SCHEDULE 13G

Amendment No. 3 
Steel Dynamics Incorporated 
Common Stock 
Cusip #858119100 


Cusip #858119100 
Item 1:	Reporting Person - FMR Corp.
Item 4:	Delaware
Item 5:	89,819 
Item 6:	0 
Item 7:	1,675,146 
Item 8:	0 
Item 9:	1,675,146 
Item 11:	3.517% 
Item 12:	    HC


Cusip #858119100  
Item 1:	Reporting Person - Edward C. Johnson 3d 
Item 4:	United States of America
Item 5:	0 
Item 6:	0 
Item 7:	1,675,146 
Item 8:	0 
Item 9:	1,675,146 
Item 11:	3.517% 
Item 12:	IN 


Cusip #858119100 
Item 1:	Reporting Person - Abigail P. Johnson 
Item 4:	United States of America
Item 5:	0 
Item 6:	0 
Item 7:	1,675,146 
Item 8:	0 
Item 9:	1,675,146 
Item 11:	3.517% 
Item 12:	    IN



	SCHEDULE 13G - TO BE INCLUDED IN 
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)




Item 1(a).	Name of Issuer:	

		Steel Dynamics Incorporated 

Item 1(b).	Name of Issuer's Principal Executive Offices:

		6714 Pointe Inverness Way
		Suite 200 
		Fort Wayne, IN  46804
		  

Item 2(a).	Name of Person Filing: 

		FMR Corp.

Item 2(b).	Address or Principal Business Office or, if None, 
Residence:

		82 Devonshire Street, Boston, 
Massachusetts  02109

Item 2(c).	Citizenship:

		Not applicable

Item 2(d).	Title of Class of Securities:

		Common Stock 

Item 2(e).	CUSIP Number:  

		858119100 

Item 3.	This statement is filed pursuant to Rule 13d-1(b) or 13d-2(b) 
and the person filing, FMR Corp., is a parent holding company 
in accordance with Section 240.13d-1(b)(ii)(G).  (Note:  See 
Item 7).

Item 4.	Ownership

	(a)	Amount Beneficially Owned:	1,675,146 

	(b)	Percent of Class:	3.517% 

	(c)	Number of shares as to which such 
person has:  

	(i)	sole power to vote or to direct 
the vote:	89,819 

	(ii)	shared power to vote or to 
direct the vote:	0 

	(iii)	sole power to dispose or to 
direct the disposition of:	1,675,146 

	(iv)	shared power to dispose or to 
direct the disposition of:	0 



Item 5.	Ownership of Five Percent or Less of a Class.

	If this statement is being filed to report the fact that as of the 
date hereof, the reporting person has ceased to be the 
beneficial owner of more than five percent of the class of 
securities, check the following (X).

Item 6.	Ownership of More than Five Percent on Behalf of Another 
Person.

	Not applicable

Item 7.	Identification and Classification of the Subsidiary Which 
Acquired the Security Being Reported on By the Parent 
Holding Company.

	See attached Exhibit(s) A, B. 

Item 8.	Identification and Classification of Members of 
the Group.

	Not Applicable. See attached Exhibit A. 

Item 9.	Notice of Dissolution of Group.

	Not applicable.

Item 10.	Certification.

	Inasmuch as the reporting persons are no longer the beneficial 
owners of more than five percent of the number of shares 
outstanding, the reporting persons have no further reporting 
obligation under Section 13(d) of the Securities and Exchange 
Commission thereunder, and the reporting persons have no 
obligation to amend this Statement if any material change 
occurs in the facts set forth herein.

Signature

	After reasonable inquiry and to the best of my knowledge and 
belief, I certify that the information set forth in this Schedule 
13G in connection with FMR Corp.'s beneficial ownership of 
the Common Stock of Steel Dynamics Incorporated at March 
31, 2005 is true, complete and correct. 

