CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jul. 02, 2023	Jun. 30, 2024	Jul. 02, 2023
Statement of Comprehensive Income [Abstract]
Net income	$ 85.6	$ 93.5	$ 154.0	$ 180.1
Currency translation adjustment
Unrealized net change arising during the period	(3.7)	(8.1)	(10.3)	(4.8)
Derivatives
Net derivatives loss on hedge transactions	(0.6)	(2.4)	(2.4)	(17.4)
Reclassification to net income of net realized loss (gain)	2.6	3.8	4.3	(1.7)
Income taxes on derivative transactions	1.2	0.0	1.2	0.0
Total	0.8	1.4	0.7	(19.1)
Postretirement benefit plans
Amortization of net actuarial loss	1.3	1.5	2.6	3.0
Amortization to net income of net prior service credits	(0.2)	(0.2)	(0.3)	(0.3)
Income taxes on postretirement benefit plans	0.2	0.0	0.5	0.0
Total	0.9	1.3	1.8	2.7
Other comprehensive loss, net of tax	(2.0)	(5.4)	(7.8)	(21.2)
Comprehensive income	83.6	88.1	146.2	158.9
Less: Comprehensive income (loss) attributable to noncontrolling interests	3.1	(2.6)	5.2	3.9
Comprehensive income attributable to ATI	$ 80.5	$ 90.7	$ 141.0	$ 155.0