00%	8.00%
Precision rolled strip
Disaggregation of Revenue [Line Items]
Percent of total revenue	8.00%	10.00%	14.00%
China
Disaggregation of Revenue [Line Items]
Revenue	$ 299.6	$ 333.3	$ 351.8
GERMANY
Disaggregation of Revenue [Line Items]
Revenue	259.7	243.0	200.9
UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenue	257.9	259.1	217.7
Rest of World
Disaggregation of Revenue [Line Items]
Revenue	676.0	757.1	602.4
France
Disaggregation of Revenue [Line Items]
Revenue	227.5	219.4	157.2
United States
Disaggregation of Revenue [Line Items]
Revenue	2,525.2	2,250.8	2,218.6
CANADA
Disaggregation of Revenue [Line Items]
Revenue	116.2	111.0	87.4
Total Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue	2,720.9	2,474.8	1,873.6
Automotive
Disaggregation of Revenue [Line Items]
Revenue	259.4	210.7	302.1
Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue	158.5	162.9	176.4
Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	224.9	176.9	163.1
Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	194.3	159.9	200.0
Other
Disaggregation of Revenue [Line Items]
Revenue	217.5	300.7	367.5
Jet Engines- Commercial
Disaggregation of Revenue [Line Items]
Revenue	1,457.8	1,333.5	1,063.5
Airframes- Commercial
Disaggregation of Revenue [Line Items]
Revenue	772.9	739.4	468.9
Defense
Disaggregation of Revenue [Line Items]
Revenue	490.2	401.9	341.2
Specialty Energy
Disaggregation of Revenue [Line Items]
Revenue	284.6	273.2	276.6
Conventional Energy
Disaggregation of Revenue [Line Items]
Revenue	302.0	414.6	476.7
Industrial Markets
Disaggregation of Revenue [Line Items]
Revenue	937.4	1,088.9	1,322.7
Other Core Markets
Disaggregation of Revenue [Line Items]
Revenue	703.8	610.0	639.7
Core End Markets
Disaggregation of Revenue [Line Items]
Revenue	3,424.7	3,084.8	2,513.3
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	4,884.9	4,638.9	4,249.4
Operating Segments \| HPMC
Disaggregation of Revenue [Line Items]
Revenue	$ 2,526.2	$ 2,302.0	$ 1,815.7
Percent of total revenue	100.00%	100.00%	100.00%
Operating Segments \| HPMC \| Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue	41.00%	44.00%	49.00%
Operating Segments \| HPMC \| Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of total revenue	36.00%	33.00%	34.00%
Operating Segments \| HPMC \| Titanium and Titanium-based Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of total revenue	23.00%	22.00%	17.00%
Operating Segments \| HPMC \| Zirconium and Related Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of total revenue	0.00%	0.00%	0.00%
Operating Segments \| HPMC \| Precision rolled strip
Disaggregation of Revenue [Line Items]
Percent of total revenue	0.00%	1.00%	0.00%
Operating Segments \| HPMC \| China
Disaggregation of Revenue [Line Items]
Revenue	$ 57.6	$ 70.1	$ 59.8
Operating Segments \| HPMC \| GERMANY
Disaggregation of Revenue [Line Items]
Revenue	202.5	204.2	148.4
Operating Segments \| HPMC \| UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenue	217.0	224.8	165.7
Operating Segments \| HPMC \| Rest of World
Disaggregation of Revenue [Line Items]
Revenue	419.6	468.5	352.5
Operating Segments \| HPMC \| France
Disaggregation of Revenue [Line Items]
Revenue	186.9	172.4	125.7
Operating Segments \| HPMC \| United States
Disaggregation of Revenue [Line Items]
Revenue	1,134.0	915.3	742.9
Operating Segments \| HPMC \| CANADA
Disaggregation of Revenue [Line Items]
Revenue	60.9	64.9	46.2
Operating Segments \| HPMC \| Total Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue	1,959.9	1,786.9	1,318.0
Operating Segments \| HPMC \| Automotive
Disaggregation of Revenue [Line Items]
Revenue	15.2	24.6	11.2
Operating Segments \| HPMC \| Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue	26.3	35.0	34.1
Operating Segments \| HPMC \| Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	115.5	102.6	73.2
Operating Segments \| HPMC \| Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	3.0	3.1	2.4
Operating Segments \| HPMC \| Other
Disaggregation of Revenue [Line Items]
Revenue	52.0	63.5	53.7
Operating Segments \| HPMC \| Jet Engines- Commercial
Disaggregation of Revenue [Line Items]
Revenue	1,365.4	1,255.3	975.7
Operating Segments \| HPMC \| Airframes- Commercial