April 11, 2005 			
Date

/s/Eric D. Roiter				
Signature

Eric D. Roiter				
Duly authorized under Power of Attorney
dated December 30, 1997 by and on behalf
of FMR Corp. and its direct and indirect
subsidiaries


	SCHEDULE 13G - TO BE INCLUDED IN 
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)



	Pursuant to the instructions in Item 7 of Schedule 
13G, Fidelity Management & Research Company 
("Fidelity"), 82 Devonshire Street, Boston, Massachusetts 
02109, a wholly-owned subsidiary of FMR Corp. and an 
investment adviser registered under Section 203 of the 
Investment Advisers Act of 1940, is the beneficial owner of 
1,585,647 shares or 3.329% of the Common Stock 
outstanding of Steel Dynamics Incorporated  ("the 
Company") as a result of acting as investment adviser to 
various investment companies registered under Section 8 of 
the Investment Company Act of 1940.

	Edward C. Johnson 3d, FMR Corp., through its 
control of Fidelity, and the funds each has sole power to 
dispose of the 1,585,647 shares owned by the Funds.

	Neither FMR Corp. nor Edward C. Johnson 3d, 
Chairman of FMR Corp., has the sole power to vote or direct 
the voting of the shares owned directly by the Fidelity Funds, 
which power resides with the Funds' Boards of Trustees.  
Fidelity carries out the voting of the shares under written 
guidelines established by the Funds' Boards of Trustees.

	Fidelity Management Trust Company, 82 Devonshire 
Street, Boston, Massachusetts 02109, a wholly-owned 
subsidiary of FMR Corp. and a bank as defined in Section 
3(a)(6) of the Securities Exchange Act of 1934, is the 
beneficial owner of  89,499 shares or 0.188% of the Common 
Stock outstanding of the Company as a result of its serving as 
investment manager of the institutional account(s).

	Edward C. Johnson 3d and FMR Corp., through its 
control of Fidelity Management Trust Company, each has 
sole dispositive power over 89,499 shares and sole power to 
vote or to direct the voting of 89,499 shares of Common 
Stock owned by the institutional account(s) as reported 
above.

	Members of the Edward C. Johnson 3d family are the 
predominant owners of Class B shares of common stock of 
FMR Corp., representing approximately 49% of the voting 
power of FMR Corp.  Mr. Johnson 3d owns 12.0% and 
Abigail Johnson owns 24.5% of the aggregate outstanding 
voting stock of FMR Corp.  Mr. Johnson 3d is Chairman of 
FMR Corp. and Abigail P. Johnson is a Director of FMR 
Corp.  The Johnson family group and all other Class B 
shareholders have entered into a shareholders' voting 
agreement under which all Class B shares will be voted in 
accordance with the majority vote of Class B shares.  
Accordingly, through their ownership of voting common 
stock and the execution of the shareholders' voting 
agreement, members of the Johnson family may be deemed, 
under the Investment Company Act of 1940, to form a 
controlling group with respect to FMR Corp.

 


	SCHEDULE 13G - TO BE INCLUDED IN 
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)
	RULE 13d-1(f)(1)  AGREEMENT

	The undersigned persons, on April 11, 2005, agree and 
consent to the joint filing on their behalf of this Schedule 13G 
in connection with their beneficial ownership of the Common 
Stock of Steel Dynamics Incorporated at March 31, 2005.

	FMR Corp.

	By /s/ Eric D. Roiter			
	Eric D. Roiter
	Duly authorized under Power of Attorney
	dated December 30, 1997, by and on behalf
	of FMR Corp. and its direct and indirect 
	subsidiaries

	Edward C. Johnson 3d

	By /s/ Eric D. Roiter			
	Eric D. Roiter
	Duly authorized under Power of Attorney
	dated December 30, 1997, by and on behalf
	of Edward C. Johnson 3d

	Abigail P. Johnson

	By /s/ Eric D. Roiter			
	Eric D. Roiter
	Duly authorized under Power of Attorney
	dated December 30, 1997, by and on behalf
	of Abigail P. Johnson

	Fidelity Management & Research Company

	By /s/ Eric D. Roiter			
	Eric D. Roiter
	Senior V.P. and General Counsel