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Debt - Schedule of Debt (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Sep. 30, 2018	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jul. 31, 2018	Feb. 15, 2016	[1]	Dec. 31, 2013
Debt Instrument [Line Items]
Debt Issuance Costs, Net				$ 10.4	$ 13.1
Total short-term and long-term debt				1,542.1	1,540.7
Short-term debt and current portion of long-term debt				6.6	10.1
Total long-term debt				$ 1,535.5	$ 1,530.6
Allegheny Technologies $500 million 5.875% Senior Notes due 2023 [Member]
Debt Instrument [Line Items]
Debt instrument issuer	[1]			Allegheny Technologies	Allegheny Technologies
Debt instrument interest rate stated percentage								7.875%		5.875%
Debt instrument carrying amount	[1]			$ 500.0	$ 500.0
Allegheny Technologies $500 million 5.95% Senior Notes due 2021 [Member]
Debt Instrument [Line Items]
Debt instrument issuer				Allegheny Technologies	Allegheny Technologies
Debt instrument interest rate stated percentage				5.95%	5.95%
Debt instrument carrying amount				$ 500.0	$ 500.0
Allegheny Technologies $350 million 9.375% Senior Notes due 2019 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage						9.375%
Debt instrument carrying amount						$ 350.0
Allegheny Technologies, $287.5 million Convertible Senior Notes, 4.75%, Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument issuer				Allegheny Technologies	Allegheny Technologies
Debt instrument interest rate stated percentage				4.75%	4.75%
Debt instrument carrying amount				$ 287.5	$ 287.5
2022 Term Loan [Member]
Debt Instrument [Line Items]
Debt instrument issuer				Allegheny Technologies	Allegheny Technologies
Debt instrument interest rate stated percentage							5.44%
Debt instrument carrying amount				$ 100.0	$ 100.0
Allegheny Ludlum 6.95% Debentures due 2025 [Member]
Debt Instrument [Line Items]
Debt instrument issuer				Allegheny Ludlum	Allegheny Ludlum
Debt instrument interest rate stated percentage				6.95%	6.95%
Debt instrument carrying amount				$ 150.0	$ 150.0
Domestic Bank Group $400 million asset-based credit facility [Member]
Debt Instrument [Line Items]
Debt instrument carrying amount				0.0	0.0
Other [Member]
Debt Instrument [Line Items]
Debt instrument carrying amount				15.0	10.0
Foreign credit agreements [Member]
Debt Instrument [Line Items]
Debt instrument carrying amount				$ 0.0	$ 6.3
Maximum [Member] | Allegheny Technologies $500 million 5.875% Senior Notes due 2023 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage				7.875%
London Interbank Offered Rate (LIBOR) [Member] | 2022 Term Loan [Member]
Debt Instrument [Line Items]
Variable rate spread		2.50%	3.00%			3.50%
Revolving Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member] | Domestic Bank Group $400 million asset-based credit facility [Member]
Debt Instrument [Line Items]
Variable rate spread				1.75%		2.00%
Revolving Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member] | Domestic Bank Group $400 million asset-based credit facility [Member]
Debt Instrument [Line Items]
Variable rate spread				2.25%		2.50%
[1] Bearing interest at 7.875% effective February 15, 2016.

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