CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement of Comprehensive Income [Abstract]
Net loss	$ (44.4)	$ (419.9)	$ (46.8)	$ (396.3)
Currency translation adjustment
Unrealized net change arising during the period	7.0	14.2	2.2	(9.2)
Derivatives
Net derivatives gain (loss) on hedge transactions	7.0	2.0	7.4	(10.3)
Reclassification to net loss of net realized (gain) loss	(1.5)	4.2	(3.2)	6.0
Income taxes on derivative transactions	0.0	2.5	0.0	0.0
Total	5.5	3.7	4.2	(4.3)
Postretirement benefit plans
Amortization of net actuarial loss	22.2	21.3	44.4	42.6
Amortization to net loss of net prior service credits	(0.5)	(0.8)	(1.0)	(1.5)
Income taxes on postretirement benefit plans	0.0	(4.9)	0.0	0.0
Total	21.7	25.4	43.4	41.1
Other comprehensive income, net of tax	34.2	43.3	49.8	27.6
Comprehensive income (loss)	(10.2)	(376.6)	3.0	(368.7)
Less: Comprehensive income attributable to noncontrolling interests	6.7	2.2	12.8	3.9
Comprehensive loss attributable to ATI	$ (16.9)	$ (378.8)	$ (9.8)	$ (372.6)