CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 115.3	$ 55.6	$ 210.1	$ 191.7
Currency translation adjustment
Unrealized net change arising during the period	(16.3)	(13.8)	(13.8)	(16.4)
Derivatives
Net derivatives gain (loss) on hedge transactions	14.3	(10.8)	19.1	2.4
Reclassification to net income of net realized (gain) loss	(0.8)	(2.6)	0.6	(10.3)
Income taxes on derivative transactions	0.0	0.0	0.0	0.0
Total	13.5	(13.4)	19.7	(7.9)
Postretirement benefit plans
Amortization of net actuarial loss	21.9	19.2	65.4	57.5
Amortization to net income of net prior service credits	(0.7)	(0.7)	(1.9)	(1.9)
Income taxes on postretirement benefit plans	0.0	0.0	0.0	0.0
Total	21.2	18.5	63.5	55.6
Other comprehensive income (loss), net of tax	18.4	(8.7)	69.4	31.3
Comprehensive income	133.7	46.9	279.5	223.0
Less: Comprehensive income (loss) attributable to noncontrolling interests	(0.3)	(3.9)	6.0	5.2
Comprehensive income attributable to ATI	$ 134.0	$ 50.8	$ 273.5	$ 217.8