CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended
	Mar. 30, 2025	Mar. 31, 2024
Statement of Comprehensive Income [Abstract]
Net income	$ 100.5	$ 68.4
Currency translation adjustment
Unrealized net change arising during the period	7.4	(6.6)
Reclassification adjustment included in net income	5.1	0.0
Total	12.5	(6.6)
Derivatives
Net derivatives gain (loss) on hedge transactions	6.4	(1.8)
Reclassification to net income of net realized loss	1.0	1.7
Income taxes on derivative transactions	1.7	0.0
Total	5.7	(0.1)
Postretirement benefit plans
Amortization of net actuarial loss	1.3	1.3
Amortization to net income of net prior service credits	(0.1)	(0.1)
Income taxes on postretirement benefit plans	0.3	0.3
Total	0.9	0.9
Net current-period OCI	19.1	(5.8)
Comprehensive income	119.6	62.6
Less: Comprehensive income attributable to noncontrolling interests	4.7	2.1
Comprehensive income attributable to ATI	$ 114.9	$ 60.5