CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 35.2	$ (2.4)
Currency translation adjustment
Unrealized net change arising during the period	(6.8)	(4.8)
Derivatives
Net derivatives gain on hedge transactions	24.6	0.4
Reclassification to net income (loss) of net realized gain	(5.0)	(1.7)
Income taxes on derivative transactions	0.0	0.0
Total	19.6	(1.3)
Postretirement benefit plans
Amortization of net actuarial loss	19.3	22.2
Amortization to net income (loss) of net prior service credits	(0.2)	(0.5)
Income taxes on postretirement benefit plans	0.0	0.0
Total	19.1	21.7
Other comprehensive income, net of tax	31.9	15.6
Comprehensive income	67.1	13.2
Less: Comprehensive income attributable to noncontrolling interests	5.0	6.1
Comprehensive income attributable to ATI	$ 62.1	$ 7.1