Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ (16.2)	$ (1,559.6)	$ 265.0
Currency translation adjustment
Unrealized net change arising during the period	(4.6)	32.5	(4.0)
Derivatives
Amount of Gain (Loss) Recognized in OCI on Derivatives	15.5	(2.8)	9.7
Reclassification to net income (loss) of net realized (gain) loss	(11.4)	6.0	(4.1)
Income taxes on derivative transactions	0.0	0.0	(4.7)
Total	4.1	3.2	10.3
Postretirement benefit plans- Actuarial loss
Amortization of net actuarial loss	89.5	85.3	87.2
Net gain (loss) arising during the period	155.9	(128.4)	(180.5)
Postretirement benefit plans- Prior Service Cost
Amortization to net income (loss) of net prior service credits	(1.8)	(3.1)	(2.6)
Settlement gain	(21.9)	0.0	0.0
Income taxes on postretirement benefit plans	(15.5)	0.0	(20.4)
Total	237.2	(46.2)	(75.5)
Other comprehensive income (loss), net of tax	236.7	(10.5)	(69.2)
Comprehensive income (loss)	220.5	(1,570.1)	195.8
Less: Comprehensive income attributable to noncontrolling interests	26.8	24.4	11.2
Comprehensive income (loss) attributable to ATI	$ 193.7	$ (1,594.5)	$ 184.6