Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2015	Oct. 30, 2015
Document and Entity Information
Entity Registrant Name	STEEL DYNAMICS INC
Entity Central Index Key	0001022671
Document Type	10-Q
Document Period End Date	Sep. 30, 2015
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		242,123,707
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q3