Cover - USD ($)	12 Months Ended
	Dec. 31, 2019	Feb. 24, 2020	Jun. 28, 2019
Cover page.
Document Type	10-K/A
Document Period End Date	Dec. 31, 2019
Entity File Number	1-5415
Entity Registrant Name	A. M. CASTLE & CO.
Entity Incorporation, State or Country Code	MD
Entity Tax Identification Number	36-0879160
Entity Address, Postal Zip Code	60523
City Area Code	847
Local Phone Number	455-7111
Title of 12(g) Security	Common Stock, Par Value $0.01 Per Share
Trading Symbol	CTAM
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 1,584,996
Entity Common Stock, Shares Outstanding		3,649,658
Documents Incorporated by Reference	None
Entity Central Index Key	0000018172
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Amendment Flag	true
Document Annual Report	true
Document Transition Report	false
Entity Address, Address Line One	1420 Kensington Road, Suite 220,
Entity Address, City or Town	Oak Brook,
Entity Address, State or Province	IL
Entity Filer Category	Non-accelerated Filer
Amendment Description	This Amendment No. 1 to Form 10-K (the “Amendment”) amends the Annual Report on Form 10-K for the fiscal year ended December 31, 2019 originally filed with the U.S. Security and Exchange Commission on February 27, 2020 (the “Original Filing”) by A. M. Castle & Co., a Maryland corporation ("A.M. Castle & Co.," the “Company,” “we,” or “us”). We are filing the Amendment to present the information required by Part III of Form 10-K as we will not file our definitive proxy statement within 120 days of the end of our year ended December 31, 2019. In addition, the Company is revising Item 1A. “Risk Factors” to add a risk factor regarding the Coronavirus Disease 2019 (COVID-19). The reference on the cover page of the 2019 Form 10-K to the incorporation by reference of portions of the Company's definitive proxy statement into Part III of the 2019 Form 10-K is hereby deleted.