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Debt - Schedule of Debt (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Feb. 15, 2016	[1]	Dec. 31, 2013
Debt Instrument [Line Items]
Debt Issuance Costs, Net			$ 13.1	$ 17.1
Total short-term and long-term debt			1,540.7	1,877.0
Short-term debt and current portion of long-term debt			10.1	105.1
Total long-term debt			$ 1,530.6	$ 1,771.9
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 issuer				Allegheny Technologies
Debt instrument interest rate stated percentage				9.375%
Debt instrument carrying amount			$ 0.0	$ 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
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			10.0	2.2
Foreign credit agreements [Member]
Debt Instrument [Line Items]
Debt instrument carrying amount			$ 6.3	$ 4.4
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		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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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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