CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ (34.3)	$ (44.4)	$ 0.9	$ (46.8)
Currency translation adjustment
Unrealized net change arising during the period	(22.4)	7.0	(29.2)	2.2
Reclassification adjustment included in net income (loss)	20.0	0.0	20.0	0.0
Total	(2.4)	7.0	(9.2)	2.2
Derivatives
Net derivatives gain on hedge transactions	16.1	7.0	40.7	7.4
Reclassification to net income (loss) of net realized gain	(20.4)	(1.5)	(25.4)	(3.2)
Income taxes on derivative transactions	0.0	0.0	0.0	0.0
Total	(4.3)	5.5	15.3	4.2
Postretirement benefit plans
Amortization of net actuarial loss	19.2	22.2	38.5	44.4
Amortization to net income (loss) of net prior service credits	(0.1)	(0.5)	(0.3)	(1.0)
Settlement loss included in net income (loss)	29.5	0.0	29.5	0.0
Income taxes on postretirement benefit plans	0.0	0.0	0.0	0.0
Total	48.6	21.7	67.7	43.4
Other comprehensive income, net of tax	41.9	34.2	73.8	49.8
Comprehensive income (loss)	7.6	(10.2)	74.7	3.0
Less: Comprehensive income (loss) attributable to noncontrolling interests	(6.0)	6.7	(1.0)	12.8
Comprehensive income (loss) attributable to ATI	$ 13.6	$ (16.9)	$ 75.7	$ (9.8)