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Contractual interest rate for funds borrowed, under the debt agreement.

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Business Segments - Additional Information (Details) $ in Millions		3 Months Ended		9 Months Ended
	Jul. 12, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Restructuring Cost and Reserve [Line Items]
Number of business segments \| segment				2
Impairment of goodwill		$ 0.0	$ 114.4	$ 0.0	$ 114.4
Goodwill		536.4		536.4		$ 531.4
Allegheny Technologies, Senior Notes, 9.375, Due 2019 [Member]
Restructuring Cost and Reserve [Line Items]
Interest rate			9.375%		9.375%
Addaero Manufacturing [Member]
Restructuring Cost and Reserve [Line Items]
Payments to Acquire Businesses, Gross	$ 10.0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles		2.0		2.0
Goodwill		$ 6.0		$ 6.0