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Debt Schedule of Debt (Details) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2017	Dec. 31, 2016	Feb. 15, 2016	Dec. 31, 2013
Debt Instrument [Line Items]
Debt Issuance Costs, Net		$ 15.2	$ 17.1
Total Debt		1,914.1	1,877.0
Short term debt and current portion of long-term debt		36.4	105.1
Long-term debt		1,877.7	1,771.9
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 287.5	$ 287.5
Debt Instrument, Issuer		Allegheny Technologies
Debt Instrument, Maturity Date		Jul. 01, 2022	Jul. 01, 2022
Interest rate		4.75%	4.75%
Allegheny Technologies 5.875% Notes due 2023 (a)
Debt Instrument [Line Items]
Long-term Debt, Gross	[1]	$ 500.0	$ 500.0
Debt Instrument, Issuer		Allegheny Technologies
Debt Instrument, Maturity Date		Aug. 15, 2023	Aug. 15, 2023
Interest rate		7.875%	7.875%	7.875%	5.875%
Allegheny Technologies 5.95% Notes due 2021
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 500.0	$ 500.0
Debt Instrument, Issuer		Allegheny Technologies
Debt Instrument, Maturity Date		Jan. 15, 2021	Jan. 15, 2021
Interest rate		5.95%	5.95%
Allegheny Technologies 9.375% Notes due 2019
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 350.0	$ 350.0
Debt Instrument, Issuer		Allegheny Technologies
Debt Instrument, Maturity Date		Jun. 01, 2019	Jun. 01, 2019
Interest rate		9.375%	9.375%
Allegheny Ludlum 6.95% debentures due 2025
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 150.0	$ 150.0
Debt Instrument, Issuer		Allegheny Ludlum
Debt Instrument, Maturity Date		Dec. 15, 2025	Dec. 15, 2025
Interest rate		6.95%	6.95%
2022 Term Loan [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 100.0	$ 100.0
Debt Instrument, Issuer		Allegheny Technologies
Debt Instrument, Maturity Date		Feb. 28, 2022	Nov. 13, 2017
Domestic Bank Group $400 million asset-based credit facility
Debt Instrument [Line Items]
Short-term Debt		$ 25.0	$ 0.0
Foreign credit facilities
Debt Instrument [Line Items]
Short-term Debt		7.7	4.4
Other
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 9.1	$ 2.2
[1] Bearing interest at 7.875% effective February 15, 2016.

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total of both long-term debt, short-term debt and current portion of long-term debt

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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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Identification of the contractual issuer of the debt instrument (for example, a subsidiary of the reporting entity).

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