Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2023	Oct. 01, 2023	Jul. 02, 2023	Apr. 02, 2023	Jan. 01, 2023	Oct. 02, 2022	Jul. 03, 2022	Apr. 03, 2022	Dec. 31, 2023	Jan. 01, 2023	Jan. 02, 2022
Disaggregation of Revenue [Line Items]
Revenue	$ 1,064.0	$ 1,025.6	$ 1,046.0	$ 1,038.1	$ 1,010.4	$ 1,032.0	$ 959.5	$ 834.1	$ 4,173.7	$ 3,836.0	$ 2,799.8
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									49.00%	52.00%	43.00%
Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of total revenue									17.00%	15.00%	16.00%
Titanium and Titanium-based Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of total revenue									17.00%	11.00%	12.00%
Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									7.00%	8.00%	10.00%
Precision rolled strip
Disaggregation of Revenue [Line Items]
Percent of total revenue									10.00%	14.00%	19.00%
China
Disaggregation of Revenue [Line Items]
Revenue									$ 333.3	$ 351.8	$ 370.4
Germany
Disaggregation of Revenue [Line Items]
Revenue									243.0	200.9	121.3
UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenue									259.1	217.7	153.9
Rest of World
Disaggregation of Revenue [Line Items]
Revenue									740.4	609.7	497.6
France
Disaggregation of Revenue [Line Items]
Revenue									219.4	157.2	58.5
MEXICO
Disaggregation of Revenue [Line Items]
Revenue									127.7	80.1	63.2
United States
Disaggregation of Revenue [Line Items]
Revenue									2,250.8	2,218.6	1,534.9
Total Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue									2,474.8	1,873.6	1,132.7
Total Energy
Disaggregation of Revenue [Line Items]
Revenue									687.8	753.3	591.9
Automotive
Disaggregation of Revenue [Line Items]
Revenue									210.7	302.1	305.1
Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue									71.9	158.5	153.1
Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue									162.9	176.4	122.2
Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue									176.9	163.1	131.5
Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue									159.9	200.0	215.1
Other
Disaggregation of Revenue [Line Items]
Revenue									228.8	209.0	148.2
Jet Engines- Commercial
Disaggregation of Revenue [Line Items]
Revenue									1,333.5	1,063.5	517.2
Airframes- Commercial
Disaggregation of Revenue [Line Items]
Revenue									739.4	468.9	262.7
Defense
Disaggregation of Revenue [Line Items]
Revenue									401.9	341.2	352.8
Oil & Gas
Disaggregation of Revenue [Line Items]
Revenue									414.6	476.7	332.3
Specialty Energy
Disaggregation of Revenue [Line Items]
Revenue									273.2	276.6	259.6
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue									4,638.9	4,249.4	3,010.7
Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue									$ 2,302.0	$ 1,815.7	$ 1,248.3
Percent of total revenue									100.00%	100.00%	100.00%
Operating Segments | High Performance Materials & Components | Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									44.00%	49.00%	43.00%
Operating Segments | High Performance Materials & Components | Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of total revenue									33.00%	34.00%	38.00%
Operating Segments | High Performance Materials & Components | Titanium and Titanium-based Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of total revenue									22.00%	17.00%	19.00%
Operating Segments | High Performance Materials & Components | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									0.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | Precision rolled strip
Disaggregation of Revenue [Line Items]
Percent of total revenue									1.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | China
Disaggregation of Revenue [Line Items]
Revenue									$ 70.1	$ 59.8	$ 49.5
Operating Segments | High Performance Materials & Components | Germany
Disaggregation of Revenue [Line Items]
Revenue									204.2	148.4	74.1
Operating Segments | High Performance Materials & Components | UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenue									224.8	165.7	136.7
Operating Segments | High Performance Materials & Components | Rest of World
Disaggregation of Revenue [Line Items]
Revenue									431.1	342.0	249.2
Operating Segments | High Performance Materials & Components | France
Disaggregation of Revenue [Line Items]
Revenue									172.4	125.7	48.7
Operating Segments | High Performance Materials & Components | MEXICO
Disaggregation of Revenue [Line Items]
Revenue									102.3	56.7	25.6
Operating Segments | High Performance Materials & Components | United States
Disaggregation of Revenue [Line Items]
Revenue									915.3	742.9	571.3
Operating Segments | High Performance Materials & Components | Total Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue									1,786.9	1,318.0	835.5
Operating Segments | High Performance Materials & Components | Total Energy
Disaggregation of Revenue [Line Items]
Revenue									104.5	148.6	178.3
Operating Segments | High Performance Materials & Components | Automotive
Disaggregation of Revenue [Line Items]
Revenue									24.6	11.2	8.7
Operating Segments | High Performance Materials & Components | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue									0.0	0.2	0.1
Operating Segments | High Performance Materials & Components | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue									35.0	34.1	24.0
