Basis of Presentation and Significant Accounting Policies - Narrative (Details) $ / shares in Units, shares in Thousands				1 Months Ended	4 Months Ended	8 Months Ended	12 Months Ended
	Sep. 01, 2017 shares	Aug. 31, 2017 USD ($) subsidiary shares	Jun. 18, 2017 subsidiary	Sep. 30, 2017 $ / shares	Dec. 31, 2017 USD ($) Service_Center shares	Aug. 31, 2017 USD ($) shares	Dec. 31, 2017 USD ($) Service_Center $ / shares shares	Dec. 31, 2016 USD ($) shares	Aug. 31, 2016
Accounting Policies [Line Items]
Service centers | Service_Center					22		22
Number of subsidiaries | subsidiary		4	4
Net loss from continuing operations					$ (13,327,000)
Shipping and handling costs					8,120,000
Original maturity cash and equivalents							90 days
Depreciation					3,213,000
Goodwill recognized in fresh start accounting		$ 2,675,000				$ 2,675,000
Fresh start adjustment intangible assets		$ 24,000			5,500,000	24,000	$ 5,500,000
Royalty rate (as a percent)							0.10%
Undistributed earnings of foreign subsidiaries					0		$ 0
Net loss					$ (13,327,000)
Outstanding common stock equivalents (in shares) | shares					0
Reorganization shares issued to debt holders (in shares) | shares		2,000
Predecessor
Accounting Policies [Line Items]
Net loss from continuing operations						36,190,000		$ (114,090,000)
Shipping and handling costs						16,292,000		26,370,000
Depreciation						6,062,000		10,252,000
Net loss						$ 36,190,000		$ (107,982,000)
Outstanding common stock equivalents (in shares) | shares						0		0
Minimum | Customer relationships
Accounting Policies [Line Items]
Intangible assets useful lives							4 years
Maximum
Accounting Policies [Line Items]
Percentage of revenue recognized at time of shipment (less than)					1.00%
Maximum | Customer Concentration Risk
Accounting Policies [Line Items]
Concentration of sales (as a percent)					5.00%
Maximum | Predecessor
Accounting Policies [Line Items]
Percentage of revenue recognized at time of shipment (less than)						1.00%		1.00%
Maximum | Predecessor | Customer Concentration Risk
Accounting Policies [Line Items]
Concentration of sales (as a percent)						4.00%
Maximum | Customer relationships
Accounting Policies [Line Items]
Intangible assets useful lives							12 years
North America
Accounting Policies [Line Items]
Service centers | Service_Center					17		17
Europe
Accounting Policies [Line Items]
Service centers | Service_Center					3		3
Asia
Accounting Policies [Line Items]
Service centers | Service_Center					2		2
United States | Geographic Concentration Risk
Accounting Policies [Line Items]
Concentration of sales (as a percent)					65.00%
United States | Predecessor | Geographic Concentration Risk
Accounting Policies [Line Items]
Concentration of sales (as a percent)						63.00%
A.M. Castle & Co. 2017 Management Incentive Plan | Restricted Stock
Accounting Policies [Line Items]
Reorganization shares issued to debt holders (in shares) | shares	1,734
Number of shares convertible (in shares) | shares	638
Granted (in dollars per share) | $ / shares				$ 3.14
5.25% Convertible Notes due December 30, 2019
Accounting Policies [Line Items]
Convertible debt					$ 0		$ 0
5.25% Convertible Notes due December 30, 2019 | Convertible Debt Securities
Accounting Policies [Line Items]
Earnings per share for dilutive shares (in dollars per share) | $ / shares							$ 3.77
Potentially dilutive shares (in shares) | shares							43,700
5.25% Convertible Notes due December 30, 2019 | Predecessor
Accounting Policies [Line Items]
Convertible debt								$ 22,323,000
Second Lien Notes | A.M. Castle & Co. 2017 Management Incentive Plan | Restricted Stock
Accounting Policies [Line Items]
Convertible debt					$ 2,400,000		$ 2,400,000
Kreher Steel Company, LLC | Predecessor
Accounting Policies [Line Items]
Joint venture ownership (as a percent)									50.00%