Debt - 2025 Convertible Notes - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		4 Months Ended	12 Months Ended
	Sep. 28, 2025	Sep. 29, 2024 USD ($) $ / shares shares	Sep. 29, 2024 USD ($) $ / shares	Dec. 28, 2025 USD ($)	Dec. 29, 2024 USD ($)	Dec. 31, 2023 USD ($)
Debt Instrument [Line Items]
Conversion of convertible notes		$ 291.0
Stock issued during period, shares, conversion of convertible securities | shares		18.8
Payments on long-term debt and finance leases				$ 182.6	$ 29.6	$ 25.2
Receipt of convertible note capped call		$ 76.1		$ 0.0	$ 76.1	$ 0.0
ATI Convertible Senior Notes, 3.5%, Due 2025
Debt Instrument [Line Items]
Payments on long-term debt and finance leases		0.4
ATI Convertible Senior Notes, 3.5%, Due 2025 | Convertible Debt
Debt Instrument [Line Items]
Debt instrument carrying amount		$ 291.4	$ 291.4
Interest rate		3.50%	3.50%
Convertible debt conversion rate in shares of stock	0.0645745	0.0647178
Debt instrument, convertible, conversion price with premium (in dollars per share) | $ / shares		$ 15.45	$ 15.45
Conversion price of convertible notes (in dollars per share) | $ / shares		15.49	$ 15.49
Debt instrument, increase, accrued interest			$ 2.3
Write off of deferred debt issuance cost			$ 1.6
Debt instrument, convertible, cap price (in dollars per share) | $ / shares		$ 19.76	$ 19.76
Debt instrument, interest rate, effective percentage					4.20%	4.20%