Cover - USD ($)	12 Months Ended
	Dec. 31, 2019	Feb. 24, 2020	Jun. 28, 2019
Cover page.
Document Type	10-K
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	This Annual Report on Form 10-K incorporates by reference portions of the registrant’s Definitive Proxy Statement for its 2020 Annual Meeting of Stockholders. In the event the registrant does not file a Proxy Statement for its 2020 Annual Meeting of Stockholders with the United States Securities and Exchange Commission within 120 days after the end of the registrant’s 2019 fiscal year, this information will be included in an amendment to this Annual Report on Form 10-K.
Entity Central Index Key	0000018172
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Amendment Flag	false
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