CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 75.6	$ 13.4	$ 136.1	$ 34.5
Currency translation adjustment
Unrealized net change arising during the period	(26.2)	4.4	(2.6)	14.8
Derivatives
Net derivatives gain (loss) on hedge transactions	9.9	(8.2)	13.2	(10.8)
Reclassification to net income of net realized gain	(4.7)	(1.4)	(7.7)	(2.3)
Income taxes on derivative transactions	0.0	(5.0)	0.0	(5.0)
Total	5.2	(4.6)	5.5	(8.1)
Postretirement benefit plans
Amortization of net actuarial loss	19.1	18.0	38.3	35.8
Amortization to net income of net prior service credits	(0.6)	(0.4)	(1.2)	(0.8)
Income taxes on postretirement benefit plans	0.0	13.0	0.0	13.0
Total	18.5	4.6	37.1	22.0
Other comprehensive income (loss), net of tax	(2.5)	4.4	40.0	28.7
Comprehensive income	73.1	17.8	176.1	63.2
Less: Comprehensive income (loss) attributable to noncontrolling interests	(0.3)	3.5	9.1	9.1
Comprehensive income attributable to ATI	$ 73.4	$ 14.3	$ 167.0	$ 54.1