CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Net (loss) income	$ (419.9)	$ 78.5	$ (396.3)	$ 94.8
Currency translation adjustment
Unrealized net change arising during the period	14.2	(8.5)	(9.2)	2.5
Derivatives
Net derivatives gain (loss) on hedge transactions	2.0	(5.7)	(10.3)	4.8
Reclassification to net income (loss) of net realized loss	4.2	0.5	6.0	1.4
Income taxes on derivative transactions	2.5	0.0	0.0	0.0
Total	3.7	(5.2)	(4.3)	6.2
Postretirement benefit plans
Amortization of net actuarial loss	21.3	21.8	42.6	43.5
Amortization to net income (loss) of net prior service credits	(0.8)	(0.6)	(1.5)	(1.2)
Income taxes on postretirement benefit plans	(4.9)	0.0	0.0	0.0
Total	25.4	21.2	41.1	42.3
Other comprehensive income, net of tax	43.3	7.5	27.6	51.0
Comprehensive (loss) income	(376.6)	86.0	(368.7)	145.8
Less: Comprehensive income (loss) attributable to noncontrolling interests	2.2	(0.2)	3.9	6.3
Comprehensive (loss) income attributable to ATI	$ (378.8)	$ 86.2	$ (372.6)	$ 139.5