Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net loss	$ (79.7)	$ (627.1)	$ (365.9)
Currency translation adjustment
Unrealized net change arising during the period	39.1	(47.1)	(37.0)
Reclassification adjustment included in net loss	0.0	0.0	0.0
Total	39.1	(47.1)	(37.0)
Unrealized holding gain (loss) on securities
Net gain (loss) arising during the period	0.0	0.0	0.0
Derivatives
Net derivatives gain (loss) on hedge transactions	14.3	21.2	(33.3)
Reclassification to net income of net realized loss (gain)	(7.2)	7.9	(18.2)
Income taxes on derivative transactions	0.0	9.5	(19.5)
Total	7.1	19.6	(32.0)
Postretirement benefit plans- Actuarial loss
Amortization of net actuarial loss	71.6	75.0	75.0
Net loss arising during the period	(42.7)	(92.1)	(95.8)
Postretirement benefit plans- Prior Service Cost
Amortization to net income (loss) of net prior service cost (credits)	(1.6)	(0.4)	6.2
Income taxes on postretirement benefit plans	0.0	43.9	5.1
Total	27.3	(61.4)	(19.7)
Other comprehensive income (loss), net of tax	73.5	(88.9)	(88.7)
Comprehensive loss	(6.2)	(716.0)	(454.6)
Less: Comprehensive income attributable to noncontrolling interests	19.8	4.1	6.4
Comprehensive loss attributable to ATI	$ (26.0)	$ (720.1)	$ (461.0)