Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 146.5	$ (16.2)	$ (1,559.6)
Currency translation adjustment
Unrealized net change arising during the period	(43.5)	(4.6)	32.5
Reclassification adjustment included in net income (loss)	20.0	0.0	0.0
Total	(23.5)	(4.6)	32.5
Derivatives
Net derivatives gain (loss) on hedge transactions	53.8	15.5	(2.8)
Reclassification to net income (loss) of net realized (gain) loss	(42.8)	(11.4)	6.0
Income taxes on derivative transactions	0.0	0.0	0.0
Total	11.0	4.1	3.2
Postretirement benefit plans- Actuarial loss
Amortization of net actuarial loss	76.7	89.5	85.3
Net gain (loss) arising during the period	155.0	155.9	(128.4)
Postretirement benefit plans- Prior Service Cost
Amortization to net income (loss) of net prior service credits	(0.5)	(1.8)	(3.1)
Settlement loss (gain) included in net income (loss)	29.5	(21.9)	0.0
Income taxes on postretirement benefit plans	0.0	(15.5)	0.0
Total	260.7	237.2	(46.2)
Other comprehensive income (loss)	248.2	236.7	(10.5)
Comprehensive income (loss)	394.7	220.5	(1,570.1)
Less: Comprehensive income (loss) attributable to noncontrolling interests	(2.7)	26.8	24.4
Comprehensive income (loss) attributable to ATI	$ 397.4	$ 193.7	$ (1,594.5)