Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 28, 2025	Dec. 29, 2024	Dec. 31, 2023
Statement of Comprehensive Income [Abstract]
Net income	$ 418.6	$ 382.7	$ 423.4
Currency translation adjustment
Unrealized net change arising during the period	28.9	(13.0)	1.3
Reclassification adjustment included in net income	5.1	0.0	0.0
Total	34.0	(13.0)	1.3
Derivatives
Net derivatives loss on hedge transactions	(1.3)	(5.9)	(28.5)
Reclassification to net income of net realized loss	5.3	11.0	2.5
Income taxes on derivative transactions	0.9	2.0	(6.1)
Total	3.1	3.1	(19.9)
Actuarial gain/loss
Amortization of net actuarial loss	5.1	5.2	6.0
Net loss arising during the period	(6.7)	(2.2)	(3.8)
Prior service cost
Amortization to net income of net prior service credits	(0.5)	(0.5)	(0.6)
Settlement loss included in net income	0.0	0.0	1.1
Income taxes on postretirement benefit plans	(0.4)	0.5	0.3
Total	(1.7)	2.0	2.4
Other comprehensive income (loss), net of tax	35.4	(7.9)	(16.2)
Comprehensive income	454.0	374.8	407.2
Less: Comprehensive income attributable to noncontrolling interests	20.6	13.3	12.2
Comprehensive income attributable to ATI	$ 433.4	$ 361.5	$ 395.0