CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement of Comprehensive Income [Abstract]
Net loss	$ (141.3)	$ 2.9	$ (142.6)	$ (15.7)
Currency translation adjustment
Unrealized net change arising during the period	(16.2)	(16.6)	(18.7)	(12.2)
Unrealized holding gain on securities
Net gain (loss) arising during the period	0.0	0.0	(0.1)	0.1
Derivatives
Net derivatives gain (loss) on hedge transactions	(26.8)	28.0	(18.7)	50.1
Reclassification to net loss of net realized gain	(3.3)	(2.6)	(8.1)	(1.0)
Income taxes on derivative transactions	(11.6)	9.8	(10.3)	18.9
Total	(18.5)	15.6	(16.5)	30.2
Postretirement benefit plans
Amortization of net actuarial loss	18.8	22.1	56.2	66.1
Amortization to net loss of net prior service cost (credits)	1.5	(0.2)	4.5	(0.5)
Income taxes on postretirement benefit plans	7.8	8.5	23.2	25.3
Total	12.5	13.4	37.5	40.3
Other comprehensive income (loss), net of tax	(22.2)	12.4	2.2	58.4
Comprehensive income (loss)	(163.5)	15.3	(140.4)	42.7
Less: Comprehensive income attributable to noncontrolling interests	(0.8)	5.4	4.6	7.8
Comprehensive income (loss) attributable to ATI	$ (162.7)	$ 9.9	$ (145.0)	$ 34.9