Financial Information for Subsidiary and Guarantor Parent (Comp Inc.) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	$ 45.0	$ 55.6	$ 75.6	$ 60.5	$ 5.2	$ (119.4)	$ 13.4	$ 21.1	$ 236.7	$ (79.7)	$ (627.1)
Currency translation adjustment arising during the period									(26.6)	39.1	(47.1)
Net derivative gain on hedge transactions									(18.1)	7.1	19.6
Pension and postretirement benefits									(67.5)	27.3	(61.4)
Other comprehensive income (loss), net of tax									(112.2)	73.5	(88.9)
Comprehensive income (loss)									124.5	(6.2)	(716.0)
Less: Comprehensive income attributable to noncontrolling interest									8.1	19.8	4.1
Comprehensive income (loss) attributable to ATI									116.4	(26.0)	(720.1)
Legal Entities [Member] | Guarantor Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									236.7	(79.7)	(627.1)
Currency translation adjustment arising during the period									(26.6)	39.1	(47.1)
Net derivative gain on hedge transactions									(18.1)	7.1	19.6
Pension and postretirement benefits									(67.5)	27.3	(61.4)
Other comprehensive income (loss), net of tax									(112.2)	73.5	(88.9)
Comprehensive income (loss)									124.5	(6.2)	(716.0)
Less: Comprehensive income attributable to noncontrolling interest									0.0	0.0	0.0
Comprehensive income (loss) attributable to ATI									124.5	(6.2)	(716.0)
Legal Entities [Member] | Subsidiary [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									(34.9)	(34.0)	(160.0)
Currency translation adjustment arising during the period									0.0	0.0	0.0
Net derivative gain on hedge transactions									0.0	0.0	0.0
Pension and postretirement benefits									(19.1)	(5.8)	17.2
Other comprehensive income (loss), net of tax									(19.1)	(5.8)	17.2
Comprehensive income (loss)									(54.0)	(39.8)	(142.8)
Less: Comprehensive income attributable to noncontrolling interest									0.0	0.0	0.0
Comprehensive income (loss) attributable to ATI									(54.0)	(39.8)	(142.8)
Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									474.3	158.9	(140.2)
Currency translation adjustment arising during the period									(26.6)	39.1	(47.1)
Net derivative gain on hedge transactions									0.0	0.0	0.0
Pension and postretirement benefits									0.7	(10.7)	1.8
Other comprehensive income (loss), net of tax									(25.9)	28.4	(45.3)
Comprehensive income (loss)									448.4	187.3	(185.5)
Less: Comprehensive income attributable to noncontrolling interest									8.1	19.8	4.1
Comprehensive income (loss) attributable to ATI									440.3	167.5	(189.6)
Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									(439.4)	(124.9)	300.2
Currency translation adjustment arising during the period									26.6	(39.1)	47.1
Net derivative gain on hedge transactions									0.0	0.0	0.0
Pension and postretirement benefits									18.4	16.5	(19.0)
Other comprehensive income (loss), net of tax									45.0	(22.6)	28.1
Comprehensive income (loss)									(394.4)	(147.5)	328.3
Less: Comprehensive income attributable to noncontrolling interest									0.0	0.0	0.0
Comprehensive income (loss) attributable to ATI									$ (394.4)	$ (147.5)	$ 328.3