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Financial Information for Subsidiary and Guarantor Parent (Comp Inc.) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	$ (223.3)	$ (141.3)	$ (13.9)	$ 12.6	$ 25.3	$ 2.9	$ (0.7)	$ (17.9)	$ (365.9)	$ 9.6	$ 161.6
Currency translation adjustment arising during the period									(37.0)	(33.6)	15.3
Unrealized holding gain (loss) on securities									0.0		0.1
Net derivative loss on hedge transactions									(32.0)	25.9	(6.9)
Pension and postretirement benefits									(19.7)	(212.6)	311.1
Other comprehensive income (loss), net of tax									(88.7)	(220.3)	319.6
Comprehensive income (loss)									(454.6)	(210.7)	481.2
Less: Comprehensive income attributable to noncontrolling interest									6.4	10.1	11.0
Comprehensive income (loss) attributable to ATI									(461.0)	(220.8)	470.2
Guarantor Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									(365.9)	9.6	161.6
Currency translation adjustment arising during the period									(37.0)	(33.6)	15.3
Unrealized holding gain (loss) on securities									0.0		0.1
Net derivative loss on hedge transactions									(32.0)	25.9	(6.9)
Pension and postretirement benefits									(19.7)	(212.6)	311.1
Other comprehensive income (loss), net of tax									(88.7)	(220.3)	319.6
Comprehensive income (loss)									(454.6)	(210.7)	481.2
Less: Comprehensive income attributable to noncontrolling interest									0.0	0.0	0.0
Comprehensive income (loss) attributable to ATI									(454.6)	(210.7)	481.2
Subsidiary [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									(288.1)	(55.3)	(46.3)
Currency translation adjustment arising during the period									0.0	0.0	0.0
Unrealized holding gain (loss) on securities									0.0		0.0
Net derivative loss on hedge transactions									0.0	0.0	0.0
Pension and postretirement benefits									29.8	1.8	22.0
Other comprehensive income (loss), net of tax									29.8	1.8	22.0
Comprehensive income (loss)									(258.3)	(53.5)	(24.3)
Less: Comprehensive income attributable to noncontrolling interest									0.0	0.0	0.0
Comprehensive income (loss) attributable to ATI									(258.3)	(53.5)	(24.3)
Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									158.9	228.9	387.9
Currency translation adjustment arising during the period									(37.0)	(33.6)	15.3
Unrealized holding gain (loss) on securities									0.0		0.1
Net derivative loss on hedge transactions									0.0	0.0	0.0
Pension and postretirement benefits									3.1	(28.4)	27.6
Other comprehensive income (loss), net of tax									(33.9)	(62.0)	43.0
Comprehensive income (loss)									125.0	166.9	430.9
Less: Comprehensive income attributable to noncontrolling interest									6.4	10.1	11.0
Comprehensive income (loss) attributable to ATI									118.6	156.8	419.9
Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									129.2	(173.6)	(341.6)
Currency translation adjustment arising during the period									37.0	33.6	(15.3)
Unrealized holding gain (loss) on securities									0.0		(0.1)
Net derivative loss on hedge transactions									0.0	0.0	0.0
Pension and postretirement benefits									(32.9)	26.6	(49.6)
Other comprehensive income (loss), net of tax									4.1	60.2	(65.0)
Comprehensive income (loss)									133.3	(113.4)	(406.6)
Less: Comprehensive income attributable to noncontrolling interest									0.0	0.0	0.0
Comprehensive income (loss) attributable to ATI									$ 133.3	$ (113.4)	$ (406.6)