Disaggregation of Revenue [Line Items]
Revenue	369.7	350.6	184.1
Operating Segments \| HPMC \| Defense
Disaggregation of Revenue [Line Items]
Revenue	224.8	181.0	158.2
Operating Segments \| HPMC \| Specialty Energy
Disaggregation of Revenue [Line Items]
Revenue	96.8	93.9	113.6
Operating Segments \| HPMC \| Conventional Energy
Disaggregation of Revenue [Line Items]
Revenue	9.8	10.6	35.0
Operating Segments \| HPMC \| Industrial Markets
Disaggregation of Revenue [Line Items]
Revenue	103.3	133.7	134.0
Operating Segments \| HPMC \| Other Core Markets
Disaggregation of Revenue [Line Items]
Revenue	215.3	199.6	189.2
Operating Segments \| HPMC \| Core End Markets
Disaggregation of Revenue [Line Items]
Revenue	2,175.2	1,986.5	1,507.2
Operating Segments \| AA&S
Disaggregation of Revenue [Line Items]
Revenue	$ 2,358.7	$ 2,336.9	$ 2,433.7
Percent of total revenue	100.00%	100.00%	100.00%
Operating Segments \| AA&S \| Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue	49.00%	54.00%	54.00%
Operating Segments \| AA&S \| Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of total revenue	0.00%	0.00%	0.00%
Operating Segments \| AA&S \| Titanium and Titanium-based Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of total revenue	13.00%	12.00%	7.00%
Operating Segments \| AA&S \| Zirconium and Related Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of total revenue	19.00%	15.00%	14.00%
Operating Segments \| AA&S \| Precision rolled strip
Disaggregation of Revenue [Line Items]
Percent of total revenue	19.00%	19.00%	25.00%
Operating Segments \| AA&S \| China
Disaggregation of Revenue [Line Items]
Revenue	$ 242.0	$ 263.2	$ 292.0
Operating Segments \| AA&S \| GERMANY
Disaggregation of Revenue [Line Items]
Revenue	57.2	38.8	52.5
Operating Segments \| AA&S \| UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenue	40.9	34.3	52.0
Operating Segments \| AA&S \| Rest of World
Disaggregation of Revenue [Line Items]
Revenue	256.4	288.6	249.9
Operating Segments \| AA&S \| France
Disaggregation of Revenue [Line Items]
Revenue	40.6	47.0	31.5
Operating Segments \| AA&S \| United States
Disaggregation of Revenue [Line Items]
Revenue	1,391.2	1,335.5	1,475.7
Operating Segments \| AA&S \| CANADA
Disaggregation of Revenue [Line Items]
Revenue	55.3	46.1	41.2
Operating Segments \| AA&S \| Total Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue	761.0	687.9	555.6
Operating Segments \| AA&S \| Automotive
Disaggregation of Revenue [Line Items]
Revenue	244.2	186.1	290.9
Operating Segments \| AA&S \| Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue	132.2	127.9	142.3
Operating Segments \| AA&S \| Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	109.4	74.3	89.9
Operating Segments \| AA&S \| Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue	191.3	156.8	197.6
Operating Segments \| AA&S \| Other
Disaggregation of Revenue [Line Items]
Revenue	165.5	237.2	313.8
Operating Segments \| AA&S \| Jet Engines- Commercial
Disaggregation of Revenue [Line Items]
Revenue	92.4	78.2	87.8
Operating Segments \| AA&S \| Airframes- Commercial
Disaggregation of Revenue [Line Items]
Revenue	403.2	388.8	284.8
Operating Segments \| AA&S \| Defense
Disaggregation of Revenue [Line Items]
Revenue	265.4	220.9	183.0
Operating Segments \| AA&S \| Specialty Energy
Disaggregation of Revenue [Line Items]
Revenue	187.8	179.3	163.0
Operating Segments \| AA&S \| Conventional Energy
Disaggregation of Revenue [Line Items]
Revenue	292.2	404.0	441.7
Operating Segments \| AA&S \| Industrial Markets
Disaggregation of Revenue [Line Items]
Revenue	834.1	955.2	1,188.7
Operating Segments \| AA&S \| Other Core Markets
Disaggregation of Revenue [Line Items]
Revenue	488.5	410.4	450.5
Operating Segments \| AA&S \| Core End Markets
Disaggregation of Revenue [Line Items]
Revenue	1,249.5	1,098.3	1,006.1
External Customers \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	4,362.1	4,173.7	3,836.0
External Customers \| Operating Segments \| HPMC
Disaggregation of Revenue [Line Items]
Revenue	2,278.5	2,120.2	1,641.2
External Customers \| Operating Segments \| AA&S
Disaggregation of Revenue [Line Items]
Revenue	$ 2,083.6	$ 2,053.5	$ 2,194.8