Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2020	Feb. 19, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
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 Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
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
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 4.1
Entity Common Stock, Shares Outstanding		211,005,100
Entity Central Index Key	0001022671
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false