Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ (627.1)	$ (365.9)	$ 9.6
Currency translation adjustment
Unrealized net change arising during the period	(47.1)	(37.0)	(34.1)
Reclassification adjustment included in net income (loss)	0.0	0.0	0.5
Total	(47.1)	(37.0)	(33.6)
Unrealized holding gain (loss) on securities
Net gain arising during the period	0.0	0.0	0.0
Derivatives
Net derivatives gain (loss) on hedge transactions	21.2	(33.3)	45.7
Reclassification to net income of net realized loss (gain)	7.9	(18.2)	(3.6)
Income taxes on derivative transactions	9.5	(19.5)	16.2
Total	19.6	(32.0)	25.9
Postretirement benefit plans- Actuarial loss
Amortization of net actuarial loss	75.0	75.0	88.1
Net loss arising during the period	(92.1)	(95.8)	(424.5)
Postretirement benefit plans- Prior Service Cost
Amortization to net income (loss) of net prior service cost (credits)	(0.4)	6.2	(0.7)
Income taxes on postretirement benefit plans	43.9	5.1	(124.5)
Total	(61.4)	(19.7)	(212.6)
Other comprehensive loss, net of tax	(88.9)	(88.7)	(220.3)
Comprehensive loss	(716.0)	(454.6)	(210.7)
Less: Comprehensive income attributable to noncontrolling interests	4.1	6.4	10.1
Comprehensive loss attributable to ATI	$ (720.1)	$ (461.0)	$ (220.8)