CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 12.6	$ (17.9)
Currency translation adjustment
Unrealized net change arising during the period	(21.8)	(3.3)
Unrealized holding gain on securities
Net gain (loss) arising during the period	0	0
Derivatives
Net derivatives gain on hedge transactions	18.5	9.3
Reclassification to net income of net realized loss (gain)	(2.7)	2.1
Income taxes on derivative transactions	6.1	4.4
Total	9.7	7.0
Postretirement benefit plans
Amortization of net actuarial loss	18.7	22.0
Amortization to net income of net prior service cost (credits)	1.5	(0.2)
Income taxes on postretirement benefit plans	7.7	8.4
Total	12.5	13.4
Other comprehensive income, net of tax	0.4	17.1
Comprehensive income (loss)	13.0	(0.8)
Less: Comprehensive income attributable to noncontrolling interests	2.4	0.2
Comprehensive income (loss) attributable to ATI	$ 10.6	$ (1.0)