Share-based Compensation Authorized Shares (Details) In Thousands, unless otherwise specified	Dec. 31, 2012
Stock Options | 1995 Directors Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	188
Stock Options | 2008 A. M. Castle & Co. Omnibus Incentive Plan (amended and restated as of April 28, 2011)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	2,750
Restricted Stock | 1996 Restricted Stock and Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	938
Restricted Stock | 2000 Restricted Stock and Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	1,200
Restricted Stock | 2004 Restricted Stock, Stock Option and Equity Compensation Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	1,350