Financial Information for Subsidiary and Guarantor Parent (Narrative) (Details) - Allegheny Ludlum 6.95% debentures due 2025 - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 150.0	$ 150.0
Interest rate	6.95%	6.95%

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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