Consolidated Statements of Comprehensive Income (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net income	$ 9.6us-gaap_ProfitLoss	$ 161.6us-gaap_ProfitLoss	$ 167.8us-gaap_ProfitLoss
Currency translation adjustment
Unrealized net change arising during the period	(34.1)	13.8	14.3
Reclassification adjustment included in net income	0.5	1.5	0
Total	(33.6)	15.3	14.3
Unrealized holding gain (loss) on securities
Net gain arising during the period	0	0.1	0
Derivatives
Net derivatives gain (loss) on hedge transactions	45.7	(25.2)	(9.8)
Reclassification to net income of net realized loss (gain)	(3.6)	14.0	5.2
Income taxes on derivative transactions	16.2	(4.3)	(1.8)
Total	25.9	(6.9)	(2.8)
Postretirement benefit plans- Actuarial loss
Amortization of net actuarial loss	88.1	129.0	119.8
Net gain (loss) arising during the period	(424.5)	384.9	(272.7)
Postretirement benefit plans- Prior Service Cost
Amortization to net income of net prior service credits	(0.7)	(15.2)	(11.8)
Income taxes on postretirement benefit plans	(124.5)	187.6	(67.3)
Total	(212.6)	311.1	(97.4)
Other comprehensive income (loss), net of tax	(220.3)	319.6	(85.9)
Comprehensive income (loss)	(210.7)	481.2	81.9
Less: Comprehensive income attributable to noncontrolling interests	10.1	11.0	11.3
Comprehensive income (loss) attributable to ATI	$ (220.8)us-gaap_ComprehensiveIncomeNetOfTax	$ 470.2us-gaap_ComprehensiveIncomeNetOfTax	$ 70.6us-gaap_ComprehensiveIncomeNetOfTax

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