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Selected Quarterly Financial Data (Unaudited) - Narrative (Details) - USD ($) shares in Millions, $ in Millions		3 Months Ended				12 Months Ended
	Nov. 07, 2017	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Unusual Or Infrequent Item [Line Items]
Gain on joint venture deconsolidation		$ 15.9				$ 15.9	$ 0.0	$ 0.0
Deconsolidation, Gain (Loss), Amount, Net of tax		$ 14.7
NRV charge						(27.9)	(54.0)	(39.9)
Impairment of goodwill				$ 114.4		0.0	114.4	0.0
Impairment of goodwill net of tax				$ 113.6
Debt extinguishment charge			$ 37.0			$ 0.0	$ 37.0	0.0
Impact of U.S. tax reform			$ (4.1)
Shares issued	17
SEC Schedule, 12-09, Reserve, Inventory | Titanium sponge inventory
Unusual Or Infrequent Item [Line Items]
Inventory valuation charges					$ 11.3
Allegheny Technologies $350 million 9.375% Senior Notes due 2019 [Member]
Unusual Or Infrequent Item [Line Items]
Debt instrument carrying amount								$ 350.0
Debt Instrument, Interest Rate, Stated Percentage								9.375%
Allegheny & Tsingshan Stainless [Member] | Tsingshan Group [Member]
Unusual Or Infrequent Item [Line Items]
Joint venture ownership percentage by unaffiliated entity						50.00%

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