Cover Page - shares	6 Months Ended
	Jun. 30, 2020	Aug. 12, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2020
Document Transition Report	false
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, Address Line One	1420 Kensington Road, Suite 220,
Entity Address, City or Town	Oak Brook,
Entity Address, State or Province	IL
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 Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		73,910,334
Entity Central Index Key	0000018172
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q2
Current Fiscal Year End Date	--12-31