CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Statement of Comprehensive Income [Abstract]
Net income	$ 79.6	$ 64.4	$ 230.9	$ 65.3
Currency translation adjustment
Unrealized net change arising during the period	(9.2)	(30.0)	(14.0)	(59.2)
Reclassification adjustment included in net income	0.0	0.0	0.0	20.0
Total	(9.2)	(30.0)	(14.0)	(39.2)
Derivatives
Net derivatives gain (loss) on hedge transactions	(3.2)	7.7	(20.6)	48.4
Reclassification to net income of net realized loss (gain)	1.8	(7.1)	0.1	(32.5)
Income taxes on derivative transactions	0.0	0.0	0.0	0.0
Total	(1.4)	0.6	(20.5)	15.9
Postretirement benefit plans
Amortization of net actuarial loss	16.0	19.1	47.8	57.6
Amortization to net income of net prior service credits	(0.1)	(0.1)	(0.4)	(0.4)
Settlement loss included in net income	0.0	0.0	0.0	29.5
Income taxes on postretirement benefit plans	0.0	0.0	0.0	0.0
Total	15.9	19.0	47.4	86.7
Other comprehensive income (loss), net of tax	5.3	(10.4)	12.9	63.4
Comprehensive income	84.9	54.0	243.8	128.7
Less: Comprehensive income (loss) attributable to noncontrolling interests	1.8	(6.1)	5.7	(7.1)
Comprehensive income attributable to ATI	$ 83.1	$ 60.1	$ 238.1	$ 135.8