CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ (47.0)	$ 115.3	$ (443.3)	$ 210.1
Currency translation adjustment
Unrealized net change arising during the period	23.2	(16.3)	14.0	(13.8)
Derivatives
Net derivatives gain (loss) on hedge transactions	6.1	14.3	(4.2)	19.1
Reclassification to net income (loss) of net realized (gain) loss	0.4	(0.8)	6.4	0.6
Income taxes on derivative transactions	0.0	0.0	0.0	0.0
Total	6.5	13.5	2.2	19.7
Postretirement benefit plans
Amortization of net actuarial loss	21.3	21.9	63.9	65.4
Amortization to net income (loss) of net prior service credits	(0.8)	(0.7)	(2.3)	(1.9)
Income taxes on postretirement benefit plans	0.0	0.0	0.0	0.0
Total	20.5	21.2	61.6	63.5
Other comprehensive income, net of tax	50.2	18.4	77.8	69.4
Comprehensive income (loss)	3.2	133.7	(365.5)	279.5
Less: Comprehensive income (loss) attributable to noncontrolling interests	9.1	(0.3)	13.0	6.0
Comprehensive income (loss) attributable to ATI	$ (5.9)	$ 134.0	$ (378.5)	$ 273.5