CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 23.6	$ 16.3
Currency translation adjustment
Unrealized net change arising during the period	(23.4)	11.0
Derivatives
Net derivatives (loss) gain on hedge transactions	(12.3)	10.5
Reclassification to net income of net realized loss	1.8	0.9
Income taxes on derivative transactions	(2.5)	0.0
Total	(8.0)	11.4
Postretirement benefit plans
Amortization of net actuarial loss	21.3	21.7
Amortization to net income of net prior service credits	(0.7)	(0.6)
Income taxes on postretirement benefit plans	4.9	0.0
Total	15.7	21.1
Other comprehensive (loss) income, net of tax	(15.7)	43.5
Comprehensive income	7.9	59.8
Less: Comprehensive income attributable to noncontrolling interests	1.7	6.5
Comprehensive income attributable to ATI	$ 6.2	$ 53.3