CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 13.4	$ (15.5)	$ 34.5	$ (113.6)
Currency translation adjustment
Unrealized net change arising during the period	4.4	(9.8)	14.8	(15.2)
Derivatives
Net derivatives gain (loss) on hedge transactions	(8.2)	26.3	(10.8)	17.1
Reclassification to net income (loss) of net realized loss (gain)	(1.4)	2.9	(2.3)	7.9
Income taxes on derivative transactions	(5.0)	11.1	(5.0)	9.5
Total	(4.6)	18.1	(8.1)	15.5
Postretirement benefit plans
Amortization of net actuarial loss	18.0	18.8	35.8	37.5
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)	(0.5)	(0.8)	0.4
Income taxes on postretirement benefit plans	13.0	6.9	13.0	22.9
Total	4.6	11.4	22.0	37.5
Other comprehensive income, net of tax	4.4	19.7	28.7	37.8
Comprehensive income (loss)	17.8	4.2	63.2	(75.8)
Less: Comprehensive income attributable to noncontrolling interests	3.5	1.8	9.1	3.4
Comprehensive income (loss) attributable to ATI	$ 14.3	$ 2.4	$ 54.1	$ (79.2)