CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended
	Mar. 29, 2026	Mar. 30, 2025
Statement of Comprehensive Income [Abstract]
Net income	$ 120.5	$ 100.5
Currency translation adjustment
Unrealized net change arising during the period	(1.8)	7.4
Reclassification adjustment included in net income	0.0	5.1
Total	(1.8)	12.5
Derivatives
Net derivatives loss on hedge transactions	2.4	6.4
Reclassification to net income of net realized loss	(2.9)	1.0
Less: Income taxes on derivative transactions	(0.1)	1.7
Total	(0.4)	5.7
Actuarial loss
Amortization of net actuarial loss	1.3	1.3
Prior service cost
Amortization to net income of net prior service credits	0.1	(0.1)
Less: Income taxes on postretirement benefit plans	0.4	0.3
Total	1.0	0.9
Other comprehensive income (loss), net of tax	(1.2)	19.1
Comprehensive income	119.3	119.6
Less: Comprehensive income attributable to noncontrolling interests	3.6	4.7
Comprehensive income attributable to ATI	$ 115.7	$ 114.9