CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Statement of Comprehensive Income [Abstract]
Net income	$ 79.1	$ (34.3)	$ 151.3	$ 0.9
Currency translation adjustment
Unrealized net change arising during the period	(8.1)	(22.4)	(4.8)	(29.2)
Reclassification adjustment included in net income (loss)	0.0	20.0	0.0	20.0
Total	(8.1)	(2.4)	(4.8)	(9.2)
Derivatives
Net derivatives gain (loss) on hedge transactions	(2.4)	16.1	(17.4)	40.7
Reclassification to net income (loss) of net realized loss (gain)	3.8	(20.4)	(1.7)	(25.4)
Income taxes on derivative transactions	0.0	0.0	0.0	0.0
Total	1.4	(4.3)	(19.1)	15.3
Postretirement benefit plans
Amortization of net actuarial loss	15.9	19.2	31.8	38.5
Amortization to net income (loss) of net prior service credits	(0.2)	(0.1)	(0.3)	(0.3)
Settlement loss included in net income (loss)	0.0	29.5	0.0	29.5
Income taxes on postretirement benefit plans	0.0	0.0	0.0	0.0
Total	15.7	48.6	31.5	67.7
Other comprehensive income, net of tax	9.0	41.9	7.6	73.8
Comprehensive income	88.1	7.6	158.9	74.7
Less: Comprehensive income (loss) attributable to noncontrolling interests	(2.6)	(6.0)	3.9	(1.0)
Comprehensive income attributable to ATI	$ 90.7	$ 13.6	$ 155.0	$ 75.7