CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 29, 2025	Jun. 30, 2024	Jun. 29, 2025	Jun. 30, 2024
Statement of Comprehensive Income [Abstract]
Net income	$ 104.0	$ 85.6	$ 204.5	$ 154.0
Currency translation adjustment
Unrealized net change arising during the period	18.3	(3.7)	25.7	(10.3)
Reclassification adjustment included in net income	0.0	0.0	5.1	0.0
Total	18.3	(3.7)	30.8	(10.3)
Derivatives
Net derivatives gain (loss) on hedge transactions	(4.4)	(0.6)	2.0	(2.4)
Reclassification to net income of net realized loss	1.6	2.6	2.6	4.3
Income taxes on derivative transactions	(0.6)	1.2	1.1	1.2
Total	(2.2)	0.8	3.5	0.7
Postretirement benefit plans
Amortization of net actuarial loss	1.3	1.3	2.6	2.6
Amortization to net income of net prior service credits	(0.1)	(0.2)	(0.2)	(0.3)
Income taxes on postretirement benefit plans	0.4	0.2	0.7	0.5
Total	0.8	0.9	1.7	1.8
Net current-period OCI	16.9	(2.0)	36.0	(7.8)
Comprehensive income	120.9	83.6	240.5	146.2
Less: Comprehensive income attributable to noncontrolling interests	5.1	3.1	9.8	5.2
Comprehensive income attributable to ATI	$ 115.8	$ 80.5	$ 230.7	$ 141.0