CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 16.3	$ 60.5
Currency translation adjustment
Unrealized net change arising during the period	11.0	23.6
Derivatives
Net derivatives gain on hedge transactions	10.5	3.3
Reclassification to net income of net realized loss (gain)	0.9	(3.0)
Income taxes on derivative transactions	0.0	0.0
Total	11.4	0.3
Postretirement benefit plans
Amortization of net actuarial loss	21.7	19.2
Amortization to net income of net prior service credits	(0.6)	(0.6)
Income taxes on postretirement benefit plans	0.0	0.0
Total	21.1	18.6
Other comprehensive income, net of tax	43.5	42.5
Comprehensive income	59.8	103.0
Less: Comprehensive income attributable to noncontrolling interests	6.5	9.4
Comprehensive income attributable to ATI	$ 53.3	$ 93.6