Revenue from Contracts with Customers - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 28, 2025	Dec. 29, 2024	Dec. 31, 2023
Disaggregation of Revenue [Line Items]
Sales	$ 4,587.4	$ 4,362.1	$ 4,173.7
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	46.00%	45.00%	49.00%
Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of total revenue	22.00%	20.00%	17.00%
Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue	18.00%	18.00%	17.00%
Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue	9.00%	9.00%	7.00%
Precision rolled strip
Disaggregation of Revenue [Line Items]
Percent of total revenue	5.00%	8.00%	10.00%
United States
Disaggregation of Revenue [Line Items]
Sales	$ 2,639.8	$ 2,525.2	$ 2,250.8
China
Disaggregation of Revenue [Line Items]
Sales	324.3	299.6	333.3
United Kingdom
Disaggregation of Revenue [Line Items]
Sales	277.2	257.9	259.1
Germany
Disaggregation of Revenue [Line Items]
Sales	227.4	259.7	243.0
France
Disaggregation of Revenue [Line Items]
Sales	246.2	227.5	219.4
Canada
Disaggregation of Revenue [Line Items]
Sales	169.5	116.2	111.0
Rest of World
Disaggregation of Revenue [Line Items]
Sales	703.0	676.0	757.1
Total Aerospace & Defense
Disaggregation of Revenue [Line Items]
Sales	3,111.5	2,720.9	2,474.8
Jet-Engines- Commercial
Disaggregation of Revenue [Line Items]
Sales	1,762.9	1,457.8	1,333.5
Airframes- Commercial
Disaggregation of Revenue [Line Items]
Sales	790.9	772.9	739.4
Defense
Disaggregation of Revenue [Line Items]
Sales	557.7	490.2	401.9
Total Other Markets
Disaggregation of Revenue [Line Items]
Sales	1,218.6	1,356.6	1,425.7
Specialty Energy
Disaggregation of Revenue [Line Items]
Sales	257.3	284.6	273.2
Electronics
Disaggregation of Revenue [Line Items]
Sales	184.8	194.3	159.9
Medical
Disaggregation of Revenue [Line Items]
Sales	139.4	224.9	176.9
Automotive
Disaggregation of Revenue [Line Items]
Sales	244.6	259.4	210.7
Conventional Energy
Disaggregation of Revenue [Line Items]
Sales	328.4	302.0	414.6
Construction/Mining
Disaggregation of Revenue [Line Items]
Sales	145.4	158.5	162.9
Other
Disaggregation of Revenue [Line Items]
Sales	176.0	217.5	300.7
Operating Segments
Disaggregation of Revenue [Line Items]
Sales	5,010.6	4,884.9	4,638.9
Operating Segments | HPMC
Disaggregation of Revenue [Line Items]
Sales	$ 2,664.4	$ 2,526.2	$ 2,302.0
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	43.00%	41.00%	44.00%
Operating Segments | HPMC | Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of total revenue	40.00%	36.00%	33.00%
Operating Segments | HPMC | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue	17.00%	23.00%	22.00%
Operating Segments | HPMC | Zirconium and related alloys
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%	0.00%	1.00%
Operating Segments | HPMC | United States
Disaggregation of Revenue [Line Items]
Sales	$ 1,336.2	$ 1,134.0	$ 915.3
Operating Segments | HPMC | China
Disaggregation of Revenue [Line Items]
Sales	81.5	57.6	70.1
Operating Segments | HPMC | United Kingdom
Disaggregation of Revenue [Line Items]
Sales	222.0	217.0	224.8
Operating Segments | HPMC | Germany
Disaggregation of Revenue [Line Items]
Sales	160.2	202.5	204.2
Operating Segments | HPMC | France
Disaggregation of Revenue [Line Items]
Sales	156.0	186.9	172.4
Operating Segments | HPMC | Canada
Disaggregation of Revenue [Line Items]
Sales	90.7	60.9	64.9
Operating Segments | HPMC | Rest of World
Disaggregation of Revenue [Line Items]
Sales	395.1	419.6	468.5
Operating Segments | HPMC | Total Aerospace & Defense
Disaggregation of Revenue [Line Items]
Sales	2,239.4	1,959.9	1,786.9
Operating Segments | HPMC | Jet-Engines- Commercial
Disaggregation of Revenue [Line Items]
Sales	1,649.6	1,365.4	1,255.3
Operating Segments | HPMC | Airframes- Commercial
Disaggregation of Revenue [Line Items]
Sales	312.1	369.7	350.6
Operating Segments | HPMC | Defense
Disaggregation of Revenue [Line Items]
