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Debt - Long-term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 01, 2017	Aug. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Other, primarily capital leases	$ 316
Plus: derivative liability for embedded conversion feature	61,608
Less: unamortized discount	(68,318)
Less: unamortized debt issuance costs	0
Total long-term debt	244,465
Less: current portion	118
Total long-term portion	244,347
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	$ 165,896
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]
Less: Unvested restricted Second Lien Notes	$ (2,334)
Second Lien Notes \| Predecessor
Debt Instrument [Line Items]
Less: Unvested restricted Second Lien Notes	0
Line of Credit \| Floating rate ABL Credit Facility due February 28, 2022
Debt Instrument [Line Items]
Line of credit	87,297
Line of Credit \| Floating rate 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	3,581
Restricted Stock
Debt Instrument [Line Items]
Less: Unvested restricted Second Lien Notes	(5,294)
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	$ (2,236)