CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net loss	$ (119.4)	$ (527.2)	$ (84.9)	$ (640.8)
Currency translation adjustment
Unrealized net change arising during the period	24.2	(4.4)	39.0	(19.6)
Derivatives
Net derivatives gain (loss) on hedge transactions	9.2	4.8	(1.6)	21.9
Reclassification to net income (loss) of net realized loss (gain)	(1.2)	0.6	(3.5)	8.5
Income taxes on derivative transactions	4.8	2.0	(0.2)	11.5
Total	3.2	3.4	(4.9)	18.9
Postretirement benefit plans
Amortization of net actuarial loss	17.8	18.7	53.6	56.2
Net gain arising during period	0.0	0.0	0.0	22.5
Amortization to net income (loss) of net prior service cost (credits)	(0.4)	(0.4)	(1.2)	0.0
Income taxes on postretirement benefit plans	(11.1)	6.9	1.9	29.8
Total	28.5	11.4	50.5	48.9
Other comprehensive income, net of tax	55.9	10.4	84.6	48.2
Comprehensive loss	(63.5)	(516.8)	(0.3)	(592.6)
Less: Comprehensive income attributable to noncontrolling interests	6.8	2.0	15.9	5.4
Comprehensive loss attributable to ATI	$ (70.3)	$ (518.8)	$ (16.2)	$ (598.0)