CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Statement of Comprehensive Income [Abstract]
Net loss	$ (15.5)	$ (13.9)	$ (113.6)	$ (1.3)
Currency translation adjustment
Unrealized net change arising during the period	(9.8)	19.3	(15.2)	(2.5)
Unrealized holding gain on securities
Net loss arising during the period	0.0	(0.1)	0.0	(0.1)
Derivatives
Net derivatives gain (loss) on hedge transactions	26.3	(10.4)	17.1	8.1
Reclassification to net income (loss) of net realized loss (gain)	2.9	(2.1)	7.9	(4.8)
Income taxes on derivative transactions	11.1	(4.8)	9.5	1.3
Total	18.1	(7.7)	15.5	2.0
Postretirement benefit plans
Amortization of net actuarial loss	18.8	18.7	37.5	37.4
Net gain arising during period	0.0	0.0	22.5	0.0
Amortization to net income (loss) of net prior service cost (credit)	(0.5)	1.5	0.4	3.0
Income taxes on postretirement benefit plans	6.9	7.7	22.9	15.4
Total	11.4	12.5	37.5	25.0
Other comprehensive income, net of tax	19.7	24.0	37.8	24.4
Comprehensive income (loss)	4.2	10.1	(75.8)	23.1
Less: Comprehensive income attributable to noncontrolling interests	1.8	3.0	3.4	5.4
Comprehensive income (loss) attributable to ATI	$ 2.4	$ 7.1	$ (79.2)	$ 17.7