Operating Segments | High Performance Materials & Components | Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue									102.6	73.2	60.3
Operating Segments | High Performance Materials & Components | Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue									3.1	2.4	1.2
Operating Segments | High Performance Materials & Components | Other
Disaggregation of Revenue [Line Items]
Revenue									63.5	53.5	47.0
Operating Segments | High Performance Materials & Components | Jet Engines- Commercial
Disaggregation of Revenue [Line Items]
Revenue									1,255.3	975.7	480.9
Operating Segments | High Performance Materials & Components | Airframes- Commercial
Disaggregation of Revenue [Line Items]
Revenue									350.6	184.1	132.8
Operating Segments | High Performance Materials & Components | Defense
Disaggregation of Revenue [Line Items]
Revenue									181.0	158.2	221.8
Operating Segments | High Performance Materials & Components | Oil & Gas
Disaggregation of Revenue [Line Items]
Revenue									10.6	35.0	42.2
Operating Segments | High Performance Materials & Components | Specialty Energy
Disaggregation of Revenue [Line Items]
Revenue									93.9	113.6	136.1
Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue									$ 2,336.9	$ 2,433.7	$ 1,762.4
Percent of total revenue									100.00%	100.00%	100.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									54.00%	54.00%	44.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Precision Forgings, Casting and Components [Member]
Disaggregation of Revenue [Line Items]
Percent of total revenue									0.00%	0.00%	0.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Titanium and Titanium-based Alloys [Member]
Disaggregation of Revenue [Line Items]
Percent of total revenue									12.00%	7.00%	6.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									15.00%	14.00%	17.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Precision rolled strip
Disaggregation of Revenue [Line Items]
Percent of total revenue									19.00%	25.00%	33.00%
Operating Segments | Advanced Alloys & Solutions [Member] | China
Disaggregation of Revenue [Line Items]
Revenue									$ 263.2	$ 292.0	$ 320.9
Operating Segments | Advanced Alloys & Solutions [Member] | Germany
Disaggregation of Revenue [Line Items]
Revenue									38.8	52.5	47.2
Operating Segments | Advanced Alloys & Solutions [Member] | UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenue									34.3	52.0	17.2
Operating Segments | Advanced Alloys & Solutions [Member] | Rest of World
Disaggregation of Revenue [Line Items]
Revenue									309.3	267.7	248.4
Operating Segments | Advanced Alloys & Solutions [Member] | France
Disaggregation of Revenue [Line Items]
Revenue									47.0	31.5	9.8
Operating Segments | Advanced Alloys & Solutions [Member] | MEXICO
Disaggregation of Revenue [Line Items]
Revenue									25.4	23.4	37.6
Operating Segments | Advanced Alloys & Solutions [Member] | United States
Disaggregation of Revenue [Line Items]
Revenue									1,335.5	1,475.7	963.6
Operating Segments | Advanced Alloys & Solutions [Member] | Total Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue									687.9	555.6	297.2
Operating Segments | Advanced Alloys & Solutions [Member] | Total Energy
Disaggregation of Revenue [Line Items]
Revenue									583.3	604.7	413.6
Operating Segments | Advanced Alloys & Solutions [Member] | Automotive
Disaggregation of Revenue [Line Items]
Revenue									186.1	290.9	296.4
Operating Segments | Advanced Alloys & Solutions [Member] | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue									71.9	158.3	153.0
Operating Segments | Advanced Alloys & Solutions [Member] | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue									127.9	142.3	98.2
Operating Segments | Advanced Alloys & Solutions [Member] | Medical Market [Member]
Disaggregation of Revenue [Line Items]
Revenue									74.3	89.9	71.2
Operating Segments | Advanced Alloys & Solutions [Member] | Electronic Devices Market [Member]
Disaggregation of Revenue [Line Items]
Revenue									156.8	197.6	213.9
Operating Segments | Advanced Alloys & Solutions [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue									165.3	155.5	101.2
Operating Segments | Advanced Alloys & Solutions [Member] | Jet Engines- Commercial
Disaggregation of Revenue [Line Items]
Revenue									78.2	87.8	36.3
Operating Segments | Advanced Alloys & Solutions [Member] | Airframes- Commercial
Disaggregation of Revenue [Line Items]
Revenue									388.8	284.8	129.9
Operating Segments | Advanced Alloys & Solutions [Member] | Defense
Disaggregation of Revenue [Line Items]
Revenue									220.9	183.0	131.0
Operating Segments | Advanced Alloys & Solutions [Member] | Oil & Gas
Disaggregation of Revenue [Line Items]
Revenue									404.0	441.7	290.1
Operating Segments | Advanced Alloys & Solutions [Member] | Specialty Energy
Disaggregation of Revenue [Line Items]
Revenue									179.3	163.0	123.5
External Customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue									4,173.7	3,836.0	2,799.8
External Customers | Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue									2,120.2	1,641.2	1,155.1
External Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue									$ 2,053.5	$ 2,194.8	$ 1,644.7