Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2019	Oct. 28, 2019
Document And Entity Information Abstract
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2019
Document Transition Report	false
Entity File Number	0-21719
Entity Registrant Name	Steel Dynamics, Inc
Entity Incorporation, State or Country Code	IN
Entity Tax Identification Number	35-1929476
Entity Address, Address Line One	7575 West Jefferson Blvd
Entity Address, City or Town	Fort Wayne
Entity Address, State or Province	IN
Entity Address, Postal Zip Code	46804
City Area Code	260
Local Phone Number	969-3500
Title of 12(b) Security	Common Stock
Trading Symbol	STLD
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		215,452,169
Entity Central Index Key	0001022671
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Amendment Flag	false