Sales	277.7	224.8	181.0
Operating Segments | HPMC | Total Other Markets
Disaggregation of Revenue [Line Items]
Sales	126.5	221.8	239.4
Operating Segments | HPMC | Specialty Energy
Disaggregation of Revenue [Line Items]
Sales	75.8	96.8	93.9
Operating Segments | HPMC | Electronics
Disaggregation of Revenue [Line Items]
Sales	0.0	3.0	3.1
Operating Segments | HPMC | Medical
Disaggregation of Revenue [Line Items]
Sales	55.2	115.5	102.6
Operating Segments | HPMC | Automotive
Disaggregation of Revenue [Line Items]
Sales	6.5	15.2	24.6
Operating Segments | HPMC | Conventional Energy
Disaggregation of Revenue [Line Items]
Sales	6.1	9.8	10.6
Operating Segments | HPMC | Construction/Mining
Disaggregation of Revenue [Line Items]
Sales	27.7	26.3	35.0
Operating Segments | HPMC | Other
Disaggregation of Revenue [Line Items]
Sales	31.0	52.0	63.5
Operating Segments | AA&S
Disaggregation of Revenue [Line Items]
Sales	$ 2,346.2	$ 2,358.7	$ 2,336.9
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	50.00%	49.00%	54.00%
Operating Segments | AA&S | Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of total revenue	0.00%	0.00%	0.00%
Operating Segments | AA&S | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue	19.00%	13.00%	12.00%
Operating Segments | AA&S | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue	19.00%	19.00%	15.00%
Operating Segments | AA&S | Precision rolled strip
Disaggregation of Revenue [Line Items]
Percent of total revenue	12.00%	19.00%	19.00%
Operating Segments | AA&S | United States
Disaggregation of Revenue [Line Items]
Sales	$ 1,303.6	$ 1,391.2	$ 1,335.5
Operating Segments | AA&S | China
Disaggregation of Revenue [Line Items]
Sales	242.8	242.0	263.2
Operating Segments | AA&S | United Kingdom
Disaggregation of Revenue [Line Items]
Sales	55.2	40.9	34.3
Operating Segments | AA&S | Germany
Disaggregation of Revenue [Line Items]
Sales	67.2	57.2	38.8
Operating Segments | AA&S | France
Disaggregation of Revenue [Line Items]
Sales	90.2	40.6	47.0
Operating Segments | AA&S | Canada
Disaggregation of Revenue [Line Items]
Sales	78.8	55.3	46.1
Operating Segments | AA&S | Rest of World
Disaggregation of Revenue [Line Items]
Sales	307.9	256.4	288.6
Operating Segments | AA&S | Total Aerospace & Defense
Disaggregation of Revenue [Line Items]
Sales	872.1	761.0	687.9
Operating Segments | AA&S | Jet-Engines- Commercial
Disaggregation of Revenue [Line Items]
Sales	113.3	92.4	78.2
Operating Segments | AA&S | Airframes- Commercial
Disaggregation of Revenue [Line Items]
Sales	478.8	403.2	388.8
Operating Segments | AA&S | Defense
Disaggregation of Revenue [Line Items]
Sales	280.0	265.4	220.9
Operating Segments | AA&S | Total Other Markets
Disaggregation of Revenue [Line Items]
Sales	1,092.1	1,134.8	1,186.3
Operating Segments | AA&S | Specialty Energy
Disaggregation of Revenue [Line Items]
Sales	181.5	187.8	179.3
Operating Segments | AA&S | Electronics
Disaggregation of Revenue [Line Items]
Sales	184.8	191.3	156.8
Operating Segments | AA&S | Medical
Disaggregation of Revenue [Line Items]
Sales	84.2	109.4	74.3
Operating Segments | AA&S | Automotive
Disaggregation of Revenue [Line Items]
Sales	238.1	244.2	186.1
Operating Segments | AA&S | Conventional Energy
Disaggregation of Revenue [Line Items]
Sales	322.3	292.2	404.0
Operating Segments | AA&S | Construction/Mining
Disaggregation of Revenue [Line Items]
Sales	117.7	132.2	127.9
Operating Segments | AA&S | Other
Disaggregation of Revenue [Line Items]
Sales	145.0	165.5	237.2
Sales to external customers | Operating Segments
Disaggregation of Revenue [Line Items]
Sales	4,587.4	4,362.1	4,173.7
Sales to external customers | Operating Segments | HPMC
Disaggregation of Revenue [Line Items]
Sales	2,441.7	2,278.5	2,120.2
Sales to external customers | Operating Segments | AA&S
Disaggregation of Revenue [Line Items]
Sales	$ 2,145.7	$ 2,083.6	$ 2,053.5