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Debt - Long-term Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Aug. 31, 2017	Dec. 31, 2016	May 19, 2016
Debt Instrument [Line Items]
Other, primarily capital leases	$ 288
Plus: derivative liability for embedded conversion feature	0
Less: unamortized discount	(67,937)
Less: unamortized debt issuance costs	0
Total long-term debt	200,021
Less: current portion	118
Total long-term portion	199,903
Predecessor
Debt Instrument [Line Items]
Other, primarily capital leases			$ 96
Plus: derivative liability for embedded conversion feature			403
Less: unamortized discount			(7,587)
Less: unamortized debt issuance costs			(5,199)
Total long-term debt			286,596
Less: current portion			137
Total long-term portion			286,459
7.0% Convertible Notes due December 15, 2017
Debt Instrument [Line Items]
Convertible debt	$ 0
Stated interest rate (as a percent)	7.00%
7.0% Convertible Notes due December 15, 2017 \| Predecessor
Debt Instrument [Line Items]
Convertible debt			41
11.0% Senior Secured Term Loan Credit Facilities due September 14, 2018
Debt Instrument [Line Items]
Loans payable	$ 0
Stated interest rate (as a percent)	11.00%	11.00%
11.0% Senior Secured Term Loan Credit Facilities due September 14, 2018 \| Predecessor
Debt Instrument [Line Items]
Loans payable			99,500
12.75% Senior Secured Notes due December 15, 2018
Debt Instrument [Line Items]
Senior notes	$ 0
Stated interest rate (as a percent)	12.75%	12.75%
12.75% Senior Secured Notes due December 15, 2018 \| Predecessor
Debt Instrument [Line Items]
Senior notes			177,019
5.25% Convertible Notes due December 30, 2019
Debt Instrument [Line Items]
Convertible debt	$ 0
Stated interest rate (as a percent)	5.25%	5.25%
5.25% Convertible Notes due December 30, 2019 \| Predecessor
Debt Instrument [Line Items]
Convertible debt			22,323
5.00% / 7.00% Convertible Notes due August 31, 2022
Debt Instrument [Line Items]
Convertible debt	$ 168,767
5.00% / 7.00% Convertible Notes due August 31, 2022 \| Minimum
Debt Instrument [Line Items]
Stated interest rate (as a percent)	5.00%
5.00% / 7.00% Convertible Notes due August 31, 2022 \| Maximum
Debt Instrument [Line Items]
Stated interest rate (as a percent)	7.00%
5.00% / 7.00% Convertible Notes due August 31, 2022 \| Predecessor
Debt Instrument [Line Items]
Convertible debt			0
Second Lien Notes
Debt Instrument [Line Items]
Plus: derivative liability for embedded conversion feature	$ 60,760
Less: Unvested restricted Second Lien Notes	(2,144)
Second Lien Notes \| Predecessor
Debt Instrument [Line Items]
Plus: derivative liability for embedded conversion feature				$ 61,608
Less: Unvested restricted Second Lien Notes			0
Line of Credit \| Floating rate New ABL Credit Facility due February 28, 2022
Debt Instrument [Line Items]
Line of credit	101,047
Line of Credit \| Floating rate New ABL Credit Facility due February 28, 2022 \| Predecessor
Debt Instrument [Line Items]
Line of credit			$ 0
Foreign Line of Credit
Debt Instrument [Line Items]
Line of credit	5,854
Restricted Stock
Debt Instrument [Line Items]
Less: Unvested restricted Second Lien Notes	(4,843)
A.M. Castle & Co. 2017 Management Incentive Plan \| Restricted Stock \| Second Lien Notes
Debt Instrument [Line Items]
Convertible debt	2,400
Less: Unvested restricted Second Lien Notes	$ (2,044)