Debt - Long-term Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Other, primarily capital leases	$ 268	$ 288
Less: unamortized discount	(67,486)	(67,937)
Total long-term debt	215,096	200,021
Less: current portion of long-term debt	119	118
Total long-term portion	214,977	199,903
5.00% / 7.00% Second Lien Notes due August 31, 2022
Debt Instrument [Line Items]
Convertible debt	$ 171,720	168,767
5.00% / 7.00% Second Lien Notes due August 31, 2022 \| Minimum
Debt Instrument [Line Items]
Stated interest rate (as a percent)	5.00%
5.00% / 7.00% Second Lien Notes due August 31, 2022 \| Maximum
Debt Instrument [Line Items]
Stated interest rate (as a percent)	7.00%
Second Lien Notes
Debt Instrument [Line Items]
Less: unvested restricted Second Lien Notes	$ (1,953)	(2,144)
Line of Credit \| Floating rate New ABL Credit Facility due February 28, 2022
Debt Instrument [Line Items]
Line of credit	112,547	$ 101,047
Foreign Line of Credit
Debt Instrument [Line Items]
Line of credit	4,803
A.M. Castle & Co. 2017 Management Incentive Plan \| Second Lien Notes
Debt Instrument [Line Items]
Convertible debt	$ 2,400

Unvested Portion of Second Lien Notes, Restricted

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Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal, through the balance sheet date and due to be paid more than one year (or one operating cycle, if longer) after the balance sheet date.

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Including the current and noncurrent portions, carrying amount of debt identified as being convertible into another form of financial instrument (typically the entity's common stock) as of the balance sheet date, which originally required full repayment more than twelve months after issuance or greater than the normal operating cycle of the company.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Amount, after accumulated amortization, of debt discount.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount of long-term debt and capital leases due within one year or the normal operating cycle, if longer.

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