Basis of Presentation	6 Months Ended
	Jun. 30, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of PresentationThe Condensed Consolidated Financial Statements of A.M. Castle & Co. and its consolidated subsidiaries (collectively, the "Company") included herein and the notes thereto have been prepared by the Company, without audit, pursuant to the rules and regulations of the U.S. Securities and Exchange Commission (“SEC”), and accounting principles generally accepted in the United States of America (“GAAP”). The Condensed Consolidated Balance Sheet at December 31, 2019 is derived from the audited financial statements at that date. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to the rules and regulations of the SEC. In the opinion of the Company's management, the unaudited statements included herein contain all adjustments (consisting of only normal recurring adjustments) necessary for a fair presentation of financial results for the interim period. These Condensed Consolidated Financial Statements should be read in conjunction with the Consolidated Financial Statements and the Notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019. The operating results for the three and six months ended June 30, 2020, as reported herein, may not necessarily be indicative of the Company’s operating results for the